PERLFAQ4
Section: Perl Programmers Reference Guide (1)Updated: 2009-08-15
Index Return to Main Contents
NAME
perlfaq4 - Data ManipulationDESCRIPTION
This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues.Data: Numbers
Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
Internally, your computer represents floating-point numbers in binary. Digital (as in powers of two) computers cannot store all numbers exactly. Some real numbers lose precision in the process. This is a problem with how computers store numbers and affects all computer languages, not just Perl.perlnumber shows the gory details of number representations and conversions.
To limit the number of decimal places in your numbers, you can use the printf or sprintf function. See the ``Floating Point Arithmetic'' for more details.
printf "%.2f", 10/3; my $number = sprintf "%.2f", 10/3;
Why is int() broken?
Your "int()" is most probably working just fine. It's the numbers that aren't quite what you think.First, see the answer to ``Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?''.
For example, this
print int(0.6/0.2-2), "\n";
will in most computers print 0, not 1, because even such simple numbers as 0.6 and 0.2 cannot be presented exactly by floating-point numbers. What you think in the above as 'three' is really more like 2.9999999999999995559.
Why isn't my octal data interpreted correctly?
(contributed by brian d foy)You're probably trying to convert a string to a number, which Perl only converts as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10:
my $string = '0644'; print $string + 0; # prints 644 print $string + 44; # prints 688, certainly not octal!
This problem usually involves one of the Perl built-ins that has the same name a unix command that uses octal numbers as arguments on the command line. In this example, "chmod" on the command line knows that its first argument is octal because that's what it does:
%prompt> chmod 644 file
If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as octal numbers either by prefixing the digits with a 0 or using "oct":
chmod( 0644, $file); # right, has leading zero chmod( oct(644), $file ); # also correct
The problem comes in when you take your numbers from something that Perl thinks is a string, such as a command line argument in @ARGV:
chmod( $ARGV[0], $file); # wrong, even if "0644" chmod( oct($ARGV[0]), $file ); # correct, treat string as octal
You can always check the value you're using by printing it in octal notation to ensure it matches what you think it should be. Print it in octal and decimal format:
printf "0%o %d", $number, $number;
Does Perl have a round() function? What about ceil() and floor()? Trig functions?
Remember that "int()" merely truncates toward 0. For rounding to a certain number of digits, "sprintf()" or "printf()" is usually the easiest route.
printf("%.3f", 3.1415926535); # prints 3.142
The "POSIX" module (part of the standard Perl distribution) implements "ceil()", "floor()", and a number of other mathematical and trigonometric functions.
use POSIX; $ceil = ceil(3.5); # 4 $floor = floor(3.5); # 3
In 5.000 to 5.003 perls, trigonometry was done in the "Math::Complex" module. With 5.004, the "Math::Trig" module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the "Math::Complex" module and some functions can break out from the real axis into the complex plane, for example the inverse sine of 2.
Rounding in financial applications can have serious implications, and the rounding method used should be specified precisely. In these cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself.
To see why, notice how you'll still have an issue on half-way-point alternation:
for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7 0.8 0.8 0.9 0.9 1.0 1.0
Don't blame Perl. It's the same as in C. IEEE says we have to do this. Perl numbers whose absolute values are integers under 2**31 (on 32 bit machines) will work pretty much like mathematical integers. Other numbers are not guaranteed.
How do I convert between numeric representations/bases/radixes?
As always with Perl there is more than one way to do it. Below are a few examples of approaches to making common conversions between number representations. This is intended to be representational rather than exhaustive.Some of the examples later in perlfaq4 use the "Bit::Vector" module from CPAN. The reason you might choose "Bit::Vector" over the perl built in functions is that it works with numbers of ANY size, that it is optimized for speed on some operations, and for at least some programmers the notation might be familiar.
- How do I convert hexadecimal into decimal
-
Using perl's built in conversion of "0x" notation:
$dec = 0xDEADBEEF;
Using the "hex" function:
$dec = hex("DEADBEEF");
Using "pack":
$dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8)));
Using the CPAN module "Bit::Vector":
use Bit::Vector; $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); $dec = $vec->to_Dec();
- How do I convert from decimal to hexadecimal
-
Using "sprintf":
$hex = sprintf("%X",<PHONE_NUMBER>); # upper case A-F $hex = sprintf("%x",<PHONE_NUMBER>); # lower case a-f
Using "unpack":
$hex = unpack("H*", pack("N",<PHONE_NUMBER>));
Using "Bit::Vector":
use Bit::Vector; $vec = Bit::Vector->new_Dec(32, -559038737); $hex = $vec->to_Hex();
And "Bit::Vector" supports odd bit counts:
use Bit::Vector; $vec = Bit::Vector->new_Dec(33,<PHONE_NUMBER>); $vec->Resize(32); # suppress leading 0 if unwanted $hex = $vec->to_Hex();
- How do I convert from octal to decimal
-
Using Perl's built in conversion of numbers with leading zeros:
$dec =<PHONE_NUMBER>57; # note the leading 0!
Using the "oct" function:
$dec = oct("33653337357");
Using "Bit::Vector":
use Bit::Vector; $vec = Bit::Vector->new(32); $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); $dec = $vec->to_Dec();
- How do I convert from decimal to octal
-
Using "sprintf":
$oct = sprintf("%o",<PHONE_NUMBER>);
Using "Bit::Vector":
use Bit::Vector; $vec = Bit::Vector->new_Dec(32, -559038737); $oct = reverse join('', $vec->Chunk_List_Read(3));
- How do I convert from binary to decimal
-
Perl 5.6 lets you write binary numbers directly with
the "0b" notation:
$number = 0b10110110;
Using "oct":
my $input = "10110110"; $decimal = oct( "0b$input" );
Using "pack" and "ord":
$decimal = ord(pack('B8', '10110110'));
Using "pack" and "unpack" for larger strings:
$int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); $dec = sprintf("%d", $int); # substr() is used to left pad a 32 character string with zeros.
Using "Bit::Vector":
$vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); $dec = $vec->to_Dec();
- How do I convert from decimal to binary
-
Using "sprintf" (perl 5.6+):
$bin = sprintf("%b",<PHONE_NUMBER>);
Using "unpack":
$bin = unpack("B*", pack("N",<PHONE_NUMBER>));
Using "Bit::Vector":
use Bit::Vector; $vec = Bit::Vector->new_Dec(32, -559038737); $bin = $vec->to_Bin();
The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader.
Why doesn't & work the way I want it to?
The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string "3" is the bit pattern 00110011). The operators work with the binary form of a number (the number 3 is treated as the bit pattern 00000011).So, saying "11 & 3" performs the ``and'' operation on numbers (yielding 3). Saying "11" & "3" performs the ``and'' operation on strings (yielding "1").
Most problems with "&" and "|" arise because the programmer thinks they have a number but really it's a string. The rest arise because the programmer says:
if ("\020\020" & "\101\101") { # ... }
but a string consisting of two null bytes (the result of "\020\020" & "\101\101") is not a false value in Perl. You need:
if ( ("\020\020" & "\101\101") !~ /[^\000]/) { # ... }
How do I multiply matrices?
Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) or the PDL extension (also available from CPAN).How do I perform an operation on a series of integers?
To call a function on each element in an array, and collect the results, use:
@results = map { my_func($_) } @array;
For example:
@triple = map { 3 * $_ } @single;
To call a function on each element of an array, but ignore the results:
foreach $iterator (@array) { some_func($iterator); }
To call a function on each integer in a (small) range, you can use:
@results = map { some_func($_) } (5 .. 25);
but you should be aware that the ".." operator creates an array of all integers in the range. This can take a lot of memory for large ranges. Instead use:
@results = (); for ($i=5; $i < 500_005; $i++) { push(@results, some_func($i)); }
This situation has been fixed in Perl5.005. Use of ".." in a "for" loop will iterate over the range, without creating the entire range.
for my $i (5 .. 500_005) { push(@results, some_func($i)); }
will not create a list of 500,000 integers.
How can I output Roman numerals?
Get the http://www.cpan.org/modules/by-module/Roman module.Why aren't my random numbers random?
If you're using a version of Perl before 5.004, you must call "srand" once at the start of your program to seed the random number generator.
BEGIN { srand() if $] < 5.004 }
5.004 and later automatically call "srand" at the beginning. Don't call "srand" more than once---you make your numbers less random, rather than more.
Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). see the random article in the ``Far More Than You Ever Wanted To Know'' collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy of Tom Phoenix, talks more about this. John von Neumann said, ``Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.''
If you want numbers that are more random than "rand" with "srand" provides, you should also check out the "Math::TrulyRandom" module from CPAN. It uses the imperfections in your system's timer to generate random numbers, but this takes quite a while. If you want a better pseudorandom generator than comes with your operating system, look at ``Numerical Recipes in C'' at http://www.nr.com/ .
How do I get a random number between X and Y?
To get a random number between two values, you can use the "rand()" built-in to get a random number between 0 and 1. From there, you shift that into the range that you want."rand($x)" returns a number such that "0 <= rand($x) < $x". Thus what you want to have perl figure out is a random number in the range from 0 to the difference between your X and Y.
That is, to get a number between 10 and 15, inclusive, you want a random number between 0 and 5 that you can then add to 10.
my $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 )
Hence you derive the following simple function to abstract that. It selects a random integer between the two given integers (inclusive), For example: "random_int_between(50,120)".
sub random_int_between { my($min, $max) = @_; # Assumes that the two arguments are integers themselves! return $min if $min == $max; ($min, $max) = ($max, $min) if $min > $max; return $min + int rand(1 + $max - $min); }
Data: Dates
How do I find the day or week of the year?
The localtime function returns the day of the year. Without an argument localtime uses the current time.
$day_of_year = (localtime)[7];
The "POSIX" module can also format a date as the day of the year or week of the year.
use POSIX qw/strftime/; my $day_of_year = strftime "%j", localtime; my $week_of_year = strftime "%W", localtime;
To get the day of year for any date, use "POSIX"'s "mktime" to get a time in epoch seconds for the argument to localtime.
use POSIX qw/mktime strftime/; my $week_of_year = strftime "%W", localtime( mktime( 0, 0, 0, 18, 11, 87 ) );
The "Date::Calc" module provides two functions to calculate these.
use Date::Calc; my $day_of_year = Day_of_Year( 1987, 12, 18 ); my $week_of_year = Week_of_Year( 1987, 12, 18 );
How do I find the current century or millennium?
Use the following simple functions:
sub get_century { return int((((localtime(shift || time))[5] + 1999))/100); } sub get_millennium { return 1+int((((localtime(shift || time))[5] + 1899))/1000); }
On some systems, the "POSIX" module's "strftime()" function has been extended in a non-standard way to use a %C format, which they sometimes claim is the ``century''. It isn't, because on most such systems, this is only the first two digits of the four-digit year, and thus cannot be used to reliably determine the current century or millennium.
How can I compare two dates and find the difference?
(contributed by brian d foy)You could just store all your dates as a number and then subtract. Life isn't always that simple though. If you want to work with formatted dates, the "Date::Manip", "Date::Calc", or "DateTime" modules can help you.
How can I take a string and turn it into epoch seconds?
If it's a regular enough string that it always has the same format, you can split it up and pass the parts to "timelocal" in the standard "Time::Local" module. Otherwise, you should look into the "Date::Calc" and "Date::Manip" modules from CPAN.How can I find the Julian Day?
(contributed by brian d foy and Dave Cross)You can use the "Time::JulianDay" module available on CPAN. Ensure that you really want to find a Julian day, though, as many people have different ideas about Julian days. See http://www.hermetic.ch/cal_stud/jdn.htm for instance.
You can also try the "DateTime" module, which can convert a date/time to a Julian Day.
$ perl -MDateTime -le'print DateTime->today->jd' 2453401.5
Or the modified Julian Day
$ perl -MDateTime -le'print DateTime->today->mjd' 53401
Or even the day of the year (which is what some people think of as a Julian day)
$ perl -MDateTime -le'print DateTime->today->doy' 31
How do I find yesterday's date?
(contributed by brian d foy)Use one of the Date modules. The "DateTime" module makes it simple, and give you the same time of day, only the day before.
use DateTime; my $yesterday = DateTime->now->subtract( days => 1 ); print "Yesterday was $yesterday\n";
You can also use the "Date::Calc" module using its "Today_and_Now" function.
use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); print "@date_time\n";
Most people try to use the time rather than the calendar to figure out dates, but that assumes that days are twenty-four hours each. For most people, there are two days a year when they aren't: the switch to and from summer time throws this off. Let the modules do the work.
If you absolutely must do it yourself (or can't use one of the modules), here's a solution using "Time::Local", which comes with Perl:
# contributed by Gunnar Hjalmarsson use Time::Local; my $today = timelocal 0, 0, 12, ( localtime )[3..5]; my ($d, $m, $y) = ( localtime $today-86400 )[3..5]; printf "Yesterday: %d-%02d-%02d\n", $y+1900, $m+1, $d;
In this case, you measure the day starting at noon, and subtract 24 hours. Even if the length of the calendar day is 23 or 25 hours, you'll still end up on the previous calendar day, although not at noon. Since you don't care about the time, the one hour difference doesn't matter and you end up with the previous date.
Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is Y2K compliant (whatever that means). The programmers you've hired to use it, however, probably are not.Long answer: The question belies a true understanding of the issue. Perl is just as Y2K compliant as your pencil---no more, and no less. Can you use your pencil to write a non-Y2K-compliant memo? Of course you can. Is that the pencil's fault? Of course it isn't.
The date and time functions supplied with Perl (gmtime and localtime) supply adequate information to determine the year well beyond 2000 (2038 is when trouble strikes for 32-bit machines). The year returned by these functions when used in a list context is the year minus 1900. For years between 1910 and 1999 this happens to be a 2-digit decimal number. To avoid the year 2000 problem simply do not treat the year as a 2-digit number. It isn't.
When gmtime() and localtime() are used in scalar context they return a timestamp string that contains a fully-expanded year. For example, "$timestamp = gmtime(1005613200)" sets $timestamp to ``Tue Nov 13 01:00:00 2001''. There's no year 2000 problem here.
That doesn't mean that Perl can't be used to create non-Y2K compliant programs. It can. But so can your pencil. It's the fault of the user, not the language. At the risk of inflaming the NRA: ``Perl doesn't break Y2K, people do.'' See http://www.perl.org/about/y2k.html for a longer exposition.
Data: Strings
How do I validate input?
(contributed by brian d foy)There are many ways to ensure that values are what you expect or want to accept. Besides the specific examples that we cover in the perlfaq, you can also look at the modules with ``Assert'' and ``Validate'' in their names, along with other modules such as "Regexp::Common".
Some modules have validation for particular types of input, such as "Business::ISBN", "Business::CreditCard", "Email::Valid", and "Data::Validate::IP".
How do I unescape a string?
It depends just what you mean by ``escape''. URL escapes are dealt with in perlfaq9. Shell escapes with the backslash ("\") character are removed with
s/\\(.)/$1/g;
This won't expand "\n" or "\t" or any other special escapes.
How do I remove consecutive pairs of characters?
(contributed by brian d foy)You can use the substitution operator to find pairs of characters (or runs of characters) and replace them with a single instance. In this substitution, we find a character in "(.)". The memory parentheses store the matched character in the back-reference "\1" and we use that to require that the same thing immediately follow it. We replace that part of the string with the character in $1.
s/(.)\1/$1/g;
We can also use the transliteration operator, "tr///". In this example, the search list side of our "tr///" contains nothing, but the "c" option complements that so it contains everything. The replacement list also contains nothing, so the transliteration is almost a no-op since it won't do any replacements (or more exactly, replace the character with itself). However, the "s" option squashes duplicated and consecutive characters in the string so a character does not show up next to itself
my $str = 'Haarlem'; # in the Netherlands $str =~ tr///cs; # Now Harlem, like in New York
How do I expand function calls in a string?
(contributed by brian d foy)This is documented in perlref, and although it's not the easiest thing to read, it does work. In each of these examples, we call the function inside the braces used to dereference a reference. If we have more than one return value, we can construct and dereference an anonymous array. In this case, we call the function in list context.
print "The time values are @{ [localtime] }.\n";
If we want to call the function in scalar context, we have to do a bit more work. We can really have any code we like inside the braces, so we simply have to end with the scalar reference, although how you do that is up to you, and you can use code inside the braces. Note that the use of parens creates a list context, so we need "scalar" to force the scalar context on the function:
print "The time is ${\(scalar localtime)}.\n" print "The time is ${ my $x = localtime; \$x }.\n";
If your function already returns a reference, you don't need to create the reference yourself.
sub timestamp { my $t = localtime; \$t } print "The time is ${ timestamp() }.\n";
The "Interpolation" module can also do a lot of magic for you. You can specify a variable name, in this case "E", to set up a tied hash that does the interpolation for you. It has several other methods to do this as well.
use Interpolation E => 'eval'; print "The time values are $E{localtime()}.\n";
In most cases, it is probably easier to simply use string concatenation, which also forces scalar context.
print "The time is " . localtime() . ".\n";
How do I find matching/nesting anything?
This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single characters, a pattern like "/x([^x]*)x/" will get the intervening bits in $1. For multiple ones, then something more like "/alpha(.*?)omega/" would be needed. But none of these deals with nested patterns. For balanced expressions using "(", "{", "[" or "<" as delimiters, use the CPAN module Regexp::Common, or see ``(??{ code })'' in perlre. For other cases, you'll have to write a parser.If you are serious about writing a parser, there are a number of modules or oddities that will make your life a lot easier. There are the CPAN modules "Parse::RecDescent", "Parse::Yapp", and "Text::Balanced"; and the "byacc" program. Starting from perl 5.8 the "Text::Balanced" is part of the standard distribution.
One simple destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time:
while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { # do something with $1 }
A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work:
# $_ contains the string to parse # BEGIN and END are the opening and closing markers for the # nested text. @( = ('(',''); @) = (')',''); ($re=$_)=~s/((BEGIN)|(END)|.)/$)[!$3]\Q$1\E$([!$2]/gs; @$ = (eval{/$re/},$@!~/unmatched/i); print<EMAIL_ADDRESS>if( $$[-1] );
How do I reverse a string?
Use "reverse()" in scalar context, as documented in ``reverse'' in perlfunc.
$reversed = reverse $string;
How do I expand tabs in a string?
You can do it yourself:
1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e;
Or you can just use the "Text::Tabs" module (part of the standard Perl distribution).
use Text::Tabs; @expanded_lines = expand(@lines_with_tabs);
How do I reformat a paragraph?
Use "Text::Wrap" (part of the standard Perl distribution):
use Text::Wrap; print wrap("\t", ' ', @paragraphs);
The paragraphs you give to "Text::Wrap" should not contain embedded newlines. "Text::Wrap" doesn't justify the lines (flush-right).
Or use the CPAN module "Text::Autoformat". Formatting files can be easily done by making a shell alias, like so:
alias fmt="perl -i -MText::Autoformat -n0777 \ -e 'print autoformat $_, {all=>1}' $*"
See the documentation for "Text::Autoformat" to appreciate its many capabilities.
How can I access or change N characters of a string?
You can access the first characters of a string with substr(). To get the first character, for example, start at position 0 and grab the string of length 1.
$string = "Just another Perl Hacker"; $first_char = substr( $string, 0, 1 ); # 'J'
To change part of a string, you can use the optional fourth argument which is the replacement string.
substr( $string, 13, 4, "Perl 5.8.0" );
You can also use substr() as an lvalue.
substr( $string, 13, 4 ) = "Perl 5.8.0";
How do I change the Nth occurrence of something?
You have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of "whoever" or "whomever" into "whosoever" or "whomsoever", case insensitively. These all assume that $_ contains the string to be altered.
$count = 0; s{((whom?)ever)}{ ++$count == 5 # is it the 5th? ? "${2}soever" # yes, swap : $1 # renege and leave it there }ige;
In the more general case, you can use the "/g" modifier in a "while" loop, keeping count of matches.
$WANT = 3; $count = 0; $_ = "One fish two fish red fish blue fish"; while (/(\w+)\s+fish\b/gi) { if (++$count == $WANT) { print "The third fish is a $1 one.\n"; } }
That prints out: "The third fish is a red one." You can also use a repetition count and repeated pattern like this:
/(?:\w+\s+fish\s+){2}(\w+)\s+fish/i;
How can I count the number of occurrences of a substring within a string?
There are a number of ways, with varying efficiency. If you want a count of a certain single character (X) within a string, you can use the "tr///" function like so:
$string = "ThisXlineXhasXsomeXx'sXinXit"; $count = ($string =~ tr/X//); print "There are $count X characters in the string";
This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, "tr///" won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers:
$string = "-9 55 48 -2 23 -76 4 14 -44"; while ($string =~ /-\d+/g) { $count++ } print "There are $count negative numbers in the string";
Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches.
$count = () = $string =~ /-\d+/g;
Does Perl have a Year 2038 problem?
No, all of Perl's built in date and time functions and modules will work to about 2 billion years before and after 1970.Many systems cannot count time past the year 2038. Older versions of Perl were dependent on the system to do date calculation and thus shared their 2038 bug.
How do I capitalize all the words on one line?
(contributed by brian d foy)Damian Conway's Text::Autoformat handles all of the thinking for you.
use Text::Autoformat; my $x = "Dr. Strangelove or: How I Learned to Stop ". "Worrying and Love the Bomb"; print $x, "\n"; for my $style (qw( sentence title highlight )) { print autoformat($x, { case => $style }), "\n"; }
How do you want to capitalize those words?
FRED AND BARNEY'S LODGE # all uppercase Fred And Barney's Lodge # title case Fred and Barney's Lodge # highlight case
It's not as easy a problem as it looks. How many words do you think are in there? Wait for it... wait for it.... If you answered 5 you're right. Perl words are groups of "\w+", but that's not what you want to capitalize. How is Perl supposed to know not to capitalize that "s" after the apostrophe? You could try a regular expression:
$string =~ s/ ( (^\w) #at the beginning of the line | # or (\s\w) #preceded by whitespace ) /\U$1/xg; $string =~ s/([\w']+)/\u\L$1/g;
Now, what if you don't want to capitalize that ``and''? Just use Text::Autoformat and get on with the next problem. :)
How can I split a [character] delimited string except when inside [character]?
Several modules can handle this sort of parsing---"Text::Balanced", "Text::CSV", "Text::CSV_XS", and "Text::ParseWords", among others.Take the example case of trying to split a string that is comma-separated into its different fields. You can't use "split(/,/)" because you shouldn't split if the comma is inside quotes. For example, take a data line like this:
SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped"
Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of Mastering Regular Expressions, to handle these for us. He suggests (assuming your string is contained in $text):
@new = (); push(@new, $+) while $text =~ m{ "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes | ([^,]+),? | , }gx; push(@new, undef) if substr($text,-1,1) eq ',';
If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, "like \"this\"".
Alternatively, the "Text::ParseWords" module (part of the standard Perl distribution) lets you say:
use Text::ParseWords; @new = quotewords(",", 0, $text);
How do I strip blank space from the beginning/end of a string?
(contributed by brian d foy)A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions.
s/^\s+//; s/\s+$//;
You can also write that as a single substitution, although it turns out the combined statement is slower than the separate ones. That might not matter to you, though.
s/^\s+|\s+$//g;
In this regular expression, the alternation matches either at the beginning or the end of the string since the anchors have a lower precedence than the alternation. With the "/g" flag, the substitution makes all possible matches, so it gets both. Remember, the trailing newline matches the "\s+", and the "$" anchor can match to the physical end of the string, so the newline disappears too. Just add the newline to the output, which has the added benefit of preserving ``blank'' (consisting entirely of whitespace) lines which the "^\s+" would remove all by itself.
while( <> ) { s/^\s+|\s+$//g; print "$_\n"; }
For a multi-line string, you can apply the regular expression to each logical line in the string by adding the "/m" flag (for ``multi-line''). With the "/m" flag, the "$" matches before an embedded newline, so it doesn't remove it. It still removes the newline at the end of the string.
$string =~ s/^\s+|\s+$//gm;
Remember that lines consisting entirely of whitespace will disappear, since the first part of the alternation can match the entire string and replace it with nothing. If need to keep embedded blank lines, you have to do a little more work. Instead of matching any whitespace (since that includes a newline), just match the other whitespace.
$string =~ s/^[\t\f ]+|[\t\f ]+$//mg;
How do I pad a string with blanks or pad a number with zeroes?
In the following examples, $pad_len is the length to which you wish to pad the string, $text or $num contains the string to be padded, and $pad_char contains the padding character. You can use a single character string constant instead of the $pad_char variable if you know what it is in advance. And in the same way you can use an integer in place of $pad_len if you know the pad length in advance.The simplest method uses the "sprintf" function. It can pad on the left or right with blanks and on the left with zeroes and it will not truncate the result. The "pack" function can only pad strings on the right with blanks and it will truncate the result to a maximum length of $pad_len.
# Left padding a string with blanks (no truncation): $padded = sprintf("%${pad_len}s", $text); $padded = sprintf("%*s", $pad_len, $text); # same thing # Right padding a string with blanks (no truncation): $padded = sprintf("%-${pad_len}s", $text); $padded = sprintf("%-*s", $pad_len, $text); # same thing # Left padding a number with 0 (no truncation): $padded = sprintf("%0${pad_len}d", $num); $padded = sprintf("%0*d", $pad_len, $num); # same thing # Right padding a string with blanks using pack (will truncate): $padded = pack("A$pad_len",$text);
If you need to pad with a character other than blank or zero you can use one of the following methods. They all generate a pad string with the "x" operator and combine that with $text. These methods do not truncate $text.
Left and right padding with any character, creating a new string:
$padded = $pad_char x ( $pad_len - length( $text ) ) . $text; $padded = $text . $pad_char x ( $pad_len - length( $text ) );
Left and right padding with any character, modifying $text directly:
substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); $text .= $pad_char x ( $pad_len - length( $text ) );
How do I extract selected columns from a string?
(contributed by brian d foy)If you know where the columns that contain the data, you can use "substr" to extract a single column.
my $column = substr( $line, $start_column, $length );
You can use "split" if the columns are separated by whitespace or some other delimiter, as long as whitespace or the delimiter cannot appear as part of the data.
my $line = ' fred barney betty '; my @columns = split /\s+/, $line; # ( '', 'fred', 'barney', 'betty' ); my $line = 'fred||barney||betty'; my @columns = split /\|/, $line; # ( 'fred', '', 'barney', '', 'betty' );
If you want to work with comma-separated values, don't do this since that format is a bit more complicated. Use one of the modules that handle that format, such as "Text::CSV", "Text::CSV_XS", or "Text::CSV_PP".
If you want to break apart an entire line of fixed columns, you can use "unpack" with the A (ASCII) format. by using a number after the format specifier, you can denote the column width. See the "pack" and "unpack" entries in perlfunc for more details.
my @fields = unpack( $line, "A8 A8 A8 A16 A4" );
Note that spaces in the format argument to "unpack" do not denote literal spaces. If you have space separated data, you may want "split" instead.
How do I find the soundex value of a string?
(contributed by brian d foy)You can use the Text::Soundex module. If you want to do fuzzy or close matching, you might also try the "String::Approx", and "Text::Metaphone", and "Text::DoubleMetaphone" modules.
How can I expand variables in text strings?
(contributed by brian d foy)If you can avoid it, don't, or if you can use a templating system, such as "Text::Template" or "Template" Toolkit, do that instead. You might even be able to get the job done with "sprintf" or "printf":
my $string = sprintf 'Say hello to %s and %s', $foo, $bar;
However, for the one-off simple case where I don't want to pull out a full templating system, I'll use a string that has two Perl scalar variables in it. In this example, I want to expand $foo and $bar to their variable's values:
my $foo = 'Fred'; my $bar = 'Barney'; $string = 'Say hello to $foo and $bar';
One way I can do this involves the substitution operator and a double "/e" flag. The first "/e" evaluates $1 on the replacement side and turns it into $foo. The second /e starts with $foo and replaces it with its value. $foo, then, turns into 'Fred', and that's finally what's left in the string:
$string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney'
The "/e" will also silently ignore violations of strict, replacing undefined variable names with the empty string. Since I'm using the "/e" flag (twice even!), I have all of the same security problems I have with "eval" in its string form. If there's something odd in $foo, perhaps something like "@{[ system "rm -rf /" ]}", then I could get myself in trouble.
To get around the security problem, I could also pull the values from a hash instead of evaluating variable names. Using a single "/e", I can check the hash to ensure the value exists, and if it doesn't, I can replace the missing value with a marker, in this case "???" to signal that I missed something:
my $string = 'This has $foo and $bar'; my %Replacements = ( foo => 'Fred', ); # $string =~ s/\$(\w+)/$Replacements{$1}/g; $string =~ s/\$(\w+)/ exists $Replacements{$1} ? $Replacements{$1} : '???' /eg; print $string;
What's wrong with always quoting $vars?
The problem is that those double-quotes force stringification---coercing numbers and references into strings---even when you don't want them to be strings. Think of it this way: double-quote expansion is used to produce new strings. If you already have a string, why do you need more?If you get used to writing odd things like these:
print "$var"; # BAD $new = "$old"; # BAD somefunc("$var"); # BAD
You'll be in trouble. Those should (in 99.8% of the cases) be the simpler and more direct:
print $var; $new = $old; somefunc($var);
Otherwise, besides slowing you down, you're going to break code when the thing in the scalar is actually neither a string nor a number, but a reference:
func(\@array); sub func { my $aref = shift; my $oref = "$aref"; # WRONG }
You can also get into subtle problems on those few operations in Perl that actually do care about the difference between a string and a number, such as the magical "++" autoincrement operator or the syscall() function.
Stringification also destroys arrays.
@lines = `command`; print "@lines"; # WRONG - extra blanks print @lines; # right
Why don't my <<HERE documents work?
Check for these three things:- There must be no space after the << part.
- There (probably) should be a semicolon at the end.
- You can't (easily) have any space in front of the tag.
If you want to indent the text in the here document, you can do this:
# all in one ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; your text goes here HERE_TARGET
But the HERE_TARGET must still be flush against the margin. If you want that indented also, you'll have to quote in the indentation.
($quote = <<' FINIS') =~ s/^\s+//gm; ...we will have peace, when you and all your works have perished--and the works of your dark master to whom you would deliver us.  --Theoden in /usr/src/perl/taint.c FINIS $quote =~ s/\s+--/\n--/;
A nice general-purpose fixer-upper function for indented here documents follows. It expects to be called with a here document as its argument. It looks to see whether each line begins with a common substring, and if so, strips that substring off. Otherwise, it takes the amount of leading whitespace found on the first line and removes that much off each subsequent line.
sub fix { local $_ = shift; my ($white, $leader); # common whitespace and common leading string if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { ($white, $leader) = ($2, quotemeta($1)); } else { ($white, $leader) = (/^(\s+)/, ''); } s/^\s*?$leader(?:$white)?//gm; return $_; }
This works with leading special strings, dynamically determined:
$remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; @@@ int @@@ runops() { @@@ SAVEI32(runlevel); @@@ runlevel++; @@@ while ( op = (*op->op_ppaddr)() ); @@@ TAINT_NOT; @@@ return 0; @@@ } MAIN_INTERPRETER_LOOP
Or with a fixed amount of leading whitespace, with remaining indentation correctly preserved:
$poem = fix<<EVER_ON_AND_ON; Now far ahead the Road has gone, And I must follow, if I can, Pursuing it with eager feet, Until it joins some larger way Where many paths and errands meet. And whither then? I cannot say. --Bilbo in /usr/src/perl/pp_ctl.c EVER_ON_AND_ON
Data: Arrays
What is the difference between a list and an array?
An array has a changeable length. A list does not. An array is something you can push or pop, while a list is a set of values. Some people make the distinction that a list is a value while an array is a variable. Subroutines are passed and return lists, you put things into list context, you initialize arrays with lists, and you "foreach()" across a list. "@" variables are arrays, anonymous arrays are arrays, arrays in scalar context behave like the number of elements in them, subroutines access their arguments through the array @_, and "push"/"pop"/"shift" only work on arrays.As a side note, there's no such thing as a list in scalar context. When you say
$scalar = (2, 5, 7, 9);
you're using the comma operator in scalar context, so it uses the scalar comma operator. There never was a list there at all! This causes the last value to be returned: 9.
What is the difference between $array[1] and @array[1]?
The former is a scalar value; the latter an array slice, making it a list with one (scalar) value. You should use $ when you want a scalar value (most of the time) and @ when you want a list with one scalar value in it (very, very rarely; nearly never, in fact).Sometimes it doesn't make a difference, but sometimes it does. For example, compare:
$good[0] = `some program that outputs several lines`;
with
@bad[0] = `same program that outputs several lines`;
The "use warnings" pragma and the -w flag will warn you about these matters.
How can I remove duplicate elements from a list or array?
(contributed by brian d foy)Use a hash. When you think the words ``unique'' or ``duplicated'', think ``hash keys''.
If you don't care about the order of the elements, you could just create the hash then extract the keys. It's not important how you create that hash: just that you use "keys" to get the unique elements.
my %hash = map { $_, 1 } @array; # or a hash slice: @hash{ @array } = (); # or a foreach: $hash{$_} = 1 foreach ( @array ); my @unique = keys %hash;
If you want to use a module, try the "uniq" function from "List::MoreUtils". In list context it returns the unique elements, preserving their order in the list. In scalar context, it returns the number of unique elements.
use List::MoreUtils qw(uniq); my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 my $unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7
You can also go through each element and skip the ones you've seen before. Use a hash to keep track. The first time the loop sees an element, that element has no key in %Seen. The "next" statement creates the key and immediately uses its value, which is "undef", so the loop continues to the "push" and increments the value for that key. The next time the loop sees that same element, its key exists in the hash and the value for that key is true (since it's not 0 or "undef"), so the next skips that iteration and the loop goes to the next element.
my @unique = (); my %seen = (); foreach my $elem ( @array ) { next if $seen{ $elem }++; push @unique, $elem; }
You can write this more briefly using a grep, which does the same thing.
my %seen = (); my @unique = grep { ! $seen{ $_ }++ } @array;
How can I tell whether a certain element is contained in a list or array?
(portions of this answer contributed by Anno Siegel and brian d foy)Hearing the word ``in'' is an indication that you probably should have used a hash, not a list or array, to store your data. Hashes are designed to answer this question quickly and efficiently. Arrays aren't.
That being said, there are several ways to approach this. In Perl 5.10 and later, you can use the smart match operator to check that an item is contained in an array or a hash:
use 5.010; if( $item ~~ @array ) { say "The array contains $item" } if( $item ~~ %hash ) { say "The hash contains $item" }
With earlier versions of Perl, you have to do a bit more work. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values:
@blues = qw/azure cerulean teal turquoise lapis-lazuli/; %is_blue = (); for (@blues) { $is_blue{$_} = 1 }
Now you can check whether $is_blue{$some_color}. It might have been a good idea to keep the blues all in a hash in the first place.
If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space:
@primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); @is_tiny_prime = (); for (@primes) { $is_tiny_prime[$_] = 1 } # or simply @istiny_prime[@primes] = (1) x @primes;
Now you check whether $is_tiny_prime[$some_number].
If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead:
@articles = ( 1..10, 150..2000, 2017 ); undef $read; for (@articles) { vec($read,$_,1) = 1 }
Now check whether "vec($read,$n,1)" is true for some $n.
These methods guarantee fast individual tests but require a re-organization of the original list or array. They only pay off if you have to test multiple values against the same array.
If you are testing only once, the standard module "List::Util" exports the function "first" for this purpose. It works by stopping once it finds the element. It's written in C for speed, and its Perl equivalent looks like this subroutine:
sub first (&@) { my $code = shift; foreach (@_) { return $_ if &{$code}(); } undef; }
If speed is of little concern, the common idiom uses grep in scalar context (which returns the number of items that passed its condition) to traverse the entire list. This does have the benefit of telling you how many matches it found, though.
my $is_there = grep $_ eq $whatever, @array;
If you want to actually extract the matching elements, simply use grep in list context.
my @matches = grep $_ eq $whatever, @array;
How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array:
@union = @intersection = @difference = (); %count = (); foreach $element (@array1, @array2) { $count{$element}++ } foreach $element (keys %count) { push @union, $element; push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; }
Note that this is the symmetric difference, that is, all elements in either A or in B but not in both. Think of it as an xor operation.
How do I test whether two arrays or hashes are equal?
With Perl 5.10 and later, the smart match operator can give you the answer with the least amount of work:
use 5.010; if( @array1 ~~ @array2 ) { say "The arrays are the same"; } if( %hash1 ~~ %hash2 ) # doesn't check values! { say "The hash keys are the same"; }
The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs.
$are_equal = compare_arrays(\@frogs, \@toads); sub compare_arrays { my ($first, $second) = @_; no warnings; # silence spurious -w undef complaints return 0 unless @$first == @$second; for (my $i = 0; $i < @$first; $i++) { return 0 if $first->[$i] ne $second->[$i]; } return 1; }
For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module "FreezeThaw":
use FreezeThaw qw(cmpStr); @a = @b = ( "this", "that", [ "more", "stuff" ] ); printf "a and b contain %s arrays\n", cmpStr(\@a, \@b) == 0 ? "the same" : "different";
This approach also works for comparing hashes. Here we'll demonstrate two different answers:
use FreezeThaw qw(cmpStr cmpStrHard); %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); $a{EXTRA} = \%b; $b{EXTRA} = \%a; printf "a and b contain %s hashes\n", cmpStr(\%a, \%b) == 0 ? "the same" : "different"; printf "a and b contain %s hashes\n", cmpStrHard(\%a, \%b) == 0 ? "the same" : "different";
The first reports that both those the hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader.
How do I find the first array element for which a condition is true?
To find the first array element which satisfies a condition, you can use the "first()" function in the "List::Util" module, which comes with Perl 5.8. This example finds the first element that contains ``Perl''.
use List::Util qw(first); my $element = first { /Perl/ } @array;
If you cannot use "List::Util", you can make your own loop to do the same thing. Once you find the element, you stop the loop with last.
my $found; foreach ( @array ) { if( /Perl/ ) { $found = $_; last } }
If you want the array index, you can iterate through the indices and check the array element at each index until you find one that satisfies the condition.
my( $found, $index ) = ( undef, -1 ); for( $i = 0; $i < @array; $i++ ) { if( $array[$i] =~ /Perl/ ) { $found = $array[$i]; $index = $i; last; } }
How do I handle linked lists?
In general, you usually don't need a linked list in Perl, since with regular arrays, you can push and pop or shift and unshift at either end, or you can use splice to add and/or remove arbitrary number of elements at arbitrary points. Both pop and shift are O(1) operations on Perl's dynamic arrays. In the absence of shifts and pops, push in general needs to reallocate on the order every log(N) times, and unshift will need to copy pointers each time.If you really, really wanted, you could use structures as described in perldsc or perltoot and do just what the algorithm book tells you to do. For example, imagine a list node like this:
$node = { VALUE => 42, LINK => undef, };
You could walk the list this way:
print "List: "; for ($node = $head; $node; $node = $node->{LINK}) { print $node->{VALUE}, " "; } print "\n";
You could add to the list this way:
my ($head, $tail); $tail = append($head, 1); # grow a new head for $value ( 2 .. 10 ) { $tail = append($tail, $value); } sub append { my($list, $value) = @_; my $node = { VALUE => $value }; if ($list) { $node->{LINK} = $list->{LINK}; $list->{LINK} = $node; } else { $_[0] = $node; # replace caller's version } return $node; }
But again, Perl's built-in are virtually always good enough.
How do I handle circular lists?
(contributed by brian d foy)If you want to cycle through an array endlessy, you can increment the index modulo the number of elements in the array:
my @array = qw( a b c ); my $i = 0; while( 1 ) { print $array[ $i++ % @array ], "\n"; last if $i > 20; }
You can also use "Tie::Cycle" to use a scalar that always has the next element of the circular array:
use Tie::Cycle; tie my $cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; print $cycle; # FFFFFF print $cycle; # 000000 print $cycle; # FFFF00
The "Array::Iterator::Circular" creates an iterator object for circular arrays:
use Array::Iterator::Circular; my $color_iterator = Array::Iterator::Circular->new( qw(red green blue orange) ); foreach ( 1 .. 20 ) { print $color_iterator->next, "\n"; }
How do I shuffle an array randomly?
If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or later installed, you can say:
use List::Util 'shuffle'; @shuffled = shuffle(@list);
If not, you can use a Fisher-Yates shuffle.
sub fisher_yates_shuffle { my $deck = shift; # $deck is a reference to an array return unless @$deck; # must not be empty! my $i = @$deck; while (--$i) { my $j = int rand ($i+1); @$deck[$i,$j] = @$deck[$j,$i]; } } # shuffle my mpeg collection # my @mpeg = <audio/*/*.mp3>; fisher_yates_shuffle( \@mpeg ); # randomize @mpeg in place print @mpeg;
Note that the above implementation shuffles an array in place, unlike the "List::Util::shuffle()" which takes a list and returns a new shuffled list.
You've probably seen shuffling algorithms that work using splice, randomly picking another element to swap the current element with
srand; @new = (); @old = 1 .. 10; # just a demo while (@old) { push(@new, splice(@old, rand @old, 1)); }
This is bad because splice is already O(N), and since you do it N times, you just invented a quadratic algorithm; that is, O(N**2). This does not scale, although Perl is so efficient that you probably won't notice this until you have rather largish arrays.
How do I process/modify each element of an array?
Use "for"/"foreach":
for (@lines) { s/foo/bar/; # change that word tr/XZ/ZX/; # swap those letters }
Here's another; let's compute spherical volumes:
for (@volumes = @radii) { # @volumes has changed parts $_ **= 3; $_ *= (4/3) * 3.14159; # this will be constant folded }
which can also be done with "map()" which is made to transform one list into another:
@volumes = map {$_ ** 3 * (4/3) * 3.14159} @radii;
If you want to do the same thing to modify the values of the hash, you can use the "values" function. As of Perl 5.6 the values are not copied, so if you modify $orbit (in this case), you modify the value.
for $orbit ( values %orbits ) { ($orbit **= 3) *= (4/3) * 3.14159; }
Prior to perl 5.6 "values" returned copies of the values, so older perl code often contains constructions such as @orbits{keys %orbits} instead of "values %orbits" where the hash is to be modified.
How do I select a random element from an array?
Use the "rand()" function (see ``rand'' in perlfunc):
$index = rand @array; $element = $array[$index];
Or, simply:
my $element = $array[ rand @array ];
How do I permute N elements of a list?
Use the "List::Permutor" module on CPAN. If the list is actually an array, try the "Algorithm::Permute" module (also on CPAN). It's written in XS code and is very efficient:
use Algorithm::Permute; my @array = 'a'..'d'; my $p_iterator = Algorithm::Permute->new ( \@array ); while (my @perm = $p_iterator->next) { print "next permutation: (@perm)\n"; }
For even faster execution, you could do:
use Algorithm::Permute; my @array = 'a'..'d'; Algorithm::Permute::permute { print "next permutation: (@array)\n"; } @array;
Here's a little program that generates all permutations of all the words on each line of input. The algorithm embodied in the "permute()" function is discussed in Volume 4 (still unpublished) of Knuth's The Art of Computer Programming and will work on any list:
#!/usr/bin/perl -n # Fischer-Krause ordered permutation generator sub permute (&@) { my $code = shift; my @idx = 0..$#_; while ( $code->(@_[@idx]) ) { my $p = $#idx; --$p while $idx[$p-1] > $idx[$p]; my $q = $p or return; push @idx, reverse splice @idx, $p; ++$q while $idx[$p-1] > $idx[$q]; @idx[$p-1,$q]=@idx[$q,$p-1]; } } permute { print "@_\n" } split;
The "Algorithm::Loops" module also provides the "NextPermute" and "NextPermuteNum" functions which efficiently find all unique permutations of an array, even if it contains duplicate values, modifying it in-place: if its elements are in reverse-sorted order then the array is reversed, making it sorted, and it returns false; otherwise the next permutation is returned.
"NextPermute" uses string order and "NextPermuteNum" numeric order, so you can enumerate all the permutations of 0..9 like this:
use Algorithm::Loops qw(NextPermuteNum); my @list= 0..9; do { print "@list\n" } while NextPermuteNum @list;
How do I sort an array by (anything)?
Supply a comparison function to sort() (described in ``sort'' in perlfunc):
@list = sort { $a <=> $b } @list;
The default sort function is cmp, string comparison, which would sort "(1, 2, 10)" into "(1, 10, 2)". "<=>", used above, is the numerical comparison operator.
If you have a complicated function needed to pull out the part you want to sort on, then don't do it inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the same element. Here's an example of how to pull out the first word after the first number on each item, and then sort those words case-insensitively.
@idx = (); for (@data) { ($item) = /\d+\s*(\S+)/; push @idx, uc($item); } @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ];
which could also be written this way, using a trick that's come to be known as the Schwartzian Transform:
@sorted = map { $_->[0] } sort { $a->[1] cmp $b->[1] } map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data;
If you need to sort on several fields, the following paradigm is useful.
@sorted = sort { field1($a) <=> field1($b) || field2($a) cmp field2($b) || field3($a) cmp field3($b) } @data;
This can be conveniently combined with precalculation of keys as given above.
See the sort article in the ``Far More Than You Ever Wanted To Know'' collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for more about this approach.
See also the question later in perlfaq4 on sorting hashes.
How do I manipulate arrays of bits?
Use "pack()" and "unpack()", or else "vec()" and the bitwise operations.For example, you don't have to store individual bits in an array (which would mean that you're wasting a lot of space). To convert an array of bits to a string, use "vec()" to set the right bits. This sets $vec to have bit N set only if $ints[N] was set:
@ints = (...); # array of bits, e.g. ( 1, 0, 0, 1, 1, 0 ... ) $vec = ''; foreach( 0 .. $#ints ) { vec($vec,$_,1) = 1 if $ints[$_]; }
The string $vec only takes up as many bits as it needs. For instance, if you had 16 entries in @ints, $vec only needs two bytes to store them (not counting the scalar variable overhead).
Here's how, given a vector in $vec, you can get those bits into your @ints array:
sub bitvec_to_list { my $vec = shift; my @ints; # Find null-byte density then select best algorithm if ($vec =~ tr/\0// / length $vec > 0.95) { use integer; my $i; # This method is faster with mostly null-bytes while($vec =~ /[^\0]/g ) { $i = -9 + 8 * pos $vec; push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); push @ints, $i if vec($vec, ++$i, 1); } } else { # This method is a fast general algorithm use integer; my $bits = unpack "b*", $vec; push @ints, 0 if $bits =~ s/^(\d)// && $1; push @ints, pos $bits while($bits =~ /1/g); } return \@ints; }
This method gets faster the more sparse the bit vector is. (Courtesy of Tim Bunce and Winfried Koenig.)
You can make the while loop a lot shorter with this suggestion from Benjamin Goldberg:
while($vec =~ /[^\0]+/g ) { push @ints, grep vec($vec, $_, 1), $-[0] * 8 .. $+[0] * 8; }
Or use the CPAN module "Bit::Vector":
$vector = Bit::Vector->new($num_of_bits); $vector->Index_List_Store(@ints); @ints = $vector->Index_List_Read();
"Bit::Vector" provides efficient methods for bit vector, sets of small integers and ``big int'' math.
Here's a more extensive illustration using vec():
# vec demo $vector = "\xff\x0f\xef\xfe"; print "Ilya's string \\xff\\x0f\\xef\\xfe represents the number ", unpack("N", $vector), "\n"; $is_set = vec($vector, 23, 1); print "Its 23rd bit is ", $is_set ? "set" : "clear", ".\n"; pvec($vector); set_vec(1,1,1); set_vec(3,1,1); set_vec(23,1,1); set_vec(3,1,3); set_vec(3,2,3); set_vec(3,4,3); set_vec(3,4,7); set_vec(3,8,3); set_vec(3,8,7); set_vec(0,32,17); set_vec(1,32,17); sub set_vec { my ($offset, $width, $value) = @_; my $vector = ''; vec($vector, $offset, $width) = $value; print "offset=$offset width=$width value=$value\n"; pvec($vector); } sub pvec { my $vector = shift; my $bits = unpack("b*", $vector); my $i = 0; my $BASE = 8; print "vector length in bytes: ", length($vector), "\n"; @bytes = unpack("A8" x length($vector), $bits); print "bits are: @bytes\n\n"; }
Why does defined() return true on empty arrays and hashes?
The short story is that you should probably only use defined on scalars or functions, not on aggregates (arrays and hashes). See ``defined'' in perlfunc in the 5.004 release or later of Perl for more detail.Data: Hashes (Associative Arrays)
How do I process an entire hash?
(contributed by brian d foy)There are a couple of ways that you can process an entire hash. You can get a list of keys, then go through each key, or grab a one key-value pair at a time.
To go through all of the keys, use the "keys" function. This extracts all of the keys of the hash and gives them back to you as a list. You can then get the value through the particular key you're processing:
foreach my $key ( keys %hash ) { my $value = $hash{$key} ... }
Once you have the list of keys, you can process that list before you process the hash elements. For instance, you can sort the keys so you can process them in lexical order:
foreach my $key ( sort keys %hash ) { my $value = $hash{$key} ... }
Or, you might want to only process some of the items. If you only want to deal with the keys that start with "text:", you can select just those using "grep":
foreach my $key ( grep /^text:/, keys %hash ) { my $value = $hash{$key} ... }
If the hash is very large, you might not want to create a long list of keys. To save some memory, you can grab one key-value pair at a time using "each()", which returns a pair you haven't seen yet:
while( my( $key, $value ) = each( %hash ) ) { ... }
The "each" operator returns the pairs in apparently random order, so if ordering matters to you, you'll have to stick with the "keys" method.
The "each()" operator can be a bit tricky though. You can't add or delete keys of the hash while you're using it without possibly skipping or re-processing some pairs after Perl internally rehashes all of the elements. Additionally, a hash has only one iterator, so if you use "keys", "values", or "each" on the same hash, you can reset the iterator and mess up your processing. See the "each" entry in perlfunc for more details.
How do I merge two hashes?
(contributed by brian d foy)Before you decide to merge two hashes, you have to decide what to do if both hashes contain keys that are the same and if you want to leave the original hashes as they were.
If you want to preserve the original hashes, copy one hash (%hash1) to a new hash (%new_hash), then add the keys from the other hash (%hash2 to the new hash. Checking that the key already exists in %new_hash gives you a chance to decide what to do with the duplicates:
my %new_hash = %hash1; # make a copy; leave %hash1 alone foreach my $key2 ( keys %hash2 ) { if( exists $new_hash{$key2} ) { warn "Key [$key2] is in both hashes!"; # handle the duplicate (perhaps only warning) ... next; } else { $new_hash{$key2} = $hash2{$key2}; } }
If you don't want to create a new hash, you can still use this looping technique; just change the %new_hash to %hash1.
foreach my $key2 ( keys %hash2 ) { if( exists $hash1{$key2} ) { warn "Key [$key2] is in both hashes!"; # handle the duplicate (perhaps only warning) ... next; } else { $hash1{$key2} = $hash2{$key2}; } }
If you don't care that one hash overwrites keys and values from the other, you could just use a hash slice to add one hash to another. In this case, values from %hash2 replace values from %hash1 when they have keys in common:
@hash1{ keys %hash2 } = values %hash2;
What happens if I add or remove keys from a hash while iterating over it?
(contributed by brian d foy)The easy answer is ``Don't do that!''
If you iterate through the hash with each(), you can delete the key most recently returned without worrying about it. If you delete or add other keys, the iterator may skip or double up on them since perl may rearrange the hash table. See the entry for "each()" in perlfunc.
How do I look up a hash element by value?
Create a reverse hash:
%by_value = reverse %by_key; $key = $by_value{$value};
That's not particularly efficient. It would be more space-efficient to use:
while (($key, $value) = each %by_key) { $by_value{$value} = $key; }
If your hash could have repeated values, the methods above will only find one of the associated keys. This may or may not worry you. If it does worry you, you can always reverse the hash into a hash of arrays instead:
while (($key, $value) = each %by_key) { push @{$key_list_by_value{$value}}, $key; }
How can I know how many entries are in a hash?
(contributed by brian d foy)This is very similar to ``How do I process an entire hash?'', also in perlfaq4, but a bit simpler in the common cases.
You can use the "keys()" built-in function in scalar context to find out have many entries you have in a hash:
my $key_count = keys %hash; # must be scalar context!
If you want to find out how many entries have a defined value, that's a bit different. You have to check each value. A "grep" is handy:
my $defined_value_count = grep { defined } values %hash;
You can use that same structure to count the entries any way that you like. If you want the count of the keys with vowels in them, you just test for that instead:
my $vowel_count = grep { /[aeiou]/ } keys %hash;
The "grep" in scalar context returns the count. If you want the list of matching items, just use it in list context instead:
my @defined_values = grep { defined } values %hash;
The "keys()" function also resets the iterator, which means that you may see strange results if you use this between uses of other hash operators such as "each()".
How do I sort a hash (optionally by value instead of key)?
(contributed by brian d foy)To sort a hash, start with the keys. In this example, we give the list of keys to the sort function which then compares them ASCIIbetically (which might be affected by your locale settings). The output list has the keys in ASCIIbetical order. Once we have the keys, we can go through them to create a report which lists the keys in ASCIIbetical order.
my @keys = sort { $a cmp $b } keys %hash; foreach my $key ( @keys ) { printf "%-20s %6d\n", $key, $hash{$key}; }
We could get more fancy in the "sort()" block though. Instead of comparing the keys, we can compute a value with them and use that value as the comparison.
For instance, to make our report order case-insensitive, we use the "\L" sequence in a double-quoted string to make everything lowercase. The "sort()" block then compares the lowercased values to determine in which order to put the keys.
my @keys = sort { "\L$a" cmp "\L$b" } keys %hash;
Note: if the computation is expensive or the hash has many elements, you may want to look at the Schwartzian Transform to cache the computation results.
If we want to sort by the hash value instead, we use the hash key to look it up. We still get out a list of keys, but this time they are ordered by their value.
my @keys = sort { $hash{$a} <=> $hash{$b} } keys %hash;
From there we can get more complex. If the hash values are the same, we can provide a secondary sort on the hash key.
my @keys = sort { $hash{$a} <=> $hash{$b} or "\L$a" cmp "\L$b" } keys %hash;
How can I always keep my hash sorted?
You can look into using the "DB_File" module and "tie()" using the $DB_BTREE hash bindings as documented in ``In Memory Databases'' in DB_File. The "Tie::IxHash" module from CPAN might also be instructive.  Are you sure you need to do this? :)What's the difference between delete and undef with hashes?
Hashes contain pairs of scalars: the first is the key, the second is the value. The key will be coerced to a string, although the value can be any kind of scalar: string, number, or reference. If a key $key is present in %hash, "exists($hash{$key})" will return true. The value for a given key can be "undef", in which case $hash{$key} will be "undef" while "exists $hash{$key}" will return true. This corresponds to ($key, "undef") being in the hash.Pictures help... here's the %hash table:
keys values +------+------+ | a | 3 | | x | 7 | | d | 0 | | e | 2 | +------+------+
And these conditions hold
$hash{'a'} is true $hash{'d'} is false defined $hash{'d'} is true defined $hash{'a'} is true exists $hash{'a'} is true (Perl 5 only) grep ($_ eq 'a', keys %hash) is true
If you now say
undef $hash{'a'}
your table now reads:
keys values +------+------+ | a | undef| | x | 7 | | d | 0 | | e | 2 | +------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is FALSE $hash{'d'} is false defined $hash{'d'} is true defined $hash{'a'} is FALSE exists $hash{'a'} is true (Perl 5 only) grep ($_ eq 'a', keys %hash) is true
Notice the last two: you have an undef value, but a defined key!
Now, consider this:
delete $hash{'a'}
your table now reads:
keys values +------+------+ | x | 7 | | d | 0 | | e | 2 | +------+------+
and these conditions now hold; changes in caps:
$hash{'a'} is false $hash{'d'} is false defined $hash{'d'} is true defined $hash{'a'} is false exists $hash{'a'} is FALSE (Perl 5 only) grep ($_ eq 'a', keys %hash) is FALSE
Why don't my tied hashes make the defined/exists distinction?
This depends on the tied hash's implementation of EXISTS(). For example, there isn't the concept of undef with hashes that are tied to DBM* files. It also means that exists() and defined() do the same thing with a DBM* file, and what they end up doing is not what they do with ordinary hashes.How do I reset an each() operation part-way through?
(contributed by brian d foy)You can use the "keys" or "values" functions to reset "each". To simply reset the iterator used by "each" without doing anything else, use one of them in void context:
keys %hash; # resets iterator, nothing else. values %hash; # resets iterator, nothing else.
See the documentation for "each" in perlfunc.
How can I get the unique keys from two hashes?
First you extract the keys from the hashes into lists, then solve the ``removing duplicates'' problem described above. For example:
%seen = (); for $element (keys(%foo), keys(%bar)) { $seen{$element}++; } @uniq = keys %seen;
Or more succinctly:
@uniq = keys %{{%foo,%bar}};
Or if you really want to save space:
%seen = (); while (defined ($key = each %foo)) { $seen{$key}++; } while (defined ($key = each %bar)) { $seen{$key}++; } @uniq = keys %seen;
How can I store a multidimensional array in a DBM file?
Either stringify the structure yourself (no fun), or else get the MLDBM (which uses Data::Dumper) module from CPAN and layer it on top of either DB_File or GDBM_File.How can I make my hash remember the order I put elements into it?
Use the "Tie::IxHash" from CPAN.
use Tie::IxHash; tie my %myhash, 'Tie::IxHash'; for (my $i=0; $i<20; $i++) { $myhash{$i} = 2*$i; } my @keys = keys %myhash; # @keys = (0,1,2,3,...)
Why does passing a subroutine an undefined element in a hash create it?
(contributed by brian d foy)Are you using a really old version of Perl?
Normally, accessing a hash key's value for a nonexistent key will not create the key.
my %hash = (); my $value = $hash{ 'foo' }; print "This won't print\n" if exists $hash{ 'foo' };
Passing $hash{ 'foo' } to a subroutine used to be a special case, though. Since you could assign directly to $_[0], Perl had to be ready to make that assignment so it created the hash key ahead of time:
my_sub( $hash{ 'foo' } ); print "This will print before 5.004\n" if exists $hash{ 'foo' }; sub my_sub { # $_[0] = 'bar'; # create hash key in case you do this 1; }
Since Perl 5.004, however, this situation is a special case and Perl creates the hash key only when you make the assignment:
my_sub( $hash{ 'foo' } ); print "This will print, even after 5.004\n" if exists $hash{ 'foo' }; sub my_sub { $_[0] = 'bar'; }
However, if you want the old behavior (and think carefully about that because it's a weird side effect), you can pass a hash slice instead. Perl 5.004 didn't make this a special case:
my_sub( @hash{ qw/foo/ } );
How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
Usually a hash ref, perhaps like this:
$record = { NAME => "Jason", EMPNO => 132, TITLE => "deputy peon", AGE => 23, SALARY => 37_000, PALS => [ "Norbert", "Rhys", "Phineas"], };
References are documented in perlref and the upcoming perlreftut. Examples of complex data structures are given in perldsc and perllol. Examples of structures and object-oriented classes are in perltoot.
How can I use a reference as a hash key?
(contributed by brian d foy and Ben Morrow)Hash keys are strings, so you can't really use a reference as the key. When you try to do that, perl turns the reference into its stringified form (for instance, "HASH(0xDEADBEEF)"). From there you can't get back the reference from the stringified form, at least without doing some extra work on your own.
Remember that the entry in the hash will still be there even if the referenced variable goes out of scope, and that it is entirely possible for Perl to subsequently allocate a different variable at the same address. This will mean a new variable might accidentally be associated with the value for an old.
If you have Perl 5.10 or later, and you just want to store a value against the reference for lookup later, you can use the core Hash::Util::Fieldhash module. This will also handle renaming the keys if you use multiple threads (which causes all variables to be reallocated at new addresses, changing their stringification), and garbage-collecting the entries when the referenced variable goes out of scope.
If you actually need to be able to get a real reference back from each hash entry, you can use the Tie::RefHash module, which does the required work for you.
Data: Misc
How do I handle binary data correctly?
Perl is binary clean, so it can handle binary data just fine. On Windows or DOS, however, you have to use "binmode" for binary files to avoid conversions for line endings. In general, you should use "binmode" any time you want to work with binary data.Also see ``binmode'' in perlfunc or perlopentut.
If you're concerned about 8-bit textual data then see perllocale. If you want to deal with multibyte characters, however, there are some gotchas. See the section on Regular Expressions.
How do I determine whether a scalar is a number/whole/integer/float?
Assuming that you don't care about IEEE notations like ``NaN'' or ``Infinity'', you probably just want to use a regular expression.
if (/\D/) { print "has nondigits\n" } if (/^\d+$/) { print "is a whole number\n" } if (/^-?\d+$/) { print "is an integer\n" } if (/^[+-]?\d+$/) { print "is a +/- integer\n" } if (/^-?\d+\.?\d*$/) { print "is a real number\n" } if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number\n" } if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) { print "a C float\n" }
There are also some commonly used modules for the task. Scalar::Util (distributed with 5.8) provides access to perl's internal function "looks_like_number" for determining whether a variable looks like a number. Data::Types exports functions that validate data types using both the above and other regular expressions. Thirdly, there is "Regexp::Common" which has regular expressions to match various types of numbers. Those three modules are available from the CPAN.
If you're on a POSIX system, Perl supports the "POSIX::strtod" function. Its semantics are somewhat cumbersome, so here's a "getnum" wrapper function for more convenient access. This function takes a string and returns the number it found, or "undef" for input that isn't a C float. The "is_numeric" function is a front end to "getnum" if you just want to say, ``Is this a float?''
sub getnum { use POSIX qw(strtod); my $str = shift; $str =~ s/^\s+//; $str =~ s/\s+$//; $! = 0; my($num, $unparsed) = strtod($str); if (($str eq '') || ($unparsed != 0) || $!) { return undef; } else { return $num; } } sub is_numeric { defined getnum($_[0]) }
Or you could check out the String::Scanf module on the CPAN instead. The "POSIX" module (part of the standard Perl distribution) provides the "strtod" and "strtol" for converting strings to double and longs, respectively.
How do I keep persistent data across program calls?
For some specific applications, you can use one of the DBM modules. See AnyDBM_File. More generically, you should consult the "FreezeThaw" or "Storable" modules from CPAN. Starting from Perl 5.8 "Storable" is part of the standard distribution. Here's one example using "Storable"'s "store" and "retrieve" functions:
use Storable; store(\%hash, "filename"); # later on... $href = retrieve("filename"); # by ref %hash = %{ retrieve("filename") }; # direct to hash
How do I print out or copy a recursive data structure?
The "Data::Dumper" module on CPAN (or the 5.005 release of Perl) is great for printing out data structures. The "Storable" module on CPAN (or the 5.8 release of Perl), provides a function called "dclone" that recursively copies its argument.
use Storable qw(dclone); $r2 = dclone($r1);
Where $r1 can be a reference to any kind of data structure you'd like. It will be deeply copied. Because "dclone" takes and returns references, you'd have to add extra punctuation if you had a hash of arrays that you wanted to copy.
%newhash = %{ dclone(\%oldhash) };
How do I define methods for every class/object?
(contributed by Ben Morrow)You can use the "UNIVERSAL" class (see UNIVERSAL). However, please be very careful to consider the consequences of doing this: adding methods to every object is very likely to have unintended consequences. If possible, it would be better to have all your object inherit from some common base class, or to use an object system like Moose that supports roles.
How do I verify a credit card checksum?
Get the "Business::CreditCard" module from CPAN.How do I pack arrays of doubles or floats for XS code?
The arrays.h/arrays.c code in the "PGPLOT" module on CPAN does just this. If you're doing a lot of float or double processing, consider using the "PDL" module from CPAN instead---it makes number-crunching easy.See <http://search.cpan.org/dist/PGPLOT> for the code.
REVISION
Revision: $Revision$Date: $Date$
See perlfaq for source control details and availability.
AUTHOR AND COPYRIGHT
Copyright (c) 1997-2009 Tom Christiansen, Nathan Torkington, and other authors as noted. All rights reserved.This documentation is free; you can redistribute it and/or modify it under the same terms as Perl itself.
Irrespective of its distribution, all code examples in this file are hereby placed into the public domain. You are permitted and encouraged to use this code in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit would be courteous but is not required.
Index
- NAME
- DESCRIPTION
- Data: Numbers
-
- Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?
- Why is int() broken?
- Why isn't my octal data interpreted correctly?
- Does Perl have a round() function? What about ceil() and floor()? Trig functions?
- How do I convert between numeric representations/bases/radixes?
- Why doesn't & work the way I want it to?
- How do I multiply matrices?
- How do I perform an operation on a series of integers?
- How can I output Roman numerals?
- Why aren't my random numbers random?
- How do I get a random number between X and Y?
- Data: Dates
-
- How do I find the day or week of the year?
- How do I find the current century or millennium?
- How can I compare two dates and find the difference?
- How can I take a string and turn it into epoch seconds?
- How can I find the Julian Day?
- How do I find yesterday's date?
- Does Perl have a Year 2000 problem? Is Perl Y2K compliant?
- Data: Strings
-
- How do I validate input?
- How do I unescape a string?
- How do I remove consecutive pairs of characters?
- How do I expand function calls in a string?
- How do I find matching/nesting anything?
- How do I reverse a string?
- How do I expand tabs in a string?
- How do I reformat a paragraph?
- How can I access or change N characters of a string?
- How do I change the Nth occurrence of something?
- How can I count the number of occurrences of a substring within a string?
- Does Perl have a Year 2038 problem?
- How do I capitalize all the words on one line?
- How can I split a [character] delimited string except when inside [character]?
- How do I strip blank space from the beginning/end of a string?
- How do I pad a string with blanks or pad a number with zeroes?
- How do I extract selected columns from a string?
- How do I find the soundex value of a string?
- How can I expand variables in text strings?
- What's wrong with always quoting $vars?
- Why don't my <<HERE documents work?
- Data: Arrays
-
- What is the difference between a list and an array?
- What is the difference between $array[1] and @array[1]?
- How can I remove duplicate elements from a list or array?
- How can I tell whether a certain element is contained in a list or array?
- How do I compute the difference of two arrays? How do I compute the intersection of two arrays?
- How do I test whether two arrays or hashes are equal?
- How do I find the first array element for which a condition is true?
- How do I handle linked lists?
- How do I handle circular lists?
- How do I shuffle an array randomly?
- How do I process/modify each element of an array?
- How do I select a random element from an array?
- How do I permute N elements of a list?
- How do I sort an array by (anything)?
- How do I manipulate arrays of bits?
- Why does defined() return true on empty arrays and hashes?
- Data: Hashes (Associative Arrays)
-
- How do I process an entire hash?
- How do I merge two hashes?
- What happens if I add or remove keys from a hash while iterating over it?
- How do I look up a hash element by value?
- How can I know how many entries are in a hash?
- How do I sort a hash (optionally by value instead of key)?
- How can I always keep my hash sorted?
- What's the difference between delete and undef with hashes?
- Why don't my tied hashes make the defined/exists distinction?
- How do I reset an each() operation part-way through?
- How can I get the unique keys from two hashes?
- How can I store a multidimensional array in a DBM file?
- How can I make my hash remember the order I put elements into it?
- Why does passing a subroutine an undefined element in a hash create it?
- How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays?
- How can I use a reference as a hash key?
- Data: Misc
-
- How do I handle binary data correctly?
- How do I determine whether a scalar is a number/whole/integer/float?
- How do I keep persistent data across program calls?
- How do I print out or copy a recursive data structure?
- How do I define methods for every class/object?
- How do I verify a credit card checksum?
- How do I pack arrays of doubles or floats for XS code?
- REVISION
- AUTHOR AND COPYRIGHT
This document was created by man2html, using the manual pages.
Time: 05:29:08 GMT, December 24, 2015


My awesome header<3
This is a link to FaceBook!
This is a link to YouTube!
This is a link to Twitter!
Hello there, awesome person!


If you can't catch the money it falls on the street
and is automaticly donated to the homeless.


//Import polymer platform file //Import pre -defined polymer components . This will be used in the card design. //For responsive UI //Importing style sheet for login card Login


Finally I got it working last night.
After watching the demo of clutter at OSiM I got interested in the OpenedHand's Clutter project. More importantly embedding a browser in cairo is of greater interest to me. So when I got it working, I couldn't help showing it off.
send ur email id to mail u errors and steps
hi
may i know how u build webkit clutter is it completed or not ..guide me to build webkit clutter.
iam trying cross compilation of webkit using mingw on linux for windows .
i tried this link https://github.com/rkudiyarov/ClutterWebkit.git
and cross compiling individual package also but no success
help me to compile webkit
Regards
Santhosh


Determined. Logical. Consistent.
About Me
My name is Jaime Harman and I've been a passionate teacher for the past ten years. I'm looking to transfer the skills I've learned in the classroom towards a career in web development.
I always maintain a mature, gracious and professional manner when communicating with people. I strive to represent myself and my work place in a positive and enthusiastic way. My Bachelor's degree in Technical/Professional Writing from the University of South Florida has helped make my communications, written and oral, very proficient.
Some of my skills include critical thinking, adaptability, problem solving, and a strong work ethic. I believe that people should be able to deliver on what they promise and as a web developer, I promise I can get the job done.


Hi, my name is David Kim.
Currently, I am a sophomore studying Software System Engineering at the University of Regina.
I spent my first year at the University of Waterloo as Honours Mathematics and transferred to UofR in 2018.
I am enthusiastic about both computer hardware and software. I have strong skills to think innovative and solve problems.
University of Regina Cougar Racing: Marketing Analyst
University of Regina Korean Student Association: Executive Member
University of Waterloo Korean Student Association: member
2018 University of Regina International Entrance Scholarship
2017 University of Waterloo President's Scholarship
2017 Governor General's Academic Medal
2017 Grade 12 Mathematics and Science Award
2012-2017 Academic Honor Roll
2015-2017 Academic Proficient Award
University of Regina Cougar Racing: Marketing Analyst
University of Regina Korean Student Association: Executive Member
University of Waterloo Korean Student Association: member


MGsolver¶
Holds the data and routines to approximately invert the linear system arising from a Poisson equation using a Multi-Grid scheme. The key routines are init and update. Like all MG methods, the residual is down-sampled to successively coarser grids and the correction up-sampled to keep convergence from stalling. Somewhat uniquely, this solver uses a single Jacobi step before down-sampling, and a Conjugate Gradient method instead of a smoother after up-sampling. Simple V-cycles are used until converged.
The default maximum number of V-Cycle is 32, with an average tolerence between scuccesive iterations on \(1e-6\). A maximum if 32 PCG iterations are perfomed at each level of the multi-grid.


Dorothy and the Wizard in Oz
by L. Frank Baum
Chapter 18: “The Trial of Eureka the Kitten”
Additional Information
- Year Published: 1908
- Language: English
- Country of Origin: United States of America
- Source: Baum, F. L. (1908). Dorothy and the Wizard in Oz. Chicago: Reilly and Britton.
-
Readability:
- Flesch–Kincaid Level: 6.0
- Word Count: 1,619
- Genre: Fantasy
- Keywords: 20th century literature, american literature, books of oz, l. frank baum, self-help
- ✎ Cite This
Downloads
Several days of festivity and merry-making followed, for such old friends did not often meet and there was much to be told and talked over between them, and many amusements to be enjoyed in this delightful country.
Ozma was happy to have Dorothy beside her, for girls of her own age with whom it was proper for the Princess to associate were very few, and often the youthful Ruler of Oz was lonely for lack of companionship.
It was the third morning after Dorothy’s arrival, and she was sitting with Ozma and their friends in a reception room, talking over old times, when the Princess said to her maid:
“Please go to my boudoir, Jellia, and get the white piglet I left on the dressing-table. I want to play with it.”
Jellia at once departed on the errand, and she was gone so long that they had almost forgotten her mission when the green robed maiden returned with a troubled face.
“The piglet is not there, your Highness,” said she.
“Not there!” exclaimed Ozma. “Are you sure?”
“I have hunted in every part of the room,” the maid replied.
“Was not the door closed?” asked the Princess.
“Yes, your Highness; I am sure it was; for when I opened it Dorothy’s white kitten crept out and ran up the stairs.”
Hearing this, Dorothy and the Wizard exchanged startled glances, for they remembered how often Eureka had longed to eat a piglet. The little girl jumped up at once.
“Come, Ozma,” she said, anxiously; “let us go ourselves to search for the piglet.”
So the two went to the dressing room of the Princess and searched carefully in every corner and among the vases and baskets and ornaments that stood about the pretty boudoir. But not a trace could they find of the tiny creature they sought.
Dorothy was nearly weeping, by this time, while Ozma was angry and indignant. When they returned to the others the Princess said:
“There is little doubt that my pretty piglet has been eaten by that horrid kitten, and if that is true the offender must be punished.”
“I don’t b’lieve Eureka would do such a dreadful thing!” cried Dorothy, much distressed. “Go and get my kitten, please, Jellia, and we’ll hear what she has to say about it.”
The green maiden hastened away, but presently returned and said:
“The kitten will not come. She threatened to scratch my eyes out if I touched her.”
“Where is she?” asked Dorothy.
“Under the bed in your own room,” was the reply.
So Dorothy ran to her room and found the kitten under the bed.
“Come here, Eureka!” she said.
“I won’t,” answered the kitten, in a surly voice.
“Oh, Eureka! Why are you so bad?”
The kitten did not reply.
“If you don’t come to me, right away,” continued Dorothy, getting provoked, “I’ll take my Magic Belt and wish you in the Country of the Gurgles.”
“Why do you want me?” asked Eureka, disturbed by this threat.
“You must go to Princess Ozma. She wants to talk to you.”
“All right,” returned the kitten, creeping out. “I’m not afraid of Ozma—or anyone else.”
Dorothy carried her in her arms back to where the others sat in grieved and thoughtful silence.

“Oh, yes you will, dear,” Dorothy declared. “The piglet is gone, and you ran out of the room when Jellia opened the door. So, if you are innocent, Eureka, you must tell the Princess how you came to be in her room, and what has become of the piglet.”
“Who accuses me?” asked the kitten, defiantly.
“No one,” answered Ozma. “Your actions alone accuse you. The fact is that I left my little pet in my dressing room lying asleep upon the table; and you must have stolen in without my knowing it. When next the door was opened you ran out and hid yourself—and the piglet was gone.”
“That’s none of my business,” growled the kitten.
“Don’t be impudent, Eureka,” admonished Dorothy.
“It is you who are impudent,” said Eureka, “for accusing me of such a crime when you can’t prove it except by guessing.”
Ozma was now greatly incensed by the kitten’s conduct. She summoned her Captain-General, and when the long, lean officer appeared she said:
“Carry this cat away to prison, and keep her in safe confinement until she is tried by law for the crime of murder.”
So the Captain-General took Eureka from the arms of the now weeping Dorothy and in spite of the kitten’s snarls and scratches carried it away to prison.
“What shall we do now?” asked the Scarecrow, with a sigh, for such a crime had cast a gloom over all the company.
“I will summon the Court to meet in the Throne Room at three o’clock,” replied Ozma. “I myself will be the judge, and the kitten shall have a fair trial.”
“What will happen if she is guilty?” asked Dorothy.
“She must die,” answered the Princess.
“Nine times?” enquired the Scarecrow.
“As many times as is necessary,” was the reply. “I will ask the Tin Woodman to defend the prisoner, because he has such a kind heart I am sure he will do his best to save her. And the Woggle-Bug shall be the Public Accuser, because he is so learned that no one can deceive him.”
“Who will be the jury?” asked the Tin Woodman.
“There ought to be several animals on the jury,” said Ozma, “because animals understand each other better than we people understand them. So the jury shall consist of the Cowardly Lion, the Hungry Tiger, Jim the Cab-horse, the Yellow Hen, the Scarecrow, the Wizard, Tik-tok the Machine Man, the Sawhorse and Zeb of Hugson’s Ranch. That makes the nine which the law requires, and all my people shall be admitted to hear the testimony.”
They now separated to prepare for the sad ceremony; for whenever an appeal is made to law sorrow is almost certain to follow—even in a fairyland like Oz. But it must be stated that the people of that Land were generally so well-behaved that there was not a single lawyer amongst them, and it had been years since any Ruler had sat in judgment upon an offender of the law. The crime of murder being the most dreadful crime of all, tremendous excitement prevailed in the Emerald City when the news of Eureka’s arrest and trial became known.
The Wizard, when he returned to his own room, was exceedingly thoughtful. He had no doubt Eureka had eaten his piglet, but he realized that a kitten cannot be depended upon at all times to act properly, since its nature is to destroy small animals and even birds for food, and the tame cat that we keep in our houses today is descended from the wild cat of the jungle—a very ferocious creature, indeed. The Wizard knew that if Dorothy’s pet was found guilty and condemned to death the little girl would be made very unhappy; so, although he grieved over the piglet’s sad fate as much as any of them, he resolved to save Eureka’s life.
Sending for the Tin Woodman the Wizard took him into a corner and whispered:
“My friend, it is your duty to defend the white kitten and try to save her, but I fear you will fail because Eureka has long wished to eat a piglet, to my certain knowledge, and my opinion is that she has been unable to resist the temptation. Yet her disgrace and death would not bring back the piglet, but only serve to make Dorothy unhappy. So I intend to prove the kitten’s innocence by a trick.”
He drew from his inside pocket one of the eight tiny piglets that were remaining and continued:
“This creature you must hide in some safe place, and if the jury decides that Eureka is guilty you may then produce this piglet and claim it is the one that was lost. All the piglets are exactly alike, so no one can dispute your word.  So I will do as you say, friend Wizard.”


RxJS
The Reactive Extensions
for JavaScript
Joan Rieu — Google I/O Extended 2016 Bordeaux
Value sources
|
Single |
Multiple |
| Synchronous |
Function |
Iterable Enumerable |
| Asynchronous |
Promise |
Rx.Observable |
Operators
(1/4)
Operators
(2/4)
Operators
(3/4)
Operators
(4/4)
Total: 178 operators
Uses
Data streaming
- À la Apache Spark
- One-by-one transformation
- Bulk operations (window)
Uses
Real-time math
- Built-in operators
- Automatic dependencies
Uses
User interface
- Angular 2
- React & Flux
- Vue.js
Compatibility
- Most UI frameworks
- DOM events
- Promises
- Node.js callbacks
- Socket.io
- …
Case study
Autocomplete
Case study
Autocomplete
- Multiple requests
- Asynchronous callbacks
- Out-of-order responses
- …
☹
Case study
Autocomplete
Rx.Observable.fromEvent(input, 'keyup')
.map(e => e.target.value)
.filter(input => input.length > 3)
.debounce(250)
.distinctUntilChanged()
.flatMapLatest(input => searchAsync(input).retry(3))
.subscribe(showResults)
TL; DR
RxJS is good for your code
- Concise
- Readable & maintainable
- Efficient
- Favors standards (ES 7)
- Compatible with everything
- Used more and more
- Supported by Microsoft
- Has a huge community
- Lots of Stack Overflow answers