perlfaq4



PERLFAQ4(1)            Perl Programmers Reference Guide            PERLFAQ4(1)




NAME

       perlfaq4 - Data Manipulation ($Revision: 1.56 $, $Date: 2004/11/03
       22:47:56 $)


DESCRIPTION

       This section of the FAQ answers questions related to manipulating num-
       bers, 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 lan-
       guages, not just Perl.

       perlnumber show the gory details of number representations and conver-
       sions.

       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 above item "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 isnt my octal data interpreted correctly?

       Perl only understands octal and hex numbers as such when they occur as
       literals in your program.  Octal literals in perl must start with a
       leading "0" and hexadecimal literals must start with a leading "0x".
       If they are read in from somewhere and assigned, no automatic conver-
       sion takes place.  You must explicitly use oct() or hex() if you want
       the values converted to decimal.  oct() interprets hex ("0x350"), octal
       ("0350" or even without the leading "0", like "377") and binary
       ("0b1010") numbers, while hex() only converts hexadecimal ones, with or
       without a leading "0x", like "0x255", "3A", "ff", or "deadbeef".  The
       inverse mapping from decimal to octal can be done with either the "%o"
       or "%O" sprintf() formats.

       This problem shows up most often when people try using chmod(),
       mkdir(), umask(), or sysopen(), which by widespread tradition typically
       take permissions in octal.

           chmod(644,  $file); # WRONG
           chmod(0644, $file); # right

       Note the mistake in the first line was specifying the decimal literal
       644, rather than the intended octal literal 0644.  The problem can be
       seen with:

           printf("%#o",644); # prints 01204

       Surely you had not intended "chmod(01204, $file);" - did you?  If you
       want to use numeric literals as arguments to chmod() et al. then please
       try to express them as octal constants, that is with a leading zero and
       with the following digits restricted to the set 0..7.

       Does Perl have a round() function?  What about ceil() and floor()?
       Trig functions?

       Remember that int() merely truncates toward 0.  For rounding to a cer-
       tain 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 below 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", 3735928559); # upper case A-F
               $hex = sprintf("%x", 3735928559); # lower case a-f

           Using unpack:

               $hex = unpack("H*", pack("N", 3735928559));

           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, 3735928559);
               $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 = 033653337357; # 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", 3735928559);

           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 nota-
           tion:

               $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", 3735928559);

           Using unpack:

               $bin = unpack("B*", pack("N", 3735928559));

           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 doesnt & 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 (yield-
       ing "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 arent 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 ran-
       dom 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" pro-
       vides, 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?

       "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 ran-
       dom number between 0 and 5 that you can then add to 10.

           my $number = 10 + int rand( 15-10+1 );

       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_in(50,120)".

          sub random_int_in ($$) {
            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 week.  Without an argu-
       ment 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 the Time::Local module to get
       a time in epoch seconds for the argument to localtime.

               use POSIX qw/strftime/;
               use Time::Local;
               my $week_of_year = strftime "%W",
                       localtime( timelocal( 0, 0, 0, 18, 11, 1987 ) );

       The Date::Calc module provides two functions for 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?

       If you’re storing your dates as epoch seconds then simply subtract one
       from the other.  If you’ve got a structured date (distinct year, day,
       month, hour, minute, seconds values), then for reasons of accessibil-
       ity, simplicity, and efficiency, merely use either timelocal or timegm
       (from the Time::Local module in the standard distribution) to reduce
       structured dates to epoch seconds.  However, if you don’t know the pre-
       cise format of your dates, then you should probably use either of the
       Date::Manip and Date::Calc modules from CPAN before you go hacking up
       your own parsing routine to handle arbitrary date formats.

       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?

       Use the Time::JulianDay module (part of the Time-modules bundle avail-
       able from CPAN.)

       Before you immerse yourself too deeply in this, be sure to verify that
       it is the Julian Day you really want.  Are you interested in a way of
       getting serial days so that you just can tell how many days they are
       apart or so that you can do also other date arithmetic?  If you are
       interested in performing date arithmetic, this can be done using mod-
       ules Date::Manip or Date::Calc.

       There is too many details and much confusion on this issue to cover in
       this FAQ, but the term is applied (correctly) to a calendar now sup-
       planted by the Gregorian Calendar, with the Julian Calendar failing to
       adjust properly for leap years on centennial years (among other annoy-
       ances).  The term is also used (incorrectly) to mean: [1] days in the
       Gregorian Calendar; and [2] days since a particular starting time or
       ‘epoch’, usually 1970 in the Unix world and 1980 in the MS-DOS/Windows
       world.  If you find that it is not the first meaning that you really
       want, then check out the Date::Manip and Date::Calc modules.  (Thanks
       to David Cassell for most of this text.)

       How do I find yesterdays date?

       If you only need to find the date (and not the same time), you can use
       the Date::Calc module.

               use Date::Calc qw(Today Add_Delta_Days);

               my @date = Add_Delta_Days( Today(), -1 );

               print "@date\n";

       Most people try to use the time rather than the calendar to figure out
       dates, but that assumes that your 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. Russ Allbery offers this solu-
       tion.

           sub yesterday {
                       my $now  = defined $_[0] ? $_[0] : time;
                       my $then = $now - 60 * 60 * 24;
                       my $ndst = (localtime $now)[8] > 0;
                       my $tdst = (localtime $then)[8] > 0;
                       $then - ($tdst - $ndst) * 60 * 60;
                       }

       Should give you "this time yesterday" in seconds since epoch relative
       to the first argument or the current time if no argument is given and
       suitable for passing to localtime or whatever else you need to do with
       it.  $ndst is whether we’re currently in daylight savings time; $tdst
       is whether the point 24 hours ago was in daylight savings time.  If
       $tdst and $ndst are the same, a boundary wasn’t crossed, and the cor-
       rection will subtract 0.  If $tdst is 1 and $ndst is 0, subtract an
       hour more from yesterday’s time since we gained an extra hour while
       going off daylight savings time.  If $tdst is 0 and $ndst is 1, sub-
       tract a negative hour (add an hour) to yesterday’s time since we lost
       an hour.

       All of this is because during those days when one switches off or onto
       DST, a "day" isn’t 24 hours long; it’s either 23 or 25.

       The explicit settings of $ndst and $tdst are necessary because local-
       time only says it returns the system tm struct, and the system tm
       struct at least on Solaris doesn’t guarantee any particular positive
       value (like, say, 1) for isdst, just a positive value.  And that value
       can potentially be negative, if DST information isn’t available (this
       sub just treats those cases like no DST).

       Note that between 2am and 3am on the day after the time zone switches
       off daylight savings time, the exact hour of "yesterday" corresponding
       to the current hour is not clearly defined.  Note also that if used
       between 2am and 3am the day after the change to daylight savings time,
       the result will be between 3am and 4am of the previous day; it’s
       arguable whether this is correct.

       This sub does not attempt to deal with leap seconds (most things
       don’t).

       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?

       The answer to this question is usually a regular expression, perhaps
       with auxiliary logic.  See the more specific questions (numbers, mail
       addresses, etc.) for details.

       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?

       To turn "abbcccd" into "abccd":

           s/(.)\1/$1/g;       # add /s to include newlines

       Here’s a solution that turns "abbcccd" to "abcd":

           y///cs;     # y == tr, but shorter :-)

       How do I expand function calls in a string?

       This is documented in perlref.  In general, this is fraught with quot-
       ing and readability problems, but it is possible.  To interpolate a
       subroutine call (in list context) into a string:

           print "My sub returned @{[mysub(1,2,3)]} that time.\n";

       See also ‘‘How can I expand variables in text strings?’’ in this sec-
       tion of the FAQ.

       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 charac-
       ters, 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 mod-
       ule 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 mod-
       ules 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 join("\n",@$[0..$#$]) if( $$[-1] );

       How do I reverse a string?

       Use reverse() in scalar context, as documented in "reverse" in perl-
       func.

           $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 new-
       lines.  Text::Wrap doesn’t justify the lines (flush-right).

       Or use the CPAN module Text::Autoformat.  Formatting files can be eas-
       ily 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 capa-
       bilities.

       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 "whoso-
       ever" 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;

       How do I capitalize all the words on one line?

       To make the first letter of each word upper case:

               $line =~ s/\b(\w)/\U$1/g;

       This has the strange effect of turning ""don’t do it"" into ""Don’T Do
       It"".  Sometimes you might want this.  Other times you might need a
       more thorough solution (Suggested by brian d foy):

           $string =~ s/ (
                        (^\w)    #at the beginning of the line
                          │      # or
                        (\s\w)   #preceded by whitespace
                          )
                       /\U$1/xg;
           $string =~ /([\w’]+)/\u\L$1/g;

       To make the whole line upper case:

               $line = uc($line);

       To force each word to be lower case, with the first letter upper case:

               $line =~ s/(\w+)/\u\L$1/g;

       You can (and probably should) enable locale awareness of those charac-
       ters by placing a "use locale" pragma in your program.  See perllocale
       for endless details on locales.

       This is sometimes referred to as putting something into "title case",
       but that’s not quite accurate.  Consider the proper capitalization of
       the movie Dr. Strangelove or: How I Learned to Stop Worrying and Love
       the Bomb, for example.

       Damian Conway’s Text::Autoformat module provides some smart case trans-
       formations:

           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 can I split a [character] delimited string except when inside
       [character]?

       Several modules can handle this sort of pasing---Text::Balanced,
       Text::CVS, Text::CVS_XS, and Text::ParseWords, among others.

       Take the example case of trying to split a string that is comma-sepa-
       rated 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 Expres-
       sions, 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-delim-
       ited 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);

       There’s also a Text::CSV (Comma-Separated Values) module on CPAN.

       How do I strip blank space from the beginning/end of a string?

       Although the simplest approach would seem to be

           $string =~ s/^\s*(.*?)\s*$/$1/;

       not only is this unnecessarily slow and destructive, it also fails with
       embedded newlines.  It is much faster to do this operation in two
       steps:

           $string =~ s/^\s+//;
           $string =~ s/\s+$//;

       Or more nicely written as:

           for ($string) {
               s/^\s+//;
               s/\s+$//;
           }

       This idiom takes advantage of the "foreach" loop’s aliasing behavior to
       factor out common code.  You can do this on several strings at once, or
       arrays, or even the values of a hash if you use a slice:

           # trim whitespace in the scalar, the array,
           # and all the values in the hash
           foreach ($scalar, @array, @hash{keys %hash}) {
               s/^\s+//;
               s/\s+$//;
           }

       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 charac-
       ter 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?

       Use substr() or unpack(), both documented in perlfunc.  If you prefer
       thinking in terms of columns instead of widths, you can use this kind
       of thing:

           # determine the unpack format needed to split Linux ps output
           # arguments are cut columns
           my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72);

           sub cut2fmt {
               my(@positions) = @_;
               my $template  = ’’;
               my $lastpos   = 1;
               for my $place (@positions) {
                   $template .= "A" . ($place - $lastpos) . " ";
                   $lastpos   = $place;
               }
               $template .= "A*";
               return $template;
           }

       How do I find the soundex value of a string?

       Use the standard Text::Soundex module distributed with Perl.  Before
       you do so, you may want to determine whether ‘soundex’ is in fact what
       you think it is.  Knuth’s soundex algorithm compresses words into a
       small space, and so it does not necessarily distinguish between two
       words which you might want to appear separately.  For example, the last
       names ‘Knuth’ and ‘Kant’ are both mapped to the soundex code K530.  If
       Text::Soundex does not do what you are looking for, you might want to
       consider the String::Approx module available at CPAN.

       How can I expand variables in text strings?

       Let’s assume that you have a string like:

           $text = ’this has a $foo in it and a $bar’;

       If those were both global variables, then this would suffice:

           $text =~ s/\$(\w+)/${$1}/g;  # no /e needed

       But since they are probably lexicals, or at least, they could be, you’d
       have to do this:

           $text =~ s/(\$\w+)/$1/eeg;
           die if $@;                  # needed /ee, not /e

       It’s probably better in the general case to treat those variables as
       entries in some special hash.  For example:

           %user_defs = (
               foo  => 23,
               bar  => 19,
           );
           $text =~ s/\$(\w+)/$user_defs{$1}/g;

       See also ‘‘How do I expand function calls in a string?’’ in this sec-
       tion of the FAQ.

       Whats wrong with always quoting "$vars"?

       The problem is that those double-quotes force stringification-- coerc-
       ing 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 sim-
       pler 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 num-
       ber, 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 dont 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. 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 indenta-
       tion 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 some-
       thing you can push or pop, while a list is a set of values.  Some peo-
       ple 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?

       There are several possible ways, depending on whether the array is
       ordered and whether you wish to preserve the ordering.

       a)  If @in is sorted, and you want @out to be sorted: (this assumes all
           true values in the array)

               $prev = "not equal to $in[0]";
               @out = grep($_ ne $prev && ($prev = $_, 1), @in);

           This is nice in that it doesn’t use much extra memory, simulating
           uniq(1)’s behavior of removing only adjacent duplicates.  The ", 1"
           guarantees that the expression is true (so that grep picks it up)
           even if the $_ is 0, "", or undef.

       b)  If you don’t know whether @in is sorted:

               undef %saw;
               @out = grep(!$saw{$_}++, @in);

       c)  Like (b), but @in contains only small integers:

               @out = grep(!$saw[$_]++, @in);

       d)  A way to do (b) without any loops or greps:

               undef %saw;
               @saw{@in} = ();
               @out = sort keys %saw;  # remove sort if undesired

       e)  Like (d), but @in contains only small positive integers:

               undef @ary;
               @ary[@in] = @in;
               @out = grep {defined} @ary;

       But perhaps you should have been using a hash all along, eh?

       How can I tell whether a certain element is contained in a list or
       array?

       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.  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.

       Please do not use

           ($is_there) = grep $_ eq $whatever, @array;

       or worse yet

           ($is_there) = grep /$whatever/, @array;

       These are slow (checks every element even if the first matches), inef-
       ficient (same reason), and potentially buggy (what if there are regex
       characters in $whatever?).  If you’re only testing once, then use:

           $is_there = 0;
           foreach $elt (@array) {
               if ($elt eq $elt_to_find) {
                   $is_there = 1;
                   last;
               }
           }
           if ($is_there) { ... }

       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?

       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 my $element ( @array )
                       {
                       if( /Perl/ ) { $found = $element; 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 both O(1) opera-
       tions 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?

       Circular lists could be handled in the traditional fashion with linked
       lists, or you could just do something like this with an array:

           unshift(@array, pop(@array));  # the last shall be first
           push(@array, shift(@array));   # and vice versa

       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
               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
               y/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-Kause 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;

       How do I sort an array by (anything)?

       Supply a comparison function to sort() (described in "sort" in perl-
       func):

           @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-insensi-
       tively.

           @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 below on sorting hashes.

       How do I manipulate arrays of bits?

       Use pack() and unpack(), or else vec() and the bitwise operations.

       For example, this sets $vec to have bit N set if $ints[N] was set:

           $vec = ’’;
           foreach(@ints) { vec($vec,$_,1) = 1 }

       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?

       Use the each() function (see "each" in perlfunc) if you don’t care
       whether it’s sorted:

           while ( ($key, $value) = each %hash) {
               print "$key = $value\n";
           }

       If you want it sorted, you’ll have to use foreach() on the result of
       sorting the keys as shown in an earlier question.

       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?

       If you mean how many keys, then all you have to do is use the keys()
       function in a scalar context:

           $num_keys = keys %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 opera-
       tors such as each().

       How do I sort a hash (optionally by value instead of key)?

       Internally, hashes are stored in a way that prevents you from imposing
       an order on key-value pairs.  Instead, you have to sort a list of the
       keys or values:

           @keys = sort keys %hash;    # sorted by key
           @keys = sort {
                           $hash{$a} cmp $hash{$b}
                   } keys %hash;       # and by value

       Here we’ll do a reverse numeric sort by value, and if two keys are
       identical, sort by length of key, or if that fails, by straight ASCII
       comparison of the keys (well, possibly modified by your locale--see
       perllocale).

           @keys = sort {
                       $hash{$b} <=> $hash{$a}
                                 ││
                       length($b) <=> length($a)
                                 ││
                             $a cmp $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.

       Whats 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 $hash{’a’}                is true (Perl5 only)
               grep ($_ 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 $hash{’a’}                is true (Perl5 only)
               grep ($_ 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 $hash{’a’}                is FALSE (Perl5 only)
               grep ($_ eq ’a’, keys %hash)     is FALSE

       See, the whole entry is gone!

       Why dont my tied hashes make the defined/exists distinction?

       This depends on the tied hash’s implementation of EXISTS().  For exam-
       ple, 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 ordi-
       nary hashes.

       How do I reset an each() operation part-way through?

       Using "keys %hash" in scalar context returns the number of keys in the
       hash and resets the iterator associated with the hash.  You may need to
       do this if you use "last" to exit a loop early so that when you re-
       enter it, the hash iterator has been reset.

       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?

       If you say something like:

           somefunc($hash{"nonesuch key here"});

       Then that element "autovivifies"; that is, it springs into existence
       whether you store something there or not.  That’s because functions get
       scalars passed in by reference.  If somefunc() modifies $_[0], it has
       to be ready to write it back into the caller’s version.

       This has been fixed as of Perl5.004.

       Normally, merely accessing a key’s value for a nonexistent key does not
       cause that key to be forever there.  This is different than awk’s
       behavior.

       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?

       You can’t do this directly, but you could use the standard Tie::RefHash
       module distributed with Perl.


Data: Misc

       How do I handle binary data correctly?

       Perl is binary clean, so this shouldn’t be a problem.  For example,
       this works fine (assuming the files are found):

           if (‘cat /vmunix‘ =~ /gzip/) {
               print "Your kernel is GNU-zip enabled!\n";
           }

       On less elegant (read: Byzantine) systems, however, you have to play
       tedious games with "text" versus "binary" files.  See "binmode" in
       perlfunc or perlopentut.

       If you’re concerned about 8-bit ASCII 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 "Infin-
       ity", 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 "Reg-
       exp::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" func-
       tion.  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 recur-
       sively 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 refer-
       ences, 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?

       Use the UNIVERSAL class (see UNIVERSAL).

       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 kgbpack.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.


AUTHOR AND COPYRIGHT

       Copyright (c) 1997-2002 Tom Christiansen and Nathan Torkington.  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.



perl v5.8.6                       2004-11-05                       PERLFAQ4(1)

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