=head1 NAME perlfaq4 - Data Manipulation (\$Revision: 1.25 \$, \$Date: 2002/05/30 07:04:25 \$) =head1 DESCRIPTION This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues. =head1 Data: Numbers =head2 Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? 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 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 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. If precision is important, such as when dealing with money, it's good to work with integers and then divide at the last possible moment. For example, work in pennies (1995) instead of dollars and cents (19.95) and divide by 100 at the end. 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? Perl only understands octal and hex numbers as such when they occur as literals in your program. Octal literals in perl must start with a leading "0" and hexadecimal literals must start with a leading "0x". If they are read in from somewhere and assigned, no automatic conversion takes place. You must explicitly use oct() or hex() if you want the values converted to decimal. 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". The inverse mapping from decimal to octal can be done with either the "%o" or "%O" sprintf() formats. To get from decimal to hex try either the "%x" or the "%X" formats to sprintf(). This problem shows up most often when people try using chmod(), mkdir(), umask(), or sysopen(), which by widespread tradition typically take permissions in octal. chmod(644, \$file); # WRONG chmod(0644, \$file); # right Note the mistake in the first line was specifying the decimal literal 644, rather than the intended octal literal 0644. The problem can be seen with: printf("%#o",644); # prints 01204 Surely you had not intended C - did you? If you want to use numeric literals as arguments to chmod() et al. then please try to express them as octal constants, that is with a leading zero and with the following digits restricted to the set 0..7. =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 between numeric representations? As always with Perl there is more than one way to do it. Below are a few examples of approaches to making common conversions between number representations. This is intended to be representational rather than exhaustive. Some of the examples below use the Bit::Vector module from CPAN. The reason you might choose Bit::Vector over the perl built in functions is that it works with numbers of ANY size, that it is optimized for speed on some operations, and for at least some programmers the notation might be familiar. =over 4 =item How do I convert hexadecimal into decimal Using perl's built in conversion of 0x notation: \$int = 0xDEADBEEF; \$dec = sprintf("%d", \$int); Using the hex function: \$int = hex("DEADBEEF"); \$dec = sprintf("%d", \$int); Using pack: \$int = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); \$dec = sprintf("%d", \$int); Using the CPAN module Bit::Vector: use Bit::Vector; \$vec = Bit::Vector->new_Hex(32, "DEADBEEF"); \$dec = \$vec->to_Dec(); =item How do I convert from decimal to hexadecimal Using sprint: \$hex = sprintf("%X", 3735928559); Using unpack \$hex = unpack("H*", pack("N", 3735928559)); Using Bit::Vector use Bit::Vector; \$vec = Bit::Vector->new_Dec(32, -559038737); \$hex = \$vec->to_Hex(); And Bit::Vector supports odd bit counts: use Bit::Vector; \$vec = Bit::Vector->new_Dec(33, 3735928559); \$vec->Resize(32); # suppress leading 0 if unwanted \$hex = \$vec->to_Hex(); =item How do I convert from octal to decimal Using Perl's built in conversion of numbers with leading zeros: \$int = 033653337357; # note the leading 0! \$dec = sprintf("%d", \$int); Using the oct function: \$int = oct("33653337357"); \$dec = sprintf("%d", \$int); Using Bit::Vector: use Bit::Vector; \$vec = Bit::Vector->new(32); \$vec->Chunk_List_Store(3, split(//, reverse "33653337357")); \$dec = \$vec->to_Dec(); =item How do I convert from decimal to octal Using sprintf: \$oct = sprintf("%o", 3735928559); Using Bit::Vector use Bit::Vector; \$vec = Bit::Vector->new_Dec(32, -559038737); \$oct = reverse join('', \$vec->Chunk_List_Read(3)); =item How do I convert from binary to decimal Perl 5.6 lets you write binary numbers directly with the 0b notation: \$number = 0b10110110; Using pack and ord \$decimal = ord(pack('B8', '10110110')); Using pack and unpack for larger strings \$int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); \$dec = sprintf("%d", \$int); # substr() is used to left pad a 32 character string with zeros. Using Bit::Vector: \$vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); \$dec = \$vec->to_Dec(); =item How do I convert from decimal to binary Using unpack; \$bin = unpack("B*", pack("N", 3735928559)); Using Bit::Vector: use Bit::Vector; \$vec = Bit::Vector->new_Dec(32, -559038737); \$bin = \$vec->to_Bin(); The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader. =back =head2 Why doesn't & work the way I want it to? The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string C<"3"> is the bit pattern C<00110011>). The operators work with the binary form of a number (the number C<3> is treated as the bit pattern C<00000011>). So, saying C<11 & 3> performs the "and" operation on numbers (yielding C<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 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 loop will iterate over the range, without creating the entire range. for my \$i (5 .. 500_005) { push(@results, some_func(\$i)); } will not create a list of 500,000 integers. =head2 How can I output Roman numerals? Get the http://www.cpan.org/modules/by-module/Roman module. =head2 Why aren't my random numbers random? If you're using a version of Perl before 5.004, you must call C 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 :-). see the F artitcle in the "Far More Than You Ever Wanted To Know" collection in http://www.cpan.org/olddoc/FMTEYEWTK.tgz , courtesy of Tom Phoenix, talks more about this. John von Neumann said, ``Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin.'' If you want numbers that are more random than C 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/ . =head2 How do I get a random number between X and Y? Use the following simple function. It selects a random integer between (and possibly including!) the two given integers, e.g., C sub random_int_in (\$\$) { my(\$min, \$max) = @_; # Assumes that the two arguments are integers themselves! return \$min if \$min == \$max; (\$min, \$max) = (\$max, \$min) if \$min > \$max; return \$min + int rand(1 + \$max - \$min); } =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): \$day_of_year = (localtime(time()))[7]; =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 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 Day you really want. Are you interested in a way of getting serial days so that you just can tell how many days they are apart or so that you can do also other date arithmetic? If you are interested in performing date arithmetic, this can be done using modules Date::Manip or Date::Calc. There is too many details and much confusion on this issue to cover in this FAQ, but the term is applied (correctly) to a calendar now 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 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 a list 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, 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 (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. 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 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 are the CPAN modules Parse::RecDescent, Parse::Yapp, and Text::Balanced; and the byacc program. Starting from perl 5.8 the Text::Balanced is part of the standard distribution. One simple destructive, inside-out approach that you might try is to pull out the smallest nesting parts one at a time: while (s/BEGIN((?:(?!BEGIN)(?!END).)*)END//gs) { # do something with \$1 } A more complicated and sneaky approach is to make Perl's regular expression engine do it for you. This is courtesy Dean Inada, and rather has the nature of an Obfuscated Perl Contest entry, but it really does work: # \$_ contains the string to parse # BEGIN and END are the opening and closing markers for the # nested text. @( = ('(',''); @) = (')',''); (\$re=\$_)=~s/((BEGIN)|(END)|.)/\$)[!\$3]\Q\$1\E\$([!\$2]/gs; @\$ = (eval{/\$re/},\$@!~/unmatched/i); print join("\n",@\$[0..\$#\$]) if( \$\$[-1] ); =head2 How do I reverse a string? 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 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). Or use the CPAN module Text::Autoformat. Formatting files can be easily done by making a shell alias, like so: alias fmt="perl -i -MText::Autoformat -n0777 \ -e 'print autoformat \$_, {all=>1}' \$*" See the documentation for Text::Autoformat to appreciate its many capabilities. =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 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? 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"; \$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 won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers: \$string = "-9 55 48 -2 23 -76 4 14 -44"; while (\$string =~ /-\d+/g) { \$count++ } print "There are \$count negative numbers in the string"; Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches. \$count = () = \$string =~ /-\d+/g; =head2 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" into "C". 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 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) 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 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 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. 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 <op_ppaddr)() ); @@@ TAINT_NOT; @@@ return 0; @@@ } MAIN_INTERPRETER_LOOP Or with a fixed amount of leading whitespace, 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<@_>, 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 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 certain element is contained in a list or array? 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 maintain a hash whose keys are the first array's values. @blues = qw/azure cerulean teal turquoise lapis-lazuli/; %is_blue = (); for (@blues) { \$is_blue{\$_} = 1 } Now you can check whether \$is_blue{\$some_color}. It might have been a good idea to keep the blues all in a hash in the first place. If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space: @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); @is_tiny_prime = (); for (@primes) { \$is_tiny_prime[\$_] = 1 } # or simply @istiny_prime[@primes] = (1) x @primes; Now you check whether \$is_tiny_prime[\$some_number]. If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead: @articles = ( 1..10, 150..2000, 2017 ); undef \$read; for (@articles) { vec(\$read,\$_,1) = 1 } Now check whether C 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, 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 or L 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? If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or later installed, you can say: use List::Util 'shuffle'; @shuffled = shuffle(@list); If not, you can use a Fisher-Yates shuffle. sub fisher_yates_shuffle { my \$deck = shift; # \$deck is a reference to an array my \$i = @\$deck; while (\$i--) { my \$j = int rand (\$i+1); @\$deck[\$i,\$j] = @\$deck[\$j,\$i]; } } # shuffle my mpeg collection # my @mpeg =