X-Git-Url: http://git.shadowcat.co.uk/gitweb/gitweb.cgi?a=blobdiff_plain;f=pod%2Fperlfaq4.pod;h=838f753fa6ae2d0f6999708bdee91eee93d1745b;hb=a3dfe201291c96fc01babd3d4782d52ba945f2a3;hp=bcf03990bc6eef7d178935d3e14118b80acdf0fb;hpb=fc36a67e8855d031b2a6921819d899eb149eee2d;p=p5sagit%2Fp5-mst-13.2.git diff --git a/pod/perlfaq4.pod b/pod/perlfaq4.pod index bcf0399..838f753 100644 --- a/pod/perlfaq4.pod +++ b/pod/perlfaq4.pod @@ -1,6 +1,6 @@ =head1 NAME -perlfaq4 - Data Manipulation ($Revision: 1.19 $, $Date: 1997/04/24 22:43:57 $) +perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $) =head1 DESCRIPTION @@ -12,9 +12,13 @@ data issues. =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? +The infinite set that a mathematician thinks of as the real numbers can +only be approximate on a computer, since the computer only has a finite +number of bits to store an infinite number of, um, numbers. + Internally, your computer represents floating-point numbers in binary. -Floating-point numbers read in from a file, or appearing as literals -in your program, are converted from their decimal floating-point +Floating-point numbers read in from a file or appearing as literals +in your program are converted from their decimal floating-point representation (eg, 19.95) to the internal binary representation. However, 19.95 can't be precisely represented as a binary @@ -27,7 +31,7 @@ representation is converted back to decimal. These decimal numbers are displayed in either the format you specify with printf(), or the current output format for numbers (see L if you use print. C<$#> has a different default value in Perl5 than it did in -Perl4. Changing C<$#> yourself is deprecated. +Perl4. Changing C<$#> yourself is deprecated.) This affects B computer languages that represent decimal floating-point numbers in binary, not just Perl. Perl provides @@ -37,6 +41,7 @@ are consequently slower. To get rid of the superfluous digits, just use a format (eg, C) to get the required precision. +See L. =head2 Why isn't my octal data interpreted correctly? @@ -54,18 +59,24 @@ umask(), or sysopen(), which all want permissions in octal. chmod(644, $file); # WRONG -- perl -w catches this chmod(0644, $file); # right -=head2 Does perl have a round function? What about ceil() and floor()? -Trig functions? +=head2 Does Perl have a round() function? What about ceil() and floor()? Trig functions? + +Remember that int() merely truncates toward 0. For rounding to a +certain number of digits, sprintf() or printf() is usually the easiest +route. -For rounding to a certain number of digits, sprintf() or printf() is -usually the easiest route. + printf("%.3f", 3.1415926535); # prints 3.142 -The POSIX module (part of the standard perl distribution) implements +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 + $floor = floor(3.5); # 3 + In 5.000 to 5.003 Perls, trigonometry was done in the Math::Complex -module. With 5.004, the Math::Trig module (part of the standard perl +module. With 5.004, the Math::Trig module (part of the standard Perl distribution) implements the trigonometric functions. Internally it uses the Math::Complex module and some functions can break out from the real axis into the complex plane, for example the inverse sine of @@ -77,17 +88,64 @@ cases, it probably pays not to trust whichever system rounding is being used by Perl, but to instead implement the rounding function you need yourself. +To see why, notice how you'll still have an issue on half-way-point +alternation: + + for ($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 bits into ints? -To turn a string of 1s and 0s like '10110110' into a scalar containing -its binary value, use the pack() function (documented in -L): +To turn a string of 1s and 0s like C<10110110> into a scalar containing +its binary value, use the pack() and unpack() functions (documented in +L and L): + + $decimal = unpack('c', pack('B8', '10110110')); + +This packs the string C<10110110> into an eight bit binary structure. +This is then unpacked as a character, which returns its ordinal value. - $decimal = pack('B8', '10110110'); +This does the same thing: + + $decimal = ord(pack('B8', '10110110')); Here's an example of going the other way: - $binary_string = join('', unpack('B*', "\x29")); + $binary_string = unpack('B*', "\x29"); + +=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<1>). 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? @@ -109,12 +167,12 @@ To call a function on each element of an array, but ignore the results: foreach $iterator (@array) { - &my_func($iterator); + some_func($iterator); } To call a function on each integer in a (small) range, you B use: - @results = map { &my_func($_) } (5 .. 25); + @results = map { some_func($_) } (5 .. 25); but you should be aware that the C<..> operator creates an array of all integers in the range. This can take a lot of memory for large @@ -122,22 +180,43 @@ ranges. Instead use: @results = (); for ($i=5; $i < 500_005; $i++) { - push(@results, &my_func($i)); + push(@results, some_func($i)); } +This situation has been fixed in Perl5.005. Use of C<..> in a C +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.perl.com/CPAN/modules/by-module/Roman module. =head2 Why aren't my random numbers random? -The short explanation is that you're getting pseudorandom numbers, not -random ones, because that's how these things work. A longer -explanation is available on +If you're using a version of Perl before 5.004, you must call C +once at the start of your program to seed the random number generator. +5.004 and later automatically call C at the beginning. Don't +call C 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 :-). http://www.perl.com/CPAN/doc/FMTEYEWTK/random, courtesy of Tom -Phoenix. +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.'' -You should also check out the Math::TrulyRandom module from CPAN. +If you want numbers that are more random than C with C +provides, you should also check out the Math::TrulyRandom module from +CPAN. It uses the imperfections in your system's timer to generate +random numbers, but this takes quite a while. If you want a better +pseudorandom generator than comes with your operating system, look at +``Numerical Recipes in C'' at http://www.nr.com/ . =head1 Data: Dates @@ -157,70 +236,192 @@ You can find the week of the year by dividing this by 7: $week_of_year = int($day_of_year / 7); -Of course, this believes that weeks start at zero. - -=head2 How can I compare two date strings? - -Use the Date::Manip or Date::DateCalc modules from CPAN. +Of course, this believes that weeks start at zero. The Date::Calc +module from CPAN has a lot of date calculation functions, including +day of the year, week of the year, and so on. Note that not +all businesses consider ``week 1'' to be the same; for example, +American businesses often consider the first week with a Monday +in it to be Work Week #1, despite ISO 8601, which considers +WW1 to be the first week with a Thursday in it. + +=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, you'll find that the POSIX module's 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? + +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 accessibility, +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 precise 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. =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 timelocal in the standard -Time::Local module. Otherwise, you should look into one of the -Date modules from CPAN. +you can split it up and pass the parts to C in the standard +Time::Local module. Otherwise, you should look into the Date::Calc +and Date::Manip modules from CPAN. =head2 How can I find the Julian Day? -Neither Date::Manip nor Date::DateCalc deal with Julian days. -Instead, there is an example of Julian date calculation in -http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz, -which should help. - -=head2 Does Perl have a year 2000 problem? - -Not unless you use Perl to create one. 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). -The year returned by these functions when used in an array context is -the year minus 1900. For years between 1910 and 1999 this I -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 a scalar context they return +You could use Date::Calc's Delta_Days function and calculate the number +of days from there. Assuming that's what you really want, that is. + +Before you immerse yourself too deeply in this, be sure to verify that it +is the I Day you really want. Are they really just interested in +a way of getting serial days so that they can do date arithmetic? If you +are interested in performing date arithmetic, this can be done using +either Date::Manip or Date::Calc, without converting to Julian Day first. + +There is too much confusion on this issue to cover in this FAQ, but the +term is applied (correctly) to a calendar now supplanted by the Gregorian +Calendar, with the Julian Calendar failing to adjust properly for leap +years on centennial years (among other annoyances). 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.) + +There is also an example of Julian date calculation that should help you in +http://www.perl.com/CPAN/authors/David_Muir_Sharnoff/modules/Time/JulianDay.pm.gz + +=head2 How do I find yesterday's date? + +The C function returns the current time in seconds since the +epoch. Take twenty-four hours off that: + + $yesterday = time() - ( 24 * 60 * 60 ); + +Then you can pass this to C and get the individual year, +month, day, hour, minute, seconds values. + +Note very carefully that the code above 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. +A solution to this issue is offered by Russ Allbery. + + 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 correction + # 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, subtract 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 localtime + # 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). + # + # Copyright relinquished 1999 by Russ Allbery + # This code is in the public domain + +=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 an array context is the year minus 1900. +For years between 1910 and 1999 this I 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://language.perl.com/news/y2k.html for +a longer exposition. + =head1 Data: Strings =head2 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, email +with auxiliary logic. See the more specific questions (numbers, mail addresses, etc.) for details. =head2 How do I unescape a string? -It depends just what you mean by "escape". URL escapes are dealt with -in L. Shell escapes with the backslash (\) +It depends just what you mean by ``escape''. URL escapes are dealt +with in L. Shell escapes with the backslash (C<\>) character are removed with: s/\\(.)/$1/g; -Note that this won't expand \n or \t or any other special escapes. +This won't expand C<"\n"> or C<"\t"> or any other special escapes. =head2 How do I remove consecutive pairs of characters? -To turn "abbcccd" into "abccd": +To turn C<"abbcccd"> into C<"abccd">: - s/(.)\1/$1/g; + s/(.)\1/$1/g; # add /s to include newlines + +Here's a solution that turns "abbcccd" to "abcd": + + y///cs; # y == tr, but shorter :-) =head2 How do I expand function calls in a string? This is documented in L. In general, this is fraught with quoting and readability problems, but it is possible. To interpolate -a subroutine call (in a list context) into a string: +a subroutine call (in list context) into a string: print "My sub returned @{[mysub(1,2,3)]} that time.\n"; @@ -229,32 +430,65 @@ arbitrary expressions: print "That yields ${\($n + 5)} widgets\n"; -See also "How can I expand variables in text strings?" in this section -of the FAQ. +Version 5.004 of Perl had a bug that gave list context to the +expression in C<${...}>, but this is fixed in version 5.005. + +See also ``How can I expand variables in text strings?'' in this +section of the FAQ. =head2 How do I find matching/nesting anything? -This isn't something that can be tackled in one regular expression, no -matter how complicated. To find something between two single characters, -a pattern like C will get the intervening bits in $1. For -multiple ones, then something more like C would -be needed. But none of these deals with nested patterns, nor can they. -For that you'll have to write a parser. +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 will get the intervening +bits in $1. For multiple ones, then something more like +C would be needed. But none of these deals with +nested patterns, nor can they. For that 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 is +the CPAN module Parse::RecDescent, the standard module Text::Balanced, +the byacc program, the CPAN module Parse::Yapp, and Mark-Jason +Dominus's excellent I tool at http://www.plover.com/%7Emjd/perl/py/ +. + +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/); + print join("\n",@$[0..$#$]) if( $$[-1] ); =head2 How do I reverse a string? -Use reverse() in a scalar context, as documented in +Use reverse() in scalar context, as documented in L. $reversed = reverse $string; =head2 How do I expand tabs in a string? -You can do it the old-fashioned way: +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 +Or you can just use the Text::Tabs module (part of the standard Perl distribution). use Text::Tabs; @@ -262,18 +496,18 @@ distribution). =head2 How do I reformat a paragraph? -Use Text::Wrap (part of the standard perl distribution): +Use Text::Wrap (part of the standard Perl distribution): use Text::Wrap; print wrap("\t", ' ', @paragraphs); -The paragraphs you give to Text::Wrap may not contain embedded +The paragraphs you give to Text::Wrap should not contain embedded newlines. Text::Wrap doesn't justify the lines (flush-right). =head2 How can I access/change the first N letters of a string? There are many ways. If you just want to grab a copy, use -substr: +substr(): $first_byte = substr($a, 0, 1); @@ -282,22 +516,41 @@ use substr() as an lvalue: substr($a, 0, 3) = "Tom"; -Although those with a regexp kind of thought process will likely prefer +Although those with a pattern matching kind of thought process will +likely prefer: $a =~ s/^.../Tom/; =head2 How do I change the Nth occurrence of something? -You have to keep track. For example, let's say you want -to change the fifth occurrence of "whoever" or "whomever" -into "whosoever" or "whomsoever", case insensitively. +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 - }igex; + }ige; + +In the more general case, you can use the C modifier in a C +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? @@ -305,9 +558,9 @@ 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 function like so: - $string = "ThisXlineXhasXsomeXx'sXinXit": + $string = "ThisXlineXhasXsomeXx'sXinXit"; $count = ($string =~ tr/X//); - print "There are $count X charcters in the string"; + 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 @@ -326,8 +579,8 @@ To make the first letter of each word upper case: $line =~ s/\b(\w)/\U$1/g; This has the strange effect of turning "C" into "C". Sometimes you might want this, instead (Suggested by Brian -Foy Ecomdog@computerdog.comE): +Do It>". Sometimes you might want this, instead (Suggested by brian d. +foy): $string =~ s/ ( (^\w) #at the beginning of the line @@ -345,6 +598,15 @@ 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 +characters by placing a C pragma in your program. +See L 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 I, for example. + =head2 How can I split a [character] delimited string except when inside [character]? (Comma-separated files) @@ -371,22 +633,25 @@ suggests (assuming your string is contained in $text): If you want to represent quotation marks inside a quotation-mark-delimited field, escape them with backslashes (eg, -C<"like \"this\""). Unescaping them is a task addressed earlier in +C<"like \"this\"">. Unescaping them is a task addressed earlier in this section. -Alternatively, the Text::ParseWords module (part of the standard perl +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 module on CPAN. + =head2 How do I strip blank space from the beginning/end of a string? -The simplest approach, albeit not the fastest, is probably like this: +Although the simplest approach would seem to be: $string =~ s/^\s*(.*?)\s*$/$1/; -It would be faster to do this in two steps: +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+//; $string =~ s/\s+$//; @@ -398,37 +663,132 @@ Or more nicely written as: s/\s+$//; } +This idiom takes advantage of the C loop's aliasing +behavior to factor out common code. You can do this +on several strings at once, or arrays, or even the +values of a hash if you use a slice: + + # trim whitespace in the scalar, the array, + # and all the values in the hash + foreach ($scalar, @array, @hash{keys %hash}) { + s/^\s+//; + s/\s+$//; + } + +=head2 How do I pad a string with blanks or pad a number with zeroes? + +(This answer contributed by Uri Guttman, with kibitzing from +Bart Lateur.) + +In the following examples, C<$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<$pad_len> if you know the pad length in advance. + +The simplest method uses the C function. It can pad on the left +or right with blanks and on the left with zeroes and it will not +truncate the result. The C function can only pad strings on the +right with blanks and it will truncate the result to a maximum length of +C<$pad_len>. + + # Left padding a string with blanks (no truncation): + $padded = sprintf("%${pad_len}s", $text); + + # Right padding a string with blanks (no truncation): + $padded = sprintf("%-${pad_len}s", $text); + + # Left padding a number with 0 (no truncation): + $padded = sprintf("%0${pad_len}d", $num); + + # Right padding a string with blanks using pack (will truncate): + $padded = pack("A$pad_len",$text); + +If you need to pad with a character other than blank or zero you can use +one of the following methods. They all generate a pad string with the +C operator and combine that with C<$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? Use substr() or unpack(), both documented in L. +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; + } =head2 How do I find the soundex value of a string? -Use the standard Text::Soundex module distributed with perl. +Use the standard Text::Soundex module distributed with Perl. +But 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. =head2 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'; - $text =~ s/\$(\w+)/${$1}/g; -Before version 5 of perl, this had to be done with a double-eval -substitution: +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 -Which is bizarre enough that you'll probably actually need an EEG -afterwards. :-) +It's probably better in the general case to treat those +variables as entries in some special hash. For example: -See also "How do I expand function calls in a string?" in this section + %user_defs = ( + foo => 23, + bar => 19, + ); + $text =~ s/\$(\w+)/$user_defs{$1}/g; + +See also ``How do I expand function calls in a string?'' in this section of the FAQ. =head2 What's wrong with always quoting "$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. +don't want them to be. 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: @@ -458,7 +818,13 @@ that actually do care about the difference between a string and a number, such as the magical C<++> autoincrement operator or the syscall() function. -=head2 Why don't my <EHERE documents work? Check for these three things: @@ -472,8 +838,95 @@ Check for these three things: =back +If you want to indent the text in the here document, you +can do this: + + # all in one + ($VAR = <op_ppaddr)() ); + @@@ TAINT_NOT; + @@@ return 0; + @@@ } + MAIN_INTERPRETER_LOOP + +Or with a fixed amount of leading white space, with remaining +indentation correctly preserved: + + $poem = fix< 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<@_>, 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. + =head2 What is the difference between $array[1] and @array[1]? The former is a scalar value, the latter an array slice, which makes @@ -492,7 +945,7 @@ with The B<-w> flag will warn you about these matters. -=head2 How can I extract just the unique elements of an array? +=head2 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. @@ -500,13 +953,15 @@ ordered and whether you wish to preserve the ordering. =over 4 =item a) If @in is sorted, and you want @out to be sorted: +(this assumes all true values in the array) $prev = 'nonesuch'; @out = grep($_ ne $prev && ($prev = $_), @in); -This is nice in that it doesn't use much extra memory, -simulating uniq(1)'s behavior of removing only adjacent -duplicates. +This is nice in that it doesn't use much extra memory, simulating +uniq(1)'s behavior of removing only adjacent duplicates. It's less +nice in that it won't work with false values like undef, 0, or ""; +"0 but true" is OK, though. =item b) If you don't know whether @in is sorted: @@ -527,15 +982,21 @@ duplicates. undef @ary; @ary[@in] = @in; - @out = @ary; + @out = grep {defined} @ary; =back -=head2 How can I tell whether an array contains a certain element? +But perhaps you should have been using a hash all along, eh? + +=head2 How can I tell whether a list or array contains a certain element? -There are several ways to approach this. If you are going to make -this query many times and the values are arbitrary strings, the -fastest way is probably to invert the original array and keep an +Hearing the word "in" is an Idication 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 keep an associative array lying about whose keys are the first array's values. @blues = qw/azure cerulean teal turquoise lapis-lazuli/; @@ -550,7 +1011,8 @@ array. This kind of an array will take up less space: @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); undef @is_tiny_prime; - for (@primes) { $is_tiny_prime[$_] = 1; } + for (@primes) { $is_tiny_prime[$_] = 1 } + # or simply @istiny_prime[@primes] = (1) x @primes; Now you check whether $is_tiny_prime[$some_number]. @@ -559,7 +1021,7 @@ quite a lot of space by using bit strings instead: @articles = ( 1..10, 150..2000, 2017 ); undef $read; - grep (vec($read,$_,1) = 1, @articles); + for (@articles) { vec($read,$_,1) = 1 } Now check whether C is true for some C<$n>. @@ -573,7 +1035,17 @@ or worse yet These are slow (checks every element even if the first matches), inefficient (same reason), and potentially buggy (what if there are -regexp characters in $whatever?). +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) { ... } =head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? @@ -588,11 +1060,63 @@ each element is unique in a given array: push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; } +Note that this is the I, 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? + +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) = @_; + local $^W = 0; # silence spurious -w undef complaints + return 0 unless @$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; + $b{EXTRA} = \%a; + + printf "a and b contain %s hashes\n", + cmpStr(\%a, \%b) == 0 ? "the same" : "different"; + + printf "a and b contain %s hashes\n", + cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; + + +The first reports that both those the hashes contain the same data, +while the second reports that they do not. Which you prefer is left as +an exercise to the reader. + =head2 How do I find the first array element for which a condition is true? You can use this if you care about the index: - for ($i=0; $i < @array; $i++) { + for ($i= 0; $i < @array; $i++) { if ($array[$i] eq "Waldo") { $found_index = $i; last; @@ -605,12 +1129,50 @@ Now C<$found_index> has what you want. 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. +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) 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 or L and do just what the algorithm book tells you -to do. +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 $node->{VALUE}, " "; + } + print "\n"; + +You could grow the list this way: + + my ($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? @@ -622,7 +1184,23 @@ lists, or you could just do something like this with an array: =head2 How do I shuffle an array randomly? -Here's a shuffling algorithm which works its way through the list, +Use this: + + # fisher_yates_shuffle( \@array ) : + # generate a random permutation of @array in place + sub fisher_yates_shuffle { + my $array = shift; + my $i; + for ($i = @$array; --$i; ) { + my $j = int rand ($i+1); + next if $i == $j; + @$array[$i,$j] = @$array[$j,$i]; + } + } + + fisher_yates_shuffle( \@array ); # permutes @array in place + +You've probably seen shuffling algorithms that work using splice, randomly picking another element to swap the current element with: srand; @@ -632,65 +1210,70 @@ randomly picking another element to swap the current element with: push(@new, splice(@old, rand @old, 1)); } -For large arrays, this avoids a lot of the reshuffling: - - srand; - @new = (); - @old = 1 .. 10000; # just a demo - for( @old ){ - my $r = rand @new+1; - push(@new,$new[$r]); - $new[$r] = $_; - } +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/C: for (@lines) { - s/foo/bar/; - tr[a-z][A-Z]; + s/foo/bar/; # change that word + y/XZ/ZX/; # swap those letters } Here's another; let's compute spherical volumes: - for (@radii) { + for (@volumes = @radii) { # @volumes has changed parts $_ **= 3; $_ *= (4/3) * 3.14159; # this will be constant folded } +If you want to do the same thing to modify the values of the hash, +you may not use the C function, oddly enough. You need a slice: + + for $orbit ( @orbits{keys %orbits} ) { + ($orbit **= 3) *= (4/3) * 3.14159; + } + =head2 How do I select a random element from an array? Use the rand() function (see L): + # at the top of the program: srand; # not needed for 5.004 and later + + # then later on $index = rand @array; $element = $array[$index]; +Make sure you I. +If you are calling it more than once (such as before each +call to rand), you're almost certainly doing something wrong. + =head2 How do I permute N elements of a list? Here's a little program that generates all permutations of all the words on each line of input. The algorithm embodied -in the permut() function should work on any list: +in the permute() function should work on any list: #!/usr/bin/perl -n - # permute - tchrist@perl.com - permut([split], []); - sub permut { - my @head = @{ $_[0] }; - my @tail = @{ $_[1] }; - unless (@head) { - # stop recursing when there are no elements in the head - print "@tail\n"; + # tsc-permute: permute each word of input + permute([split], []); + sub permute { + my @items = @{ $_[0] }; + my @perms = @{ $_[1] }; + unless (@items) { + print "@perms\n"; } else { - # for all elements in @head, move one from @head to @tail - # and call permut() on the new @head and @tail - my(@newhead,@newtail,$i); - foreach $i (0 .. $#head) { - @newhead = @head; - @newtail = @tail; - unshift(@newtail, splice(@newhead, $i, 1)); - permut([@newhead], [@newtail]); + my(@newitems,@newperms,$i); + foreach $i (0 .. $#items) { + @newitems = @items; + @newperms = @perms; + unshift(@newperms, splice(@newitems, $i, 1)); + permute([@newitems], [@newperms]); } } } @@ -724,7 +1307,7 @@ 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; + map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; If you need to sort on several fields, the following paradigm is useful. @@ -785,9 +1368,54 @@ get those bits into your @ints array: This method gets faster the more sparse the bit vector is. (Courtesy of Tim Bunce and Winfried Koenig.) +Here's a demo on how to use 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), $bits); + print "bits are: @bytes\n\n"; + } + =head2 Why does defined() return true on empty arrays and hashes? -See L in the 5.004 release or later of Perl. +The short story is that you should probably only use defined on scalars or +functions, not on aggregates (arrays and hashes). See L +in the 5.004 release or later of Perl for more detail. =head1 Data: Hashes (Associative Arrays) @@ -796,7 +1424,7 @@ See L in the 5.004 release or later of Perl. Use the each() function (see L) if you don't care whether it's sorted: - while (($key,$value) = each %hash) { + while ( ($key, $value) = each %hash) { print "$key = $value\n"; } @@ -805,7 +1433,19 @@ sorting the keys as shown in an earlier question. =head2 What happens if I add or remove keys from a hash while iterating over it? -Don't do that. +Don't do that. :-) + +[lwall] In Perl 4, you were not allowed to modify a hash at all while +iterating over it. In Perl 5 you can delete from it, but you still +can't add to it, because that might cause a doubling of the hash table, +in which half the entries get copied up to the new top half of the +table, at which point you've totally bamboozled the iterator code. +Even if the table doesn't double, there's no telling whether your new +entry will be inserted before or after the current iterator position. + +Either treasure up your changes and make them after the iterator finishes, +or use keys to fetch all the old keys at once, and iterate over the list +of keys. =head2 How do I look up a hash element by value? @@ -821,8 +1461,13 @@ to use: $by_value{$value} = $key; } -If your hash could have repeated values, the methods above will only -find one of the associated keys. This may or may not worry you. +If your hash could have repeated values, the methods above will only find +one of the associated keys. This may or may not worry you. If it does +worry you, you can always reverse the hash into a hash of arrays instead: + + while (($key, $value) = each %by_key) { + push @{$key_list_by_value{$value}}, $key; + } =head2 How can I know how many entries are in a hash? @@ -831,8 +1476,9 @@ take the scalar sense of the keys() function: $num_keys = scalar keys %hash; -In void context it just resets the iterator, which is faster -for tied hashes. +In void context, the keys() function just resets the iterator, which is +faster for tied hashes than would be iterating through the whole +hash, one key-value pair at a time. =head2 How do I sort a hash (optionally by value instead of key)? @@ -862,6 +1508,7 @@ L). You can look into using the DB_File module and tie() using the $DB_BTREE hash bindings as documented in L. +The Tie::IxHash module from CPAN might also be instructive. =head2 What's the difference between "delete" and "undef" with hashes? @@ -889,7 +1536,7 @@ And these conditions hold $ary{'d'} is false defined $ary{'d'} is true defined $ary{'a'} is true - exists $ary{'a'} is true (perl5 only) + exists $ary{'a'} is true (Perl5 only) grep ($_ eq 'a', keys %ary) is true If you now say @@ -913,7 +1560,7 @@ and these conditions now hold; changes in caps: $ary{'d'} is false defined $ary{'d'} is true defined $ary{'a'} is FALSE - exists $ary{'a'} is true (perl5 only) + exists $ary{'a'} is true (Perl5 only) grep ($_ eq 'a', keys %ary) is true Notice the last two: you have an undef value, but a defined key! @@ -937,7 +1584,7 @@ and these conditions now hold; changes in caps: $ary{'d'} is false defined $ary{'d'} is true defined $ary{'a'} is false - exists $ary{'a'} is FALSE (perl5 only) + exists $ary{'a'} is FALSE (Perl5 only) grep ($_ eq 'a', keys %ary) is FALSE See, the whole entry is gone! @@ -953,15 +1600,15 @@ they end up doing is not what they do with ordinary hashes. =head2 How do I reset an each() operation part-way through? -Using C in a scalar context returns the number of keys in +Using C in scalar context returns the number of keys in the hash I resets the iterator associated with the hash. You may need to do this if you use C to exit a loop early so that when you re-enter it, the hash iterator has been reset. =head2 How can I get the unique keys from two hashes? -First you extract the keys from the hashes into arrays, and then solve -the uniquifying the array problem described above. For example: +First you extract the keys from the hashes into lists, then solve +the "removing duplicates" problem described above. For example: %seen = (); for $element (keys(%foo), keys(%bar)) { @@ -1013,7 +1660,7 @@ whether you store something there or not. That's because functions get scalars passed in by reference. If somefunc() modifies C<$_[0]>, it has to be ready to write it back into the caller's version. -This has been fixed as of perl5.004. +This has been fixed as of Perl5.004. Normally, merely accessing a key's value for a nonexistent key does I cause that key to be forever there. This is different than @@ -1021,14 +1668,26 @@ awk's behavior. =head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? -Use references (documented in L). Examples of complex data -structures are given in L and L. Examples of -structures and object-oriented classes are in L. +Usually a hash ref, perhaps like this: + + $record = { + NAME => "Jason", + EMPNO => 132, + TITLE => "deputy peon", + AGE => 23, + SALARY => 37_000, + PALS => [ "Norbert", "Rhys", "Phineas"], + }; + +References are documented in L and the upcoming L. +Examples of complex data structures are given in L and +L. Examples of structures and object-oriented classes are +in L. =head2 How can I use a reference as a hash key? You can't do this directly, but you could use the standard Tie::Refhash -module distributed with perl. +module distributed with Perl. =head1 Data: Misc @@ -1041,8 +1700,11 @@ this works fine (assuming the files are found): print "Your kernel is GNU-zip enabled!\n"; } -On some systems, however, you have to play tedious games with "text" -versus "binary" files. See L. +On less elegant (read: Byzantine) systems, however, you have +to play tedious games with "text" versus "binary" files. See +L or L. Most of these ancient-thinking +systems are curses out of Microsoft, who seem to be committed to putting +the backward into backward compatibility. If you're concerned about 8-bit ASCII data, then see L. @@ -1054,14 +1716,37 @@ some gotchas. See the section on Regular Expressions. Assuming that you don't care about IEEE notations like "NaN" or "Infinity", you probably just want to use a regular expression. - warn "has nondigits" if /\D/; - warn "not a whole number" unless /^\d+$/; - warn "not an integer" unless /^-?\d+$/; # reject +3 - warn "not an integer" unless /^[+-]?\d+$/; - warn "not a decimal number" unless /^-?\d+\.?\d*$/; # rejects .2 - warn "not a decimal number" unless /^-?(?:\d+(?:\.\d*)?|\.\d+)$/; - warn "not a C float" - unless /^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/; + if (/\D/) { print "has nondigits\n" } + if (/^\d+$/) { 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" } + if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) + { print "a C float" } + +If you're on a POSIX system, Perl's supports the C +function. Its semantics are somewhat cumbersome, so here's a C +wrapper function for more convenient access. This function takes +a string and returns the number it found, or C for input that +isn't a C float. The C function is a front end to C +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 http://www.perl.com/CPAN/modules/by-module/String/String-Scanf-1.1.tar.gz @@ -1072,19 +1757,32 @@ 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. More generically, you should consult the -FreezeThaw, Storable, or Class::Eroot modules from CPAN. +See L. More generically, you should consult the FreezeThaw, +Storable, or Class::Eroot modules from CPAN. Here's one example using +Storable's C and C 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 Data::Dumper module on CPAN is nice for printing out -data structures, and FreezeThaw for copying them. For example: +The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great +for printing out data structures. The Storable module, found on CPAN, +provides a function called C that recursively copies its argument. - use FreezeThaw qw(freeze thaw); - $new = thaw freeze $old; + use Storable qw(dclone); + $r2 = dclone($r1); -Where $old can be (a reference to) any kind of data structure you'd like. -It will be deeply copied. +Where $r1 can be a reference to any kind of data structure you'd like. +It will be deeply copied. Because C 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? @@ -1094,8 +1792,26 @@ Use the UNIVERSAL class (see L). Get the Business::CreditCard module from CPAN. -=head1 AUTHOR AND COPYRIGHT +=head2 How do I pack arrays of doubles or floats for XS code? -Copyright (c) 1997 Tom Christiansen and Nathan Torkington. -All rights reserved. See L for distribution information. +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. + +=head1 AUTHOR AND COPYRIGHT +Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. +All rights reserved. + +When included as part of the Standard Version of Perl, or as part of +its complete documentation whether printed or otherwise, this work +may be distributed only under the terms of Perl's Artistic License. +Any distribution of this file or derivatives thereof I +of that package require that special arrangements be made with +copyright holder. + +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.