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diff --git a/contrib/perl5/pod/perlfaq4.pod b/contrib/perl5/pod/perlfaq4.pod deleted file mode 100644 index 8c570c2..0000000 --- a/contrib/perl5/pod/perlfaq4.pod +++ /dev/null @@ -1,1821 +0,0 @@ -=head1 NAME - -perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $) - -=head1 DESCRIPTION - -The section of the FAQ answers questions related to the manipulation -of data as numbers, dates, strings, arrays, hashes, and miscellaneous -data issues. - -=head1 Data: Numbers - -=head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? - -The infinite set that a mathematician thinks of as the real numbers can -only be approximated on a computer, since the computer only has a finite -number of bits to store an infinite number of, um, numbers. - -Internally, your computer represents floating-point numbers in binary. -Floating-point numbers read in from a file or appearing as literals -in your program are converted from their decimal floating-point -representation (eg, 19.95) to an internal binary representation. - -However, 19.95 can't be precisely represented as a binary -floating-point number, just like 1/3 can't be exactly represented as a -decimal floating-point number. The computer's binary representation -of 19.95, therefore, isn't exactly 19.95. - -When a floating-point number gets printed, the binary floating-point -representation is converted back to decimal. These decimal numbers -are displayed in either the format you specify with printf(), or the -current output format for numbers. (See L<perlvar/"$#"> if you use -print. C<$#> has a different default value in Perl5 than it did in -Perl4. Changing C<$#> yourself is deprecated.) - -This affects B<all> computer languages that represent decimal -floating-point numbers in binary, not just Perl. Perl provides -arbitrary-precision decimal numbers with the Math::BigFloat module -(part of the standard Perl distribution), but mathematical operations -are consequently slower. - -To get rid of the superfluous digits, just use a format (eg, -C<printf("%.2f", 19.95)>) to get the required precision. -See L<perlop/"Floating-point Arithmetic">. - -=head2 Why isn't my octal data interpreted correctly? - -Perl only understands octal and hex numbers as such when they occur -as literals in your program. If they are read in from somewhere and -assigned, no automatic conversion takes place. You must explicitly -use oct() or hex() if you want the values converted. oct() interprets -both hex ("0x350") numbers and octal ones ("0350" or even without the -leading "0", like "377"), while hex() only converts hexadecimal ones, -with or without a leading "0x", like "0x255", "3A", "ff", or "deadbeef". - -This problem shows up most often when people try using chmod(), mkdir(), -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? - -Remember that int() merely truncates toward 0. For rounding to a -certain number of digits, sprintf() or printf() is usually the easiest -route. - - printf("%.3f", 3.1415926535); # prints 3.142 - -The POSIX module (part of the standard Perl distribution) implements -ceil(), floor(), and a number of other mathematical and trigonometric -functions. - - use POSIX; - $ceil = ceil(3.5); # 4 - $floor = floor(3.5); # 3 - -In 5.000 to 5.003 perls, trigonometry was done in the Math::Complex -module. With 5.004, the Math::Trig module (part of the standard Perl -distribution) implements the trigonometric functions. Internally it -uses the Math::Complex module and some functions can break out from -the real axis into the complex plane, for example the inverse sine of -2. - -Rounding in financial applications can have serious implications, and -the rounding method used should be specified precisely. In these -cases, it probably pays not to trust whichever system rounding is -being used by Perl, but to instead implement the rounding function you -need yourself. - -To see why, notice how you'll still have an issue on half-way-point -alternation: - - for ($i = 0; $i < 1.01; $i += 0.05) { printf "%.1f ",$i} - - 0.0 0.1 0.1 0.2 0.2 0.2 0.3 0.3 0.4 0.4 0.5 0.5 0.6 0.7 0.7 - 0.8 0.8 0.9 0.9 1.0 1.0 - -Don't blame Perl. It's the same as in C. IEEE says we have to do this. -Perl numbers whose absolute values are integers under 2**31 (on 32 bit -machines) will work pretty much like mathematical integers. Other numbers -are not guaranteed. - -=head2 How do I convert bits into ints? - -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<perlfunc/"pack"> and L<perlfunc/"unpack">): - - $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. - -This does the same thing: - - $decimal = ord(pack('B8', '10110110')); - -Here's an example of going the other way: - - $binary_string = unpack('B*', "\x29"); - -=head2 Why doesn't & work the way I want it to? - -The behavior of binary arithmetic operators depends on whether they're -used on numbers or strings. The operators treat a string as a series -of bits and work with that (the string C<"3"> is the bit pattern -C<00110011>). The operators work with the binary form of a number -(the number C<3> is treated as the bit pattern C<00000011>). - -So, saying C<11 & 3> performs the "and" operation on numbers (yielding -C<1>). Saying C<"11" & "3"> performs the "and" operation on strings -(yielding C<"1">). - -Most problems with C<&> and C<|> arise because the programmer thinks -they have a number but really it's a string. The rest arise because -the programmer says: - - if ("\020\020" & "\101\101") { - # ... - } - -but a string consisting of two null bytes (the result of C<"\020\020" -& "\101\101">) is not a false value in Perl. You need: - - if ( ("\020\020" & "\101\101") !~ /[^\000]/) { - # ... - } - -=head2 How do I multiply matrices? - -Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) -or the PDL extension (also available from CPAN). - -=head2 How do I perform an operation on a series of integers? - -To call a function on each element in an array, and collect the -results, use: - - @results = map { my_func($_) } @array; - -For example: - - @triple = map { 3 * $_ } @single; - -To call a function on each element of an array, but ignore the -results: - - foreach $iterator (@array) { - some_func($iterator); - } - -To call a function on each integer in a (small) range, you B<can> use: - - @results = map { some_func($_) } (5 .. 25); - -but you should be aware that the C<..> operator creates an array of -all integers in the range. This can take a lot of memory for large -ranges. Instead use: - - @results = (); - for ($i=5; $i < 500_005; $i++) { - push(@results, some_func($i)); - } - -This situation has been fixed in Perl5.005. Use of C<..> in a C<for> -loop will iterate over the range, without creating the entire range. - - for my $i (5 .. 500_005) { - push(@results, some_func($i)); - } - -will not create a list of 500,000 integers. - -=head2 How can I output Roman numerals? - -Get the http://www.perl.com/CPAN/modules/by-module/Roman module. - -=head2 Why aren't my random numbers random? - -If you're using a version of Perl before 5.004, you must call C<srand> -once at the start of your program to seed the random number generator. -5.004 and later automatically call C<srand> at the beginning. Don't -call C<srand> more than once--you make your numbers less random, rather -than more. - -Computers are good at being predictable and bad at being random -(despite appearances caused by bugs in your programs :-). -http://www.perl.com/CPAN/doc/FMTEYEWTK/random , courtesy of Tom -Phoenix, talks more about this. John von Neumann said, ``Anyone who -attempts to generate random numbers by deterministic means is, of -course, living in a state of sin.'' - -If you want numbers that are more random than C<rand> with C<srand> -provides, you should also check out the 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 - -=head2 How do I find the week-of-the-year/day-of-the-year? - -The day of the year is in the array returned by localtime() (see -L<perlfunc/"localtime">): - - $day_of_year = (localtime(time()))[7]; - -or more legibly (in 5.004 or higher): - - use Time::localtime; - $day_of_year = localtime(time())->yday; - -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. 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 C<timelocal> 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? - -Use the Time::JulianDay module (part of the Time-modules bundle -available from CPAN.) - -Before you immerse yourself too deeply in this, be sure to verify that it -is the I<Julian> Day you really want. Are you 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.) - -=head2 How do I find yesterday's date? - -The C<time()> 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<localtime()> 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 <rra@stanford.edu> - # 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 a list context is the year minus 1900. -For years between 1910 and 1999 this I<happens> to be a 2-digit decimal -number. To avoid the year 2000 problem simply do not treat the year as -a 2-digit number. It isn't. - -When gmtime() and localtime() are used in scalar context they return -a timestamp string that contains a fully-expanded year. For example, -C<$timestamp = gmtime(1005613200)> sets $timestamp to "Tue Nov 13 01:00:00 -2001". There's no year 2000 problem here. - -That doesn't mean that Perl can't be used to create non-Y2K compliant -programs. It can. But so can your pencil. It's the fault of the user, -not the language. At the risk of inflaming the NRA: ``Perl doesn't -break Y2K, people do.'' See http://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, 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<perlfaq9>. Shell escapes with the backslash (C<\>) -character are removed with - - s/\\(.)/$1/g; - -This won't expand C<"\n"> or C<"\t"> or any other special escapes. - -=head2 How do I remove consecutive pairs of characters? - -To turn C<"abbcccd"> into C<"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 :-) - -=head2 How do I expand function calls in a string? - -This is documented in L<perlref>. In general, this is fraught with -quoting 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"; - -If you prefer scalar context, similar chicanery is also useful for -arbitrary expressions: - - print "That yields ${\($n + 5)} widgets\n"; - -Version 5.004 of Perl had a bug that gave list context to the -expression in C<${...}>, 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 done in one regular expression, no -matter how complicated. To find something between two single -characters, a pattern like C</x([^x]*)x/> will get the intervening -bits in $1. For multiple ones, then something more like -C</alpha(.*?)omega/> would be needed. But none of these deals with -nested patterns, 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 are -the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; -and the byacc program. - -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 scalar context, as documented in -L<perlfunc/reverse>. - - $reversed = reverse $string; - -=head2 How do I expand tabs in a string? - -You can do it yourself: - - 1 while $string =~ s/\t+/' ' x (length($&) * 8 - length($`) % 8)/e; - -Or you can just use the Text::Tabs module (part of the standard Perl -distribution). - - use Text::Tabs; - @expanded_lines = expand(@lines_with_tabs); - -=head2 How do I reformat a paragraph? - -Use Text::Wrap (part of the standard Perl distribution): - - use Text::Wrap; - print wrap("\t", ' ', @paragraphs); - -The paragraphs you give to Text::Wrap should not contain embedded -newlines. Text::Wrap doesn't justify the lines (flush-right). - -=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(): - - $first_byte = substr($a, 0, 1); - -If you want to modify part of a string, the simplest way is often to -use substr() as an lvalue: - - substr($a, 0, 3) = "Tom"; - -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 of N yourself. For example, let's say you want -to change the fifth occurrence of C<"whoever"> or C<"whomever"> into -C<"whosoever"> or C<"whomsoever">, case insensitively. These -all assume that $_ contains the string to be altered. - - $count = 0; - s{((whom?)ever)}{ - ++$count == 5 # is it the 5th? - ? "${2}soever" # yes, swap - : $1 # renege and leave it there - }ige; - -In the more general case, you can use the C</g> modifier in a C<while> -loop, keeping count of matches. - - $WANT = 3; - $count = 0; - $_ = "One fish two fish red fish blue fish"; - while (/(\w+)\s+fish\b/gi) { - if (++$count == $WANT) { - print "The third fish is a $1 one.\n"; - } - } - -That prints out: C<"The third fish is a red one."> You can also use a -repetition count and repeated pattern like this: - - /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; - -=head2 How can I count the number of occurrences of a substring within a string? - -There are a number of ways, with varying efficiency. If you want a -count of a certain single character (X) within a string, you can use the -C<tr///> function like so: - - $string = "ThisXlineXhasXsomeXx'sXinXit"; - $count = ($string =~ tr/X//); - print "There are $count X characters in the string"; - -This is fine if you are just looking for a single character. However, -if you are trying to count multiple character substrings within a -larger string, C<tr///> won't work. What you can do is wrap a while() -loop around a global pattern match. For example, let's count negative -integers: - - $string = "-9 55 48 -2 23 -76 4 14 -44"; - while ($string =~ /-\d+/g) { $count++ } - print "There are $count negative numbers in the string"; - -=head2 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 "C<don't do it>" into "C<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 -characters by placing a C<use locale> pragma in your program. -See L<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 I<Dr. Strangelove or: How I Learned to -Stop Worrying and Love the Bomb>, for example. - -=head2 How can I split a [character] delimited string except when inside -[character]? (Comma-separated files) - -Take the example case of trying to split a string that is comma-separated -into its different fields. (We'll pretend you said comma-separated, not -comma-delimited, which is different and almost never what you mean.) You -can't use C<split(/,/)> because you shouldn't split if the comma is inside -quotes. For example, take a data line like this: - - SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" - -Due to the restriction of the quotes, this is a fairly complex -problem. Thankfully, we have Jeffrey Friedl, author of a highly -recommended book on regular expressions, to handle these for us. He -suggests (assuming your string is contained in $text): - - @new = (); - push(@new, $+) while $text =~ m{ - "([^\"\\]*(?:\\.[^\"\\]*)*)",? # groups the phrase inside the quotes - | ([^,]+),? - | , - }gx; - push(@new, undef) if substr($text,-1,1) eq ','; - -If you want to represent quotation marks inside a -quotation-mark-delimited field, escape them with backslashes (eg, -C<"like \"this\"">. Unescaping them is a task addressed earlier in -this section. - -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. - -=head2 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 C<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 ($scalar, @array, @hash{keys %hash}) { - s/^\s+//; - s/\s+$//; - } - -=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<sprintf> function. It can pad on the left -or right with blanks and on the left with zeroes and it will not -truncate the result. The C<pack> function can only pad strings on the -right with blanks and it will truncate the result to a maximum length of -C<$pad_len>. - - # Left padding a string with blanks (no truncation): - $padded = sprintf("%${pad_len}s", $text); - - # 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",$text); - -If you need to pad with a character other than blank or zero you can use -one of the following methods. They all generate a pad string with the -C<x> operator and combine that with C<$text>. These methods do -not truncate C<$text>. - -Left and right padding with any character, creating a new string: - - $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; - $padded = $text . $pad_char x ( $pad_len - length( $text ) ); - -Left and right padding with any character, modifying C<$text> directly: - - substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); - $text .= $pad_char x ( $pad_len - length( $text ) ); - -=head2 How do I extract selected columns from a string? - -Use substr() or unpack(), both documented in L<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; - } - -=head2 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. - -=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'; - -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{$1}/g; - -See also ``How do I expand function calls in a string?'' in this section -of the FAQ. - -=head2 What's wrong with always quoting "$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 strings. Think of it this way: double-quote -expansion is used to produce new strings. If you already -have a string, why do you need more? - -If you get used to writing odd things like these: - - print "$var"; # BAD - $new = "$old"; # BAD - somefunc("$var"); # BAD - -You'll be in trouble. Those should (in 99.8% of the cases) be -the simpler and more direct: - - print $var; - $new = $old; - somefunc($var); - -Otherwise, besides slowing you down, you're going to break code when -the thing in the scalar is actually neither a string nor a number, but -a reference: - - func(\@array); - sub func { - my $aref = shift; - my $oref = "$aref"; # WRONG - } - -You can also get into subtle problems on those few operations in Perl -that actually do care about the difference between a string and a -number, such as the magical C<++> autoincrement operator or the -syscall() function. - -Stringification also destroys arrays. - - @lines = `command`; - print "@lines"; # WRONG - extra blanks - print @lines; # right - -=head2 Why don't my <<HERE documents work? - -Check for these three things: - -=over 4 - -=item 1. There must be no space after the << part. - -=item 2. There (probably) should be a semicolon at the end. - -=item 3. You can't (easily) have any space in front of the tag. - -=back - -If you want to indent the text in the here document, you -can do this: - - # all in one - ($VAR = <<HERE_TARGET) =~ s/^\s+//gm; - your text - goes here - HERE_TARGET - -But the HERE_TARGET must still be flush against the margin. -If you want that indented also, you'll have to quote -in the indentation. - - ($quote = <<' FINIS') =~ s/^\s+//gm; - ...we will have peace, when you and all your works have - perished--and the works of your dark master to whom you - would deliver us. You are a liar, Saruman, and a corrupter - of men's hearts. --Theoden in /usr/src/perl/taint.c - FINIS - $quote =~ s/\s*--/\n--/; - -A nice general-purpose fixer-upper function for indented here documents -follows. It expects to be called with a here document as its argument. -It looks to see whether each line begins with a common substring, and -if so, strips that substring off. Otherwise, it takes the amount of leading -whitespace found on the first line and removes that much off each -subsequent line. - - sub fix { - local $_ = shift; - my ($white, $leader); # common whitespace and common leading string - if (/^\s*(?:([^\w\s]+)(\s*).*\n)(?:\s*\1\2?.*\n)+$/) { - ($white, $leader) = ($2, quotemeta($1)); - } else { - ($white, $leader) = (/^(\s+)/, ''); - } - s/^\s*?$leader(?:$white)?//gm; - return $_; - } - -This works with leading special strings, dynamically determined: - - $remember_the_main = fix<<' MAIN_INTERPRETER_LOOP'; - @@@ int - @@@ runops() { - @@@ SAVEI32(runlevel); - @@@ runlevel++; - @@@ while ( op = (*op->op_ppaddr)() ); - @@@ TAINT_NOT; - @@@ return 0; - @@@ } - MAIN_INTERPRETER_LOOP - -Or with a fixed amount of leading whitespace, with remaining -indentation correctly preserved: - - $poem = fix<<EVER_ON_AND_ON; - Now far ahead the Road has gone, - And I must follow, if I can, - Pursuing it with eager feet, - Until it joins some larger way - Where many paths and errands meet. - And whither then? I cannot say. - --Bilbo in /usr/src/perl/pp_ctl.c - EVER_ON_AND_ON - -=head1 Data: Arrays - -=head2 What is the difference between a list and an array? - -An array has a changeable length. A list does not. An array is something -you can push or pop, while a list is a set of values. Some people make -the distinction that a list is a value while an array is a variable. -Subroutines are passed and return lists, you put things into list -context, you initialize arrays with lists, and you foreach() across -a list. C<@> variables are arrays, anonymous arrays are arrays, arrays -in scalar context behave like the number of elements in them, subroutines -access their arguments through the array C<@_>, and 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, making -it a list with one (scalar) value. You should use $ when you want a -scalar value (most of the time) and @ when you want a list with one -scalar value in it (very, very rarely; nearly never, in fact). - -Sometimes it doesn't make a difference, but sometimes it does. -For example, compare: - - $good[0] = `some program that outputs several lines`; - -with - - @bad[0] = `same program that outputs several lines`; - -The C<use warnings> pragma and the B<-w> flag will warn you about these -matters. - -=head2 How can I remove duplicate elements from a list or array? - -There are several possible ways, depending on whether the array is -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 = "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 $_ is 0, "", or undef. - -=item b) - -If you don't know whether @in is sorted: - - undef %saw; - @out = grep(!$saw{$_}++, @in); - -=item c) - -Like (b), but @in contains only small integers: - - @out = grep(!$saw[$_]++, @in); - -=item d) - -A way to do (b) without any loops or greps: - - undef %saw; - @saw{@in} = (); - @out = sort keys %saw; # remove sort if undesired - -=item e) - -Like (d), but @in contains only small positive integers: - - undef @ary; - @ary[@in] = @in; - @out = grep {defined} @ary; - -=back - -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? - -Hearing the word "in" is an I<in>dication that you probably should have -used a hash, not a list or array, to store your data. Hashes are -designed to answer this question quickly and efficiently. Arrays aren't. - -That being said, there are several ways to approach this. 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/; - undef %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); - undef @is_tiny_prime; - for (@primes) { $is_tiny_prime[$_] = 1 } - # or simply @istiny_prime[@primes] = (1) x @primes; - -Now you check whether $is_tiny_prime[$some_number]. - -If the values in question are integers instead of strings, you can save -quite a lot of space by using bit strings instead: - - @articles = ( 1..10, 150..2000, 2017 ); - undef $read; - for (@articles) { vec($read,$_,1) = 1 } - -Now check whether C<vec($read,$n,1)> is true for some C<$n>. - -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), -inefficient (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) { ... } - -=head2 How do I compute the difference of two arrays? How do I compute the intersection of two arrays? - -Use a hash. Here's code to do both and more. It assumes that -each element is unique in a given array: - - @union = @intersection = @difference = (); - %count = (); - foreach $element (@array1, @array2) { $count{$element}++ } - foreach $element (keys %count) { - push @union, $element; - push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element; - } - -Note that this is the I<symmetric difference>, that is, all elements in -either A or in B but not in both. Think of it as an xor operation. - -=head2 How do I test whether two arrays or hashes are equal? - -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; - $b{EXTRA} = \%a; - - printf "a and b contain %s hashes\n", - cmpStr(\%a, \%b) == 0 ? "the same" : "different"; - - printf "a and b contain %s hashes\n", - cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; - - -The first reports that both those the hashes contain the same data, -while the second reports that they do not. Which you prefer is left as -an exercise to the reader. - -=head2 How do I find the first array element for which a condition is true? - -You can use this if you care about the index: - - for ($i= 0; $i < @array; $i++) { - if ($array[$i] eq "Waldo") { - $found_index = $i; - last; - } - } - -Now C<$found_index> has what you want. - -=head2 How do I handle linked lists? - -In general, you usually don't need a linked list in Perl, since with -regular arrays, you can push and pop or shift and unshift at either end, -or you can use splice to add and/or remove arbitrary number of elements at -arbitrary points. Both pop and shift are 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<perldsc> or L<perltoot> and do just what the algorithm book tells you -to do. For example, imagine a list node like this: - - $node = { - VALUE => 42, - LINK => undef, - }; - -You could walk the list this way: - - print "List: "; - for ($node = $head; $node; $node = $node->{LINK}) { - print $node->{VALUE}, " "; - } - print "\n"; - -You could add to the list this way: - - my ($head, $tail); - $tail = append($head, 1); # grow a new head - for $value ( 2 .. 10 ) { - $tail = append($tail, $value); - } - - sub append { - my($list, $value) = @_; - my $node = { VALUE => $value }; - if ($list) { - $node->{LINK} = $list->{LINK}; - $list->{LINK} = $node; - } else { - $_[0] = $node; # replace caller's version - } - return $node; - } - -But again, Perl's built-in are virtually always good enough. - -=head2 How do I handle circular lists? - -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 - -=head2 How do I shuffle an array randomly? - -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); - @$array[$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; - @new = (); - @old = 1 .. 10; # just a demo - while (@old) { - push(@new, splice(@old, rand @old, 1)); - } - -This is bad because splice is already O(N), and since you do it N times, -you just invented a quadratic algorithm; that is, O(N**2). This does -not scale, although Perl is so efficient that you probably won't notice -this until you have rather largish arrays. - -=head2 How do I process/modify each element of an array? - -Use C<for>/C<foreach>: - - for (@lines) { - s/foo/bar/; # change that word - 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 - } - -If you want to do the same thing to modify the values of the hash, -you may not use the C<values> 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<perlfunc/rand>): - - # 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<only call srand once per program, if then>. -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 permute() function should work on any list: - - #!/usr/bin/perl -n - # tsc-permute: permute each word of input - permute([split], []); - sub permute { - my @items = @{ $_[0] }; - my @perms = @{ $_[1] }; - unless (@items) { - print "@perms\n"; - } else { - my(@newitems,@newperms,$i); - foreach $i (0 .. $#items) { - @newitems = @items; - @newperms = @perms; - unshift(@newperms, splice(@newitems, $i, 1)); - permute([@newitems], [@newperms]); - } - } - } - -=head2 How do I sort an array by (anything)? - -Supply a comparison function to sort() (described in L<perlfunc/sort>): - - @list = sort { $a <=> $b } @list; - -The default sort function is cmp, string comparison, which would -sort C<(1, 2, 10)> into C<(1, 10, 2)>. C<< <=> >>, used above, is -the numerical comparison operator. - -If you have a complicated function needed to pull out the part you -want to sort on, then don't do it inside the sort function. Pull it -out first, because the sort BLOCK can be called many times for the -same element. Here's an example of how to pull out the first word -after the first number on each item, and then sort those words -case-insensitively. - - @idx = (); - for (@data) { - ($item) = /\d+\s*(\S+)/; - push @idx, uc($item); - } - @sorted = @data[ sort { $idx[$a] cmp $idx[$b] } 0 .. $#idx ]; - -which could also be written this way, using a trick -that's come to be known as the Schwartzian Transform: - - @sorted = map { $_->[0] } - sort { $a->[1] cmp $b->[1] } - map { [ $_, uc( (/\d+\s*(\S+)/)[0]) ] } @data; - -If you need to sort on several fields, the following paradigm is useful. - - @sorted = sort { field1($a) <=> field1($b) || - field2($a) cmp field2($b) || - field3($a) cmp field3($b) - } @data; - -This can be conveniently combined with precalculation of keys as given -above. - -See http://www.perl.com/CPAN/doc/FMTEYEWTK/sort.html for more about -this approach. - -See also the question below on sorting hashes. - -=head2 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 } - -And 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.) - -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), $bits); - print "bits are: @bytes\n\n"; - } - -=head2 Why does defined() return true on empty arrays and hashes? - -The short story is that you should probably only use defined on scalars or -functions, not on aggregates (arrays and hashes). See L<perlfunc/defined> -in the 5.004 release or later of Perl for more detail. - -=head1 Data: Hashes (Associative Arrays) - -=head2 How do I process an entire hash? - -Use the each() function (see L<perlfunc/each>) if you don't care -whether it's sorted: - - while ( ($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. - -=head2 What happens if I add or remove keys from a hash while iterating over it? - -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? - -Create a reverse hash: - - %by_value = reverse %by_key; - $key = $by_value{$value}; - -That's not particularly efficient. It would be more space-efficient -to use: - - while (($key, $value) = each %by_key) { - $by_value{$value} = $key; - } - -If your hash could have repeated values, the methods above will only find -one of the associated keys. This may or may not worry you. If it does -worry you, you can always reverse the hash into a hash of arrays instead: - - while (($key, $value) = each %by_key) { - push @{$key_list_by_value{$value}}, $key; - } - -=head2 How can I know how many entries are in a hash? - -If you mean how many keys, then all you have to do is -take the scalar sense of the keys() function: - - $num_keys = scalar keys %hash; - -The keys() function also resets the iterator, which in void context 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)? - -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 -L<perllocale>). - - @keys = sort { - $hash{$b} <=> $hash{$a} - || - length($b) <=> length($a) - || - $a cmp $b - } keys %hash; - -=head2 How can I always keep my hash sorted? - -You can look into using the DB_File module and tie() using the -$DB_BTREE hash bindings as documented in L<DB_File/"In Memory Databases">. -The Tie::IxHash module from CPAN might also be instructive. - -=head2 What's the difference between "delete" and "undef" with hashes? - -Hashes are pairs of scalars: the first is the key, the second is the -value. The key will be coerced to a string, although the value can be -any kind of scalar: string, number, or reference. If a key C<$key> is -present in the array, C<exists($key)> will return true. The value for -a given key can be C<undef>, in which case C<$array{$key}> will be -C<undef> while C<$exists{$key}> will return true. This corresponds to -(C<$key>, C<undef>) being in the hash. - -Pictures help... here's the C<%ary> table: - - keys values - +------+------+ - | a | 3 | - | x | 7 | - | d | 0 | - | e | 2 | - +------+------+ - -And these conditions hold - - $ary{'a'} is true - $ary{'d'} is false - defined $ary{'d'} is true - defined $ary{'a'} is true - exists $ary{'a'} is true (Perl5 only) - grep ($_ eq 'a', keys %ary) is true - -If you now say - - undef $ary{'a'} - -your table now reads: - - - keys values - +------+------+ - | a | undef| - | x | 7 | - | d | 0 | - | e | 2 | - +------+------+ - -and these conditions now hold; changes in caps: - - $ary{'a'} is FALSE - $ary{'d'} is false - defined $ary{'d'} is true - defined $ary{'a'} is FALSE - 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! - -Now, consider this: - - delete $ary{'a'} - -your table now reads: - - keys values - +------+------+ - | x | 7 | - | d | 0 | - | e | 2 | - +------+------+ - -and these conditions now hold; changes in caps: - - $ary{'a'} is false - $ary{'d'} is false - defined $ary{'d'} is true - defined $ary{'a'} is false - exists $ary{'a'} is FALSE (Perl5 only) - grep ($_ eq 'a', keys %ary) is FALSE - -See, the whole entry is gone! - -=head2 Why don't my tied hashes make the defined/exists distinction? - -They may or may not implement the EXISTS() and DEFINED() methods -differently. For example, there isn't the concept of undef with hashes -that are tied to DBM* files. This means the true/false tables above -will give different results when used on such a hash. It also means -that exists and defined do the same thing with a DBM* file, and what -they end up doing is not what they do with ordinary hashes. - -=head2 How do I reset an each() operation part-way through? - -Using C<keys %hash> in scalar context returns the number of keys in -the hash I<and> resets the iterator associated with the hash. You may -need to do this if you use C<last> 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 lists, then solve -the "removing duplicates" problem described above. For example: - - %seen = (); - for $element (keys(%foo), keys(%bar)) { - $seen{$element}++; - } - @uniq = keys %seen; - -Or more succinctly: - - @uniq = keys %{{%foo,%bar}}; - -Or if you really want to save space: - - %seen = (); - while (defined ($key = each %foo)) { - $seen{$key}++; - } - while (defined ($key = each %bar)) { - $seen{$key}++; - } - @uniq = keys %seen; - -=head2 How can I store a multidimensional array in a DBM file? - -Either stringify the structure yourself (no fun), or else -get the MLDBM (which uses Data::Dumper) module from CPAN and layer -it on top of either DB_File or GDBM_File. - -=head2 How can I make my hash remember the order I put elements into it? - -Use the Tie::IxHash from CPAN. - - use Tie::IxHash; - tie(%myhash, Tie::IxHash); - for ($i=0; $i<20; $i++) { - $myhash{$i} = 2*$i; - } - @keys = keys %myhash; - # @keys = (0,1,2,3,...) - -=head2 Why does passing a subroutine an undefined element in a hash create it? - -If you say something like: - - somefunc($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 C<$_[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 -I<not> cause that key to be forever there. This is different than -awk's behavior. - -=head2 How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? - -Usually a hash ref, perhaps like this: - - $record = { - NAME => "Jason", - EMPNO => 132, - TITLE => "deputy peon", - AGE => 23, - SALARY => 37_000, - PALS => [ "Norbert", "Rhys", "Phineas"], - }; - -References are documented in L<perlref> and the upcoming L<perlreftut>. -Examples of complex data structures are given in L<perldsc> and -L<perllol>. Examples of structures and object-oriented classes are -in L<perltoot>. - -=head2 How can I use a reference as a hash key? - -You can't do this directly, but you could use the standard Tie::Refhash -module distributed with Perl. - -=head1 Data: Misc - -=head2 How do I handle binary data correctly? - -Perl is binary clean, so this shouldn't be a problem. For example, -this works fine (assuming the files are found): - - if (`cat /vmunix` =~ /gzip/) { - print "Your kernel is GNU-zip enabled!\n"; - } - -On less elegant (read: Byzantine) systems, however, you have -to play tedious games with "text" versus "binary" files. See -L<perlfunc/"binmode"> or L<perlopentut>. Most of these ancient-thinking -systems are curses out of Microsoft, who seem to be committed to putting -the backward into backward compatibility. - -If you're concerned about 8-bit ASCII data, then see L<perllocale>. - -If you want to deal with multibyte characters, however, there are -some gotchas. See the section on Regular Expressions. - -=head2 How do I determine whether a scalar is a number/whole/integer/float? - -Assuming that you don't care about IEEE notations like "NaN" or -"Infinity", you probably just want to use a regular expression. - - if (/\D/) { print "has nondigits\n" } - if (/^\d+$/) { print "is a whole number\n" } - if (/^-?\d+$/) { print "is an integer\n" } - if (/^[+-]?\d+$/) { print "is a +/- integer\n" } - if (/^-?\d+\.?\d*$/) { print "is a real number\n" } - if (/^-?(?:\d+(?:\.\d*)?|\.\d+)$/) { print "is a decimal number" } - if (/^([+-]?)(?=\d|\.\d)\d*(\.\d*)?([Ee]([+-]?\d+))?$/) - { print "a C float" } - -If you're on a POSIX system, Perl's supports the C<POSIX::strtod> -function. Its semantics are somewhat cumbersome, so here's a C<getnum> -wrapper function for more convenient access. This function takes -a string and returns the number it found, or C<undef> for input that -isn't a C float. The C<is_numeric> function is a front end to C<getnum> -if you just want to say, ``Is this a float?'' - - sub getnum { - use POSIX qw(strtod); - my $str = shift; - $str =~ s/^\s+//; - $str =~ s/\s+$//; - $! = 0; - my($num, $unparsed) = strtod($str); - if (($str eq '') || ($unparsed != 0) || $!) { - return undef; - } else { - return $num; - } - } - - sub is_numeric { defined getnum($_[0]) } - -Or you could check out the String::Scanf module on CPAN instead. The -POSIX module (part of the standard Perl distribution) provides the -C<strtod> and C<strtol> for converting strings to double and longs, -respectively. - -=head2 How do I keep persistent data across program calls? - -For some specific applications, you can use one of the DBM modules. -See L<AnyDBM_File>. More generically, you should consult the FreezeThaw, -Storable, or Class::Eroot modules from CPAN. Here's one example using -Storable's C<store> and C<retrieve> functions: - - use Storable; - store(\%hash, "filename"); - - # later on... - $href = retrieve("filename"); # by ref - %hash = %{ retrieve("filename") }; # direct to hash - -=head2 How do I print out or copy a recursive data structure? - -The 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<dclone> that recursively copies its argument. - - use Storable qw(dclone); - $r2 = dclone($r1); - -Where $r1 can be a reference to any kind of data structure you'd like. -It will be deeply copied. Because C<dclone> takes and returns references, -you'd have to add extra punctuation if you had a hash of arrays that -you wanted to copy. - - %newhash = %{ dclone(\%oldhash) }; - -=head2 How do I define methods for every class/object? - -Use the UNIVERSAL class (see L<UNIVERSAL>). - -=head2 How do I verify a credit card checksum? - -Get the Business::CreditCard module from CPAN. - -=head2 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. - -=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<outside> -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. |
