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diff --git a/Master/tlpkg/tlperl.straw/lib/pods/perlfaq4.pod b/Master/tlpkg/tlperl.straw/lib/pods/perlfaq4.pod deleted file mode 100755 index a951be3b4f5..00000000000 --- a/Master/tlpkg/tlperl.straw/lib/pods/perlfaq4.pod +++ /dev/null @@ -1,2534 +0,0 @@ -=head1 NAME - -perlfaq4 - Data Manipulation - -=head1 DESCRIPTION - -This section of the FAQ answers questions related to manipulating -numbers, dates, strings, arrays, hashes, and miscellaneous data issues. - -=head1 Data: Numbers - -=head2 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. - -L<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 L<"Floating Point -Arithmetic"|perlop> for more details. - - printf "%.2f", 10/3; - - my $number = sprintf "%.2f", 10/3; - -=head2 Why is int() broken? - -Your C<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. - -=head2 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, C<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 C<0> or using C<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 C<@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; - -=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? - -Remember that C<int()> merely truncates toward 0. For rounding to a -certain number of digits, C<sprintf()> or C<printf()> is usually the -easiest route. - - printf("%.3f", 3.1415926535); # prints 3.142 - -The C<POSIX> module (part of the standard Perl distribution) -implements C<ceil()>, C<floor()>, and a number of other mathematical -and trigonometric functions. - - use POSIX; - $ceil = ceil(3.5); # 4 - $floor = floor(3.5); # 3 - -In 5.000 to 5.003 perls, trigonometry was done in the C<Math::Complex> -module. With 5.004, the C<Math::Trig> module (part of the standard Perl -distribution) implements the trigonometric functions. Internally it -uses the C<Math::Complex> module and some functions can break out from -the real axis into the complex plane, for example the inverse sine of -2. - -Rounding in financial applications can have serious implications, and -the rounding method used should be specified precisely. In these -cases, it probably pays not to trust whichever system rounding is -being used by Perl, but to instead implement the rounding function you -need yourself. - -To see why, notice how you'll still have an issue on half-way-point -alternation: - - for ($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. - -=head2 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 L<perlfaq4> use the C<Bit::Vector> -module from CPAN. The reason you might choose C<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. - -=over 4 - -=item How do I convert hexadecimal into decimal - -Using perl's built in conversion of C<0x> notation: - - $dec = 0xDEADBEEF; - -Using the C<hex> function: - - $dec = hex("DEADBEEF"); - -Using C<pack>: - - $dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); - -Using the CPAN module C<Bit::Vector>: - - use Bit::Vector; - $vec = Bit::Vector->new_Hex(32, "DEADBEEF"); - $dec = $vec->to_Dec(); - -=item How do I convert from decimal to hexadecimal - -Using C<sprintf>: - - $hex = sprintf("%X", 3735928559); # upper case A-F - $hex = sprintf("%x", 3735928559); # lower case a-f - -Using C<unpack>: - - $hex = unpack("H*", pack("N", 3735928559)); - -Using C<Bit::Vector>: - - use Bit::Vector; - $vec = Bit::Vector->new_Dec(32, -559038737); - $hex = $vec->to_Hex(); - -And C<Bit::Vector> supports odd bit counts: - - use Bit::Vector; - $vec = Bit::Vector->new_Dec(33, 3735928559); - $vec->Resize(32); # suppress leading 0 if unwanted - $hex = $vec->to_Hex(); - -=item How do I convert from octal to decimal - -Using Perl's built in conversion of numbers with leading zeros: - - $dec = 033653337357; # note the leading 0! - -Using the C<oct> function: - - $dec = oct("33653337357"); - -Using C<Bit::Vector>: - - use Bit::Vector; - $vec = Bit::Vector->new(32); - $vec->Chunk_List_Store(3, split(//, reverse "33653337357")); - $dec = $vec->to_Dec(); - -=item How do I convert from decimal to octal - -Using C<sprintf>: - - $oct = sprintf("%o", 3735928559); - -Using C<Bit::Vector>: - - use Bit::Vector; - $vec = Bit::Vector->new_Dec(32, -559038737); - $oct = reverse join('', $vec->Chunk_List_Read(3)); - -=item How do I convert from binary to decimal - -Perl 5.6 lets you write binary numbers directly with -the C<0b> notation: - - $number = 0b10110110; - -Using C<oct>: - - my $input = "10110110"; - $decimal = oct( "0b$input" ); - -Using C<pack> and C<ord>: - - $decimal = ord(pack('B8', '10110110')); - -Using C<pack> and C<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 C<Bit::Vector>: - - $vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); - $dec = $vec->to_Dec(); - -=item How do I convert from decimal to binary - -Using C<sprintf> (perl 5.6+): - - $bin = sprintf("%b", 3735928559); - -Using C<unpack>: - - $bin = unpack("B*", pack("N", 3735928559)); - -Using C<Bit::Vector>: - - use Bit::Vector; - $vec = Bit::Vector->new_Dec(32, -559038737); - $bin = $vec->to_Bin(); - -The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) -are left as an exercise to the inclined reader. - -=back - -=head2 Why doesn't & work the way I want it to? - -The behavior of binary arithmetic operators depends on whether they're -used on numbers or strings. The operators treat a string as a series -of bits and work with that (the string C<"3"> is the bit pattern -C<00110011>). The operators work with the binary form of a number -(the number C<3> is treated as the bit pattern C<00000011>). - -So, saying C<11 & 3> performs the "and" operation on numbers (yielding -C<3>). Saying C<"11" & "3"> performs the "and" operation on strings -(yielding C<"1">). - -Most problems with C<&> and C<|> arise because the programmer thinks -they have a number but really it's a string. The rest arise because -the programmer says: - - if ("\020\020" & "\101\101") { - # ... - } - -but a string consisting of two null bytes (the result of C<"\020\020" -& "\101\101">) is not a false value in Perl. You need: - - if ( ("\020\020" & "\101\101") !~ /[^\000]/) { - # ... - } - -=head2 How do I multiply matrices? - -Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) -or the PDL extension (also available from CPAN). - -=head2 How do I perform an operation on a series of integers? - -To call a function on each element in an array, and collect the -results, use: - - @results = map { my_func($_) } @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 B<can> use: - - @results = map { some_func($_) } (5 .. 25); - -but you should be aware that the C<..> operator creates an array of -all integers in the range. This can take a lot of memory for large -ranges. Instead use: - - @results = (); - for ($i=5; $i < 500_005; $i++) { - push(@results, some_func($i)); - } - -This situation has been fixed in Perl5.005. Use of C<..> in a C<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. - -=head2 How can I output Roman numerals? - -Get the http://www.cpan.org/modules/by-module/Roman module. - -=head2 Why aren't my random numbers random? - -If you're using a version of Perl before 5.004, you must call C<srand> -once at the start of your program to seed the random number generator. - - BEGIN { srand() if $] < 5.004 } - -5.004 and later automatically call C<srand> at the beginning. Don't -call C<srand> more than once--you make your numbers less random, -rather than more. - -Computers are good at being predictable and bad at being random -(despite appearances caused by bugs in your programs :-). see the -F<random> article in the "Far More Than You Ever Wanted To Know" -collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz , courtesy -of Tom Phoenix, talks more about this. John von Neumann said, "Anyone -who attempts to generate random numbers by deterministic means is, of -course, living in a state of sin." - -If you want numbers that are more random than C<rand> with C<srand> -provides, you should also check out the C<Math::TrulyRandom> module from -CPAN. It uses the imperfections in your system's timer to generate -random numbers, but this takes quite a while. If you want a better -pseudorandom generator than comes with your operating system, look at -"Numerical Recipes in C" at http://www.nr.com/ . - -=head2 How do I get a random number between X and Y? - -To get a random number between two values, you can use the C<rand()> -built-in to get a random number between 0 and 1. From there, you shift -that into the range that you want. - -C<rand($x)> returns a number such that C<< 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 I<X> and I<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 $number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 ) - -Hence you derive the following simple function to abstract -that. It selects a random integer between the two given -integers (inclusive), For example: C<random_int_between(50,120)>. - - sub random_int_between { - my($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); - } - -=head1 Data: Dates - -=head2 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 C<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 C<POSIX>'s C<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 C<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 ); - -=head2 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 C<POSIX> module's C<strftime()> function has been -extended in a non-standard way to use a C<%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. - -=head2 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 C<Date::Manip>, C<Date::Calc>, or C<DateTime> -modules can help you. - -=head2 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 C<timelocal> in the standard -C<Time::Local> module. Otherwise, you should look into the C<Date::Calc> -and C<Date::Manip> modules from CPAN. - -=head2 How can I find the Julian Day? - -(contributed by brian d foy and Dave Cross) - -You can use the C<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 C<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 - -=head2 How do I find yesterday's date? -X<date> X<yesterday> X<DateTime> X<Date::Calc> X<Time::Local> -X<daylight saving time> X<day> X<Today_and_Now> X<localtime> -X<timelocal> - -(contributed by brian d foy) - -Use one of the Date modules. The C<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 C<Date::Calc> module using its C<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 C<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, $d; - -In this case, you measure the day starting at noon, and subtract 24 -hours. Even if the length of the calendar day is 23 or 25 hours, -you'll still end up on the previous calendar day, although not at -noon. Since you don't care about the time, the one hour difference -doesn't matter and you end up with the previous date. - -=head2 Does Perl have a Year 2000 problem? Is Perl Y2K compliant? - -Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is -Y2K compliant (whatever that means). The programmers you've hired to -use it, however, probably are not. - -Long answer: The question belies a true understanding of the issue. -Perl is just as Y2K compliant as your pencil--no more, and no less. -Can you use your pencil to write a non-Y2K-compliant memo? Of course -you can. Is that the pencil's fault? Of course it isn't. - -The date and time functions supplied with Perl (gmtime and localtime) -supply adequate information to determine the year well beyond 2000 -(2038 is when trouble strikes for 32-bit machines). The year returned -by these functions when used in a list context is the year minus 1900. -For years between 1910 and 1999 this I<happens> to be a 2-digit decimal -number. To avoid the year 2000 problem simply do not treat the year as -a 2-digit number. It isn't. - -When gmtime() and localtime() are used in scalar context they return -a timestamp string that contains a fully-expanded year. For example, -C<$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. - -=head1 Data: Strings - -=head2 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 C<Regexp::Common>. - -Some modules have validation for particular types of input, such -as C<Business::ISBN>, C<Business::CreditCard>, C<Email::Valid>, -and C<Data::Validate::IP>. - -=head2 How do I unescape a string? - -It depends just what you mean by "escape". URL escapes are dealt -with in L<perlfaq9>. Shell escapes with the backslash (C<\>) -character are removed with - - s/\\(.)/$1/g; - -This won't expand C<"\n"> or C<"\t"> or any other special escapes. - -=head2 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 C<(.)>. The memory parentheses -store the matched character in the back-reference C<\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 C<$1>. - - s/(.)\1/$1/g; - -We can also use the transliteration operator, C<tr///>. In this -example, the search list side of our C<tr///> contains nothing, but -the C<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 C<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 - -=head2 How do I expand function calls in a string? - -(contributed by brian d foy) - -This is documented in L<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 C<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 C<Interpolation> module can also do a lot of magic for you. You can -specify a variable name, in this case C<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"; - -=head2 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 C</x([^x]*)x/> will get the intervening -bits in $1. For multiple ones, then something more like -C</alpha(.*?)omega/> would be needed. But none of these deals with -nested patterns. For balanced expressions using C<(>, C<{>, C<[> or -C<< < >> as delimiters, use the CPAN module Regexp::Common, or see -L<perlre/(??{ code })>. 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 C<Parse::RecDescent>, C<Parse::Yapp>, and -C<Text::Balanced>; and the C<byacc> program. Starting from perl 5.8 -the C<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 join("\n",@$[0..$#$]) if( $$[-1] ); - -=head2 How do I reverse a string? - -Use C<reverse()> in scalar context, as documented in -L<perlfunc/reverse>. - - $reversed = reverse $string; - -=head2 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 C<Text::Tabs> module (part of the standard Perl -distribution). - - use Text::Tabs; - @expanded_lines = expand(@lines_with_tabs); - -=head2 How do I reformat a paragraph? - -Use C<Text::Wrap> (part of the standard Perl distribution): - - use Text::Wrap; - print wrap("\t", ' ', @paragraphs); - -The paragraphs you give to C<Text::Wrap> should not contain embedded -newlines. C<Text::Wrap> doesn't justify the lines (flush-right). - -Or use the CPAN module C<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 C<Text::Autoformat> to appreciate its many -capabilities. - -=head2 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"; - -=head2 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 C<"whoever"> or C<"whomever"> into -C<"whosoever"> or C<"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 C</g> modifier in a C<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: C<"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; - -=head2 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 -C<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, C<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; - -=head2 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. - -=head2 How do I capitalize all the words on one line? -X<Text::Autoformat> X<capitalize> X<case, title> X<case, sentence> - -(contributed by brian d foy) - -Damian Conway's L<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 C<\w+>, but that's not what -you want to capitalize. How is Perl supposed to know not to capitalize -that C<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$1/g; - -Now, what if you don't want to capitalize that "and"? Just use -L<Text::Autoformat> and get on with the next problem. :) - -=head2 How can I split a [character] delimited string except when inside [character]? - -Several modules can handle this sort of parsing--C<Text::Balanced>, -C<Text::CSV>, C<Text::CSV_XS>, and C<Text::ParseWords>, among others. - -Take the example case of trying to split a string that is -comma-separated into its different fields. You can't use C<split(/,/)> -because you shouldn't split if the comma is inside quotes. For -example, take a data line like this: - - SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" - -Due to the restriction of the quotes, this is a fairly complex -problem. Thankfully, we have Jeffrey Friedl, author of -I<Mastering Regular Expressions>, to handle these for us. He -suggests (assuming your string is contained in C<$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, -C<"like \"this\"">. - -Alternatively, the C<Text::ParseWords> module (part of the standard -Perl distribution) lets you say: - - use Text::ParseWords; - @new = quotewords(",", 0, $text); - -=head2 How do I strip blank space from the beginning/end of a string? - -(contributed by brian d foy) - -A substitution can do this for you. For a single line, you want to -replace all the leading or trailing whitespace with nothing. You -can do that with a pair of substitutions. - - s/^\s+//; - s/\s+$//; - -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+$//g; - -In this regular expression, the alternation matches either at the -beginning or the end of the string since the anchors have a lower -precedence than the alternation. With the C</g> flag, the substitution -makes all possible matches, so it gets both. Remember, the trailing -newline matches the C<\s+>, and the C<$> 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 C<^\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 C</m> flag (for -"multi-line"). With the C</m> flag, the C<$> matches I<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; - -=head2 How do I pad a string with blanks or pad a number with zeroes? - -In the following examples, C<$pad_len> is the length to which you wish -to pad the string, C<$text> or C<$num> contains the string to be padded, -and C<$pad_char> contains the padding character. You can use a single -character string constant instead of the C<$pad_char> variable if you -know what it is in advance. And in the same way you can use an integer in -place of C<$pad_len> if you know the pad length in advance. - -The simplest method uses the C<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 C<pack> function can only pad strings on the -right with blanks and it will truncate the result to a maximum length of -C<$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",$text); - -If you need to pad with a character other than blank or zero you can use -one of the following methods. They all generate a pad string with the -C<x> operator and combine that with C<$text>. These methods do -not truncate C<$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( $text ) ); - -Left and right padding with any character, modifying C<$text> directly: - - substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); - $text .= $pad_char x ( $pad_len - length( $text ) ); - -=head2 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 C<substr> to extract a single column. - - my $column = substr( $line, $start_column, $length ); - -You can use C<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 C<Text::CSV>, C<Text::CSV_XS>, or -C<Text::CSV_PP>. - -If you want to break apart an entire line of fixed columns, you can use -C<unpack> with the A (ASCII) format. by using a number after the format -specifier, you can denote the column width. See the C<pack> and C<unpack> -entries in L<perlfunc> for more details. - - my @fields = unpack( $line, "A8 A8 A8 A16 A4" ); - -Note that spaces in the format argument to C<unpack> do not denote literal -spaces. If you have space separated data, you may want C<split> instead. - -=head2 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 C<String::Approx>, and -C<Text::Metaphone>, and C<Text::DoubleMetaphone> modules. - -=head2 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 C<Text::Template> or C<Template> Toolkit, do that instead. You -might even be able to get the job done with C<sprintf> or C<printf>: - - my $string = sprintf 'Say hello to %s and %s', $foo, $bar; - -However, for the one-off simple case where I don't want to pull out a -full templating system, I'll use a string that has two Perl scalar -variables in it. In this example, I want to expand C<$foo> and C<$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 -C</e> flag. The first C</e> evaluates C<$1> on the replacement side and -turns it into C<$foo>. The second /e starts with C<$foo> and replaces -it with its value. C<$foo>, then, turns into 'Fred', and that's finally -what's left in the string: - - $string =~ s/(\$\w+)/$1/eeg; # 'Say hello to Fred and Barney' - -The C</e> will also silently ignore violations of strict, replacing -undefined variable names with the empty string. Since I'm using the -C</e> flag (twice even!), I have all of the same security problems I -have with C<eval> in its string form. If there's something odd in -C<$foo>, perhaps something like C<@{[ 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 C</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 C<???> 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 $string; - -=head2 What's wrong with always quoting "$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 C<++> autoincrement operator or the -syscall() function. - -Stringification also destroys arrays. - - @lines = `command`; - print "@lines"; # WRONG - extra blanks - print @lines; # right - -=head2 Why don't my E<lt>E<lt>HERE documents work? - -Check for these three things: - -=over 4 - -=item There must be no space after the E<lt>E<lt> part. - -=item There (probably) should be a semicolon at the end. - -=item You can't (easily) have any space in front of the tag. - -=back - -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. You are a liar, Saruman, and a corrupter - of men's hearts. --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 - -=head1 Data: Arrays - -=head2 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 C<foreach()> -across a list. C<@> 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 C<@_>, and -C<push>/C<pop>/C<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. - -=head2 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 C<use warnings> pragma and the B<-w> flag will warn you about these -matters. - -=head2 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 C<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 C<uniq> function from -C<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 C<%Seen>. The C<next> statement -creates the key and immediately uses its value, which is C<undef>, so -the loop continues to the C<push> and increments the value for that -key. The next time the loop sees that same element, its key exists in -the hash I<and> the value for that key is true (since it's not 0 or -C<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; - -=head2 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 I<in>dication 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 C<$is_blue{$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,$_,1) = 1 } - -Now check whether C<vec($read,$n,1)> is true for some C<$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 C<List::Util> exports -the function C<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; - -=head2 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 I<symmetric difference>, that is, all elements -in either A or in B but not in both. Think of it as an xor operation. - -=head2 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 C<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. - -=head2 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 C<first()> function in the C<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 C<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; - } - } - -=head2 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 -L<perldsc> or L<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. - -=head2 How do I handle circular lists? -X<circular> X<array> X<Tie::Cycle> X<Array::Iterator::Circular> -X<cycle> X<modulus> - -(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 C<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 C<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"; - } - -=head2 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 C<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. - -=head2 How do I process/modify each element of an array? - -Use C<for>/C<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; - $_ *= (4/3) * 3.14159; # this will be constant folded - } - -which can also be done with C<map()> which is made to transform -one list into another: - - @volumes = map {$_ ** 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 C<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 C<values> returned copies of the values, -so older perl code often contains constructions such as -C<@orbits{keys %orbits}> instead of C<values %orbits> where -the hash is to be modified. - -=head2 How do I select a random element from an array? - -Use the C<rand()> function (see L<perlfunc/rand>): - - $index = rand @array; - $element = $array[$index]; - -Or, simply: - - my $element = $array[ rand @array ]; - -=head2 How do I permute N elements of a list? -X<List::Permuter> X<permute> X<Algorithm::Loops> X<Knuth> -X<The Art of Computer Programming> X<Fischer-Krause> - -Use the C<List::Permutor> module on CPAN. If the list is actually an -array, try the C<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 -C<permute()> function is discussed in Volume 4 (still unpublished) of -Knuth's I<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 C<Algorithm::Loops> module also provides the C<NextPermute> and -C<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. - -C<NextPermute> uses string order and C<NextPermuteNum> numeric order, so -you can enumerate all the permutations of C<0..9> like this: - - use Algorithm::Loops qw(NextPermuteNum); - - my @list= 0..9; - do { print "@list\n" } while NextPermuteNum @list; - -=head2 How do I sort an array by (anything)? - -Supply a comparison function to sort() (described in L<perlfunc/sort>): - - @list = sort { $a <=> $b } @list; - -The default sort function is cmp, string comparison, which would -sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is -the numerical comparison operator. - -If you have a complicated function needed to pull out the part you -want to sort on, then don't do it inside the sort function. Pull it -out first, because the sort BLOCK can be called many times for the -same element. Here's an example of how to pull out the first word -after the first number on each item, and then sort those words -case-insensitively. - - @idx = (); - for (@data) { - ($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($b) - } @data; - -This can be conveniently combined with precalculation of keys as given -above. - -See the F<sort> article in the "Far More Than You Ever Wanted -To Know" collection in http://www.cpan.org/misc/olddoc/FMTEYEWTK.tgz for -more about this approach. - -See also the question later in L<perlfaq4> on sorting hashes. - -=head2 How do I manipulate arrays of bits? - -Use C<pack()> and C<unpack()>, or else C<vec()> and the bitwise -operations. - -For example, you don't have to store individual bits in an array -(which would mean that you're wasting a lot of space). To convert an -array of bits to a string, use C<vec()> to set the right bits. This -sets C<$vec> to have bit N set only if C<$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 C<$vec> only takes up as many bits as it needs. For -instance, if you had 16 entries in C<@ints>, C<$vec> only needs two -bytes to store them (not counting the scalar variable overhead). - -Here's how, given a vector in C<$vec>, you can get those bits into -your C<@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 C<Bit::Vector>: - - $vector = Bit::Vector->new($num_of_bits); - $vector->Index_List_Store(@ints); - @ints = $vector->Index_List_Read(); - -C<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"; - } - -=head2 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 L<perlfunc/defined> -in the 5.004 release or later of Perl for more detail. - -=head1 Data: Hashes (Associative Arrays) - -=head2 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 C<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 C<text:>, you can select just -those using C<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 -C<each()>, which returns a pair you haven't seen yet: - - while( my( $key, $value ) = each( %hash ) ) { - ... - } - -The C<each> operator returns the pairs in apparently random order, so if -ordering matters to you, you'll have to stick with the C<keys> method. - -The C<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 C<keys>, C<values>, or C<each> on the same hash, you can reset -the iterator and mess up your processing. See the C<each> entry in -L<perlfunc> for more details. - -=head2 How do I merge two hashes? -X<hash> X<merge> X<slice, hash> - -(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 (C<%hash1>) -to a new hash (C<%new_hash>), then add the keys from the other hash -(C<%hash2> to the new hash. Checking that the key already exists in -C<%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 C<%new_hash> to C<%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 C<%hash2> replace values from C<%hash1> when they have keys in common: - - @hash1{ keys %hash2 } = values %hash2; - -=head2 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 C<each()> in L<perlfunc>. - -=head2 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; - } - -=head2 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 -L<perlfaq4>, but a bit simpler in the common cases. - -You can use the C<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 C<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 C<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 C<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 C<each()>. - -=head2 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 C<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 C<\L> sequence in a double-quoted string to make everything -lowercase. The C<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; - -=head2 How can I always keep my hash sorted? -X<hash tie sort DB_File Tie::IxHash> - -You can look into using the C<DB_File> module and C<tie()> using the -C<$DB_BTREE> hash bindings as documented in L<DB_File/"In Memory -Databases">. The C<Tie::IxHash> module from CPAN might also be -instructive. Although this does keep your hash sorted, you might not -like the slow down you suffer from the tie interface. Are you sure you -need to do this? :) - -=head2 What's the difference between "delete" and "undef" with hashes? - -Hashes contain pairs of scalars: the first is the key, the -second is the value. The key will be coerced to a string, -although the value can be any kind of scalar: string, -number, or reference. If a key C<$key> is present in -%hash, C<exists($hash{$key})> will return true. The value -for a given key can be C<undef>, in which case -C<$hash{$key}> will be C<undef> while C<exists $hash{$key}> -will return true. This corresponds to (C<$key>, C<undef>) -being in the hash. - -Pictures help... here's the C<%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 - -See, the whole entry is gone! - -=head2 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. - -=head2 How do I reset an each() operation part-way through? - -(contributed by brian d foy) - -You can use the C<keys> or C<values> functions to reset C<each>. To -simply reset the iterator used by C<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 C<each> in L<perlfunc>. - -=head2 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; - -=head2 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. - -=head2 How can I make my hash remember the order I put elements into it? - -Use the C<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,...) - -=head2 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 -I<not> create the key. - - my %hash = (); - my $value = $hash{ 'foo' }; - print "This won't print\n" if exists $hash{ 'foo' }; - -Passing C<$hash{ 'foo' }> to a subroutine used to be a special case, though. -Since you could assign directly to C<$_[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/ } ); - -=head2 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 L<perlref> and the upcoming L<perlreftut>. -Examples of complex data structures are given in L<perldsc> and -L<perllol>. Examples of structures and object-oriented classes are -in L<perltoot>. - -=head2 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, C<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. - -=head1 Data: Misc - -=head2 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 C<binmode> for binary -files to avoid conversions for line endings. In general, you should -use C<binmode> any time you want to work with binary data. - -Also see L<perlfunc/"binmode"> or L<perlopentut>. - -If you're concerned about 8-bit textual data then see L<perllocale>. -If you want to deal with multibyte characters, however, there are -some gotchas. See the section on Regular Expressions. - -=head2 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. -L<Scalar::Util> (distributed with 5.8) provides access to perl's -internal function C<looks_like_number> for determining whether a -variable looks like a number. L<Data::Types> exports functions that -validate data types using both the above and other regular -expressions. Thirdly, there is C<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 C<POSIX::strtod> -function. Its semantics are somewhat cumbersome, so here's a -C<getnum> wrapper function for more convenient access. This function -takes a string and returns the number it found, or C<undef> for input -that isn't a C float. The C<is_numeric> function is a front end to -C<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 L<String::Scanf> module on the CPAN -instead. The C<POSIX> module (part of the standard Perl distribution) -provides the C<strtod> and C<strtol> for converting strings to double -and longs, respectively. - -=head2 How do I keep persistent data across program calls? - -For some specific applications, you can use one of the DBM modules. -See L<AnyDBM_File>. More generically, you should consult the C<FreezeThaw> -or C<Storable> modules from CPAN. Starting from Perl 5.8 C<Storable> is part -of the standard distribution. Here's one example using C<Storable>'s C<store> -and C<retrieve> functions: - - use Storable; - store(\%hash, "filename"); - - # later on... - $href = retrieve("filename"); # by ref - %hash = %{ retrieve("filename") }; # direct to hash - -=head2 How do I print out or copy a recursive data structure? - -The C<Data::Dumper> module on CPAN (or the 5.005 release of Perl) is great -for printing out data structures. The C<Storable> module on CPAN (or the -5.8 release of Perl), provides a function called C<dclone> that recursively -copies its argument. - - use Storable qw(dclone); - $r2 = dclone($r1); - -Where C<$r1> can be a reference to any kind of data structure you'd like. -It will be deeply copied. Because C<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) }; - -=head2 How do I define methods for every class/object? - -(contributed by Ben Morrow) - -You can use the C<UNIVERSAL> class (see L<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. - -=head2 How do I verify a credit card checksum? - -Get the C<Business::CreditCard> module from CPAN. - -=head2 How do I pack arrays of doubles or floats for XS code? - -The arrays.h/arrays.c code in the C<PGPLOT> module on CPAN does just this. -If you're doing a lot of float or double processing, consider using -the C<PDL> module from CPAN instead--it makes number-crunching easy. - -See L<http://search.cpan.org/dist/PGPLOT> for the code. - -=head1 REVISION - -Revision: $Revision$ - -Date: $Date$ - -See L<perlfaq> for source control details and availability. - -=head1 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. |