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 man : perlfaq4(1) Command: man perldoc info search(apropos)   ```PERLFAQ4(1) Perl Programmers Reference Guide PERLFAQ4(1) NAME perlfaq4 - Data Manipulation DESCRIPTION This section of the FAQ answers questions related to manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues. Data: Numbers Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? For the long explanation, see David Goldberg's "What Every Computer Scientist Should Know About Floating-Point Arithmetic" (http://docs.sun.com/source/806-3568/ncg_goldberg.html). Internally, your computer represents floating-point numbers in binary. Digital (as in powers of two) computers cannot store all numbers exactly. Some real numbers lose precision in the process. This is a problem with how computers store numbers and affects all computer languages, not just Perl. perlnumber shows the gory details of number representations and conversions. To limit the number of decimal places in your numbers, you can use the "printf" or "sprintf" function. See the "Floating Point Arithmetic" for more details. printf "%.2f", 10/3; my \$number = sprintf "%.2f", 10/3; Why is int() broken? Your "int()" is most probably working just fine. It's the numbers that aren't quite what you think. First, see the answer to "Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?". For example, this print int(0.6/0.2-2), "\n"; will in most computers print 0, not 1, because even such simple numbers as 0.6 and 0.2 cannot be presented exactly by floating-point numbers. What you think in the above as 'three' is really more like 2.9999999999999995559. Why isn't my octal data interpreted correctly? (contributed by brian d foy) You're probably trying to convert a string to a number, which Perl only converts as a decimal number. When Perl converts a string to a number, it ignores leading spaces and zeroes, then assumes the rest of the digits are in base 10: my \$string = '0644'; print \$string + 0; # prints 644 print \$string + 44; # prints 688, certainly not octal! This problem usually involves one of the Perl built-ins that has the same name a Unix command that uses octal numbers as arguments on the command line. In this example, "chmod" on the command line knows that its first argument is octal because that's what it does: %prompt> chmod 644 file If you want to use the same literal digits (644) in Perl, you have to tell Perl to treat them as octal numbers either by prefixing the digits with a 0 or using "oct": chmod( 0644, \$file); # right, has leading zero chmod( oct(644), \$file ); # also correct The problem comes in when you take your numbers from something that Perl thinks is a string, such as a command line argument in @ARGV: chmod( \$ARGV[0], \$file); # wrong, even if "0644" chmod( oct(\$ARGV[0]), \$file ); # correct, treat string as octal You can always check the value you're using by printing it in octal notation to ensure it matches what you think it should be. Print it in octal and decimal format: printf "0%o %d", \$number, \$number; 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. How do I convert between numeric representations/bases/radixes? As always with Perl there is more than one way to do it. Below are a few examples of approaches to making common conversions between number representations. This is intended to be representational rather than exhaustive. Some of the examples later in perlfaq4 use the "Bit::Vector" module from CPAN. The reason you might choose "Bit::Vector" over the perl built in functions is that it works with numbers of ANY size, that it is optimized for speed on some operations, and for at least some programmers the notation might be familiar. How do I convert hexadecimal into decimal Using perl's built in conversion of "0x" notation: \$dec = 0xDEADBEEF; Using the "hex" function: \$dec = hex("DEADBEEF"); Using "pack": \$dec = unpack("N", pack("H8", substr("0" x 8 . "DEADBEEF", -8))); Using the CPAN module "Bit::Vector": use Bit::Vector; \$vec = Bit::Vector->new_Hex(32, "DEADBEEF"); \$dec = \$vec->to_Dec(); How do I convert from decimal to hexadecimal Using "sprintf": \$hex = sprintf("%X", 3735928559); # upper case A-F \$hex = sprintf("%x", 3735928559); # lower case a-f Using "unpack": \$hex = unpack("H*", pack("N", 3735928559)); Using "Bit::Vector": use Bit::Vector; \$vec = Bit::Vector->new_Dec(32, -559038737); \$hex = \$vec->to_Hex(); And "Bit::Vector" supports odd bit counts: use Bit::Vector; \$vec = Bit::Vector->new_Dec(33, 3735928559); \$vec->Resize(32); # suppress leading 0 if unwanted \$hex = \$vec->to_Hex(); How do I convert from octal to decimal Using Perl's built in conversion of numbers with leading zeros: \$dec = 033653337357; # note the leading 0! Using the "oct" function: \$dec = oct("33653337357"); Using "Bit::Vector": use Bit::Vector; \$vec = Bit::Vector->new(32); \$vec->Chunk_List_Store(3, split(//, reverse "33653337357")); \$dec = \$vec->to_Dec(); How do I convert from decimal to octal Using "sprintf": \$oct = sprintf("%o", 3735928559); Using "Bit::Vector": use Bit::Vector; \$vec = Bit::Vector->new_Dec(32, -559038737); \$oct = reverse join('', \$vec->Chunk_List_Read(3)); How do I convert from binary to decimal Perl 5.6 lets you write binary numbers directly with the "0b" notation: \$number = 0b10110110; Using "oct": my \$input = "10110110"; \$decimal = oct( "0b\$input" ); Using "pack" and "ord": \$decimal = ord(pack('B8', '10110110')); Using "pack" and "unpack" for larger strings: \$int = unpack("N", pack("B32", substr("0" x 32 . "11110101011011011111011101111", -32))); \$dec = sprintf("%d", \$int); # substr() is used to left pad a 32 character string with zeros. Using "Bit::Vector": \$vec = Bit::Vector->new_Bin(32, "11011110101011011011111011101111"); \$dec = \$vec->to_Dec(); How do I convert from decimal to binary Using "sprintf" (perl 5.6+): \$bin = sprintf("%b", 3735928559); Using "unpack": \$bin = unpack("B*", pack("N", 3735928559)); Using "Bit::Vector": use Bit::Vector; \$vec = Bit::Vector->new_Dec(32, -559038737); \$bin = \$vec->to_Bin(); The remaining transformations (e.g. hex -> oct, bin -> hex, etc.) are left as an exercise to the inclined reader. Why 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 "3" is the bit pattern 00110011). The operators work with the binary form of a number (the number 3 is treated as the bit pattern 00000011). So, saying "11 & 3" performs the "and" operation on numbers (yielding 3). Saying "11" & "3" performs the "and" operation on strings (yielding "1"). Most problems with "&" and "|" arise because the programmer thinks they have a number but really it's a string. The rest arise because the programmer says: if ("\020\020" & "\101\101") { # ... } but a string consisting of two null bytes (the result of "\020\020" & "\101\101") is not a false value in Perl. You need: if ( ("\020\020" & "\101\101") !~ /[^\000]/) { # ... } How do I multiply matrices? Use the "Math::Matrix" or "Math::MatrixReal" modules (available from CPAN) or the "PDL" extension (also available from CPAN). How do I perform an operation on a series of integers? To call a function on each element in an array, and collect the results, use: @results = map { my_func(\$_) } @array; For example: @triple = map { 3 * \$_ } @single; To call a function on each element of an array, but ignore the results: foreach \$iterator (@array) { some_func(\$iterator); } To call a function on each integer in a (small) range, you can use: @results = map { some_func(\$_) } (5 .. 25); but you should be aware that the ".." operator creates an array of all integers in the range. This can take a lot of memory for large ranges. Instead use: @results = (); for (\$i=5; \$i < 500_005; \$i++) { push(@results, some_func(\$i)); } This situation has been fixed in Perl5.005. Use of ".." in a "for" loop will iterate over the range, without creating the entire range. for my \$i (5 .. 500_005) { push(@results, some_func(\$i)); } will not create a list of 500,000 integers. How can I output Roman numerals? Get the http://www.cpan.org/modules/by-module/Roman ; module. Why aren't my random numbers random? If you're using a version of Perl before 5.004, you must call "srand" once at the start of your program to seed the random number generator. BEGIN { srand() if \$] < 5.004 } 5.004 and later automatically call "srand" at the beginning. Don't call "srand" more than onceO-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 random article in the "Far More Than You Ever Wanted To Know" collection in ;, courtesy of Tom Phoenix, talks more about this. John von Neumann said, "Anyone who attempts to generate random numbers by deterministic means is, of course, living in a state of sin." If you want numbers that are more random than "rand" with "srand" 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 ;. How do I get a random number between X and Y? To get a random number between two values, you can use the "rand()" built-in to get a random number between 0 and 1. From there, you shift that into the range that you want. "rand(\$x)" returns a number such that "0 <= rand(\$x) < \$x". Thus what you want to have perl figure out is a random number in the range from 0 to the difference between your X and Y. That is, to get a number between 10 and 15, inclusive, you want a random number between 0 and 5 that you can then add to 10. my \$number = 10 + int rand( 15-10+1 ); # ( 10,11,12,13,14, or 15 ) Hence you derive the following simple function to abstract that. It selects a random integer between the two given integers (inclusive), For example: "random_int_between(50,120)". sub random_int_between { my(\$min, \$max) = @_; # Assumes that the two arguments are integers themselves! return \$min if \$min == \$max; (\$min, \$max) = (\$max, \$min) if \$min > \$max; return \$min + int rand(1 + \$max - \$min); } Data: Dates How do I find the day or week of the year? The "localtime" function returns the day of the year. Without an argument "localtime" uses the current time. \$day_of_year = (localtime)[7]; The "POSIX" module can also format a date as the day of the year or week of the year. use POSIX qw/strftime/; my \$day_of_year = strftime "%j", localtime; my \$week_of_year = strftime "%W", localtime; To get the day of year for any date, use "POSIX"'s "mktime" to get a time in epoch seconds for the argument to "localtime". use POSIX qw/mktime strftime/; my \$week_of_year = strftime "%W", localtime( mktime( 0, 0, 0, 18, 11, 87 ) ); The "Date::Calc" module provides two functions to calculate these. use Date::Calc; my \$day_of_year = Day_of_Year( 1987, 12, 18 ); my \$week_of_year = Week_of_Year( 1987, 12, 18 ); How do I find the current century or millennium? Use the following simple functions: sub get_century { return int((((localtime(shift || time))[5] + 1999))/100); } sub get_millennium { return 1+int((((localtime(shift || time))[5] + 1899))/1000); } On some systems, the "POSIX" module's "strftime()" function has been extended in a non-standard way to use a %C format, which they sometimes claim is the "century". It isn't, because on most such systems, this is only the first two digits of the four-digit year, and thus cannot be used to reliably determine the current century or millennium. How can I compare two dates and find the difference? (contributed by brian d foy) You could just store all your dates as a number and then subtract. Life isn't always that simple though. If you want to work with formatted dates, the "Date::Manip", "Date::Calc", or "DateTime" modules can help you. How can I take a string and turn it into epoch seconds? If it's a regular enough string that it always has the same format, you can split it up and pass the parts to "timelocal" in the standard "Time::Local" module. Otherwise, you should look into the "Date::Calc" and "Date::Manip" modules from CPAN. How can I find the Julian Day? (contributed by brian d foy and Dave Cross) You can use the "Time::JulianDay" module available on CPAN. Ensure that you really want to find a Julian day, though, as many people have different ideas about Julian days. See http://www.hermetic.ch/cal_stud/jdn.htm for instance. You can also try the "DateTime" module, which can convert a date/time to a Julian Day. \$ perl -MDateTime -le'print DateTime->today->jd' 2453401.5 Or the modified Julian Day \$ perl -MDateTime -le'print DateTime->today->mjd' 53401 Or even the day of the year (which is what some people think of as a Julian day) \$ perl -MDateTime -le'print DateTime->today->doy' 31 How do I find yesterday's date? (contributed by brian d foy) Use one of the Date modules. The "DateTime" module makes it simple, and give you the same time of day, only the day before. use DateTime; my \$yesterday = DateTime->now->subtract( days => 1 ); print "Yesterday was \$yesterday\n"; You can also use the "Date::Calc" module using its "Today_and_Now" function. use Date::Calc qw( Today_and_Now Add_Delta_DHMS ); my @date_time = Add_Delta_DHMS( Today_and_Now(), -1, 0, 0, 0 ); print "@date_time\n"; Most people try to use the time rather than the calendar to figure out dates, but that assumes that days are twenty-four hours each. For most people, there are two days a year when they aren't: the switch to and from summer time throws this off. Let the modules do the work. If you absolutely must do it yourself (or can't use one of the modules), here's a solution using "Time::Local", which comes with Perl: # contributed by Gunnar Hjalmarsson use Time::Local; my \$today = timelocal 0, 0, 12, ( localtime )[3..5]; my (\$d, \$m, \$y) = ( localtime \$today-86400 )[3..5]; printf "Yesterday: %d-%02d-%02d\n", \$y+1900, \$m+1, \$d; In this case, you measure the day starting at noon, and subtract 24 hours. Even if the length of the calendar day is 23 or 25 hours, you'll still end up on the previous calendar day, although not at noon. Since you don't care about the time, the one hour difference doesn't matter and you end up with the previous date. Does Perl have a Year 2000 or 2038 problem? Is Perl Y2K compliant? (contributed by brian d foy) Perl itself never had a Y2K problem, although that never stopped people from creating Y2K problems on their own. See the documentation for "localtime" for its proper use. Starting with Perl 5.11, "localtime" and "gmtime" can handle dates past 03:14:08 January 19, 2038, when a 32-bit based time would overflow. You still might get a warning on a 32-bit "perl": % perl5.11.2 -E 'say scalar localtime( 0x9FFF_FFFFFFFF )' Integer overflow in hexadecimal number at -e line 1. Wed Nov 1 19:42:39 5576711 On a 64-bit "perl", you can get even larger dates for those really long running projects: % perl5.11.2 -E 'say scalar gmtime( 0x9FFF_FFFFFFFF )' Thu Nov 2 00:42:39 5576711 You're still out of luck if you need to keep tracking of decaying protons though. Data: Strings How do I validate input? (contributed by brian d foy) There are many ways to ensure that values are what you expect or want to accept. Besides the specific examples that we cover in the perlfaq, you can also look at the modules with "Assert" and "Validate" in their names, along with other modules such as "Regexp::Common". Some modules have validation for particular types of input, such as "Business::ISBN", "Business::CreditCard", "Email::Valid", and "Data::Validate::IP". How do I unescape a string? It depends just what you mean by "escape". URL escapes are dealt with in perlfaq9. Shell escapes with the backslash ("\") character are removed with s/\\(.)/\$1/g; This won't expand "\n" or "\t" or any other special escapes. How do I remove consecutive pairs of characters? (contributed by brian d foy) You can use the substitution operator to find pairs of characters (or runs of characters) and replace them with a single instance. In this substitution, we find a character in "(.)". The memory parentheses store the matched character in the back-reference "\1" and we use that to require that the same thing immediately follow it. We replace that part of the string with the character in \$1. s/(.)\1/\$1/g; We can also use the transliteration operator, "tr///". In this example, the search list side of our "tr///" contains nothing, but the "c" option complements that so it contains everything. The replacement list also contains nothing, so the transliteration is almost a no-op since it won't do any replacements (or more exactly, replace the character with itself). However, the "s" option squashes duplicated and consecutive characters in the string so a character does not show up next to itself my \$str = 'Haarlem'; # in the Netherlands \$str =~ tr///cs; # Now Harlem, like in New York How do I expand function calls in a string? (contributed by brian d foy) This is documented in perlref, and although it's not the easiest thing to read, it does work. In each of these examples, we call the function inside the braces used to dereference a reference. If we have more than one return value, we can construct and dereference an anonymous array. In this case, we call the function in list context. print "The time values are @{ [localtime] }.\n"; If we want to call the function in scalar context, we have to do a bit more work. We can really have any code we like inside the braces, so we simply have to end with the scalar reference, although how you do that is up to you, and you can use code inside the braces. Note that the use of parens creates a list context, so we need "scalar" to force the scalar context on the function: print "The time is \${\(scalar localtime)}.\n" print "The time is \${ my \$x = localtime; \\$x }.\n"; If your function already returns a reference, you don't need to create the reference yourself. sub timestamp { my \$t = localtime; \\$t } print "The time is \${ timestamp() }.\n"; The "Interpolation" module can also do a lot of magic for you. You can specify a variable name, in this case "E", to set up a tied hash that does the interpolation for you. It has several other methods to do this as well. use Interpolation E => 'eval'; print "The time values are \$E{localtime()}.\n"; In most cases, it is probably easier to simply use string concatenation, which also forces scalar context. print "The time is " . localtime() . ".\n"; 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 "/x([^x]*)x/" will get the intervening bits in \$1. For multiple ones, then something more like "/alpha(.*?)omega/" would be needed. But none of these deals with nested patterns. For balanced expressions using "(", "{", "[" or "<" as delimiters, use the CPAN module Regexp::Common, or see "(??{ code })" in perlre. For other cases, you'll have to write a parser. If you are serious about writing a parser, there are a number of 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] ); How do I reverse a string? Use "reverse()" in scalar context, as documented in "reverse" in perlfunc. \$reversed = reverse \$string; How do I expand tabs in a string? You can do it yourself: 1 while \$string =~ s/\t+/' ' x (length(\$&) * 8 - length(\$`) % 8)/e; Or you can just use the "Text::Tabs" module (part of the standard Perl distribution). use Text::Tabs; @expanded_lines = expand(@lines_with_tabs); How do I reformat a paragraph? Use "Text::Wrap" (part of the standard Perl distribution): use Text::Wrap; print wrap("\t", ' ', @paragraphs); The paragraphs you give to "Text::Wrap" should not contain embedded 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. How can I access or change N characters of a string? You can access the first characters of a string with substr(). To get the first character, for example, start at position 0 and grab the string of length 1. \$string = "Just another Perl Hacker"; \$first_char = substr( \$string, 0, 1 ); # 'J' To change part of a string, you can use the optional fourth argument which is the replacement string. substr( \$string, 13, 4, "Perl 5.8.0" ); You can also use substr() as an lvalue. substr( \$string, 13, 4 ) = "Perl 5.8.0"; How do I change the Nth occurrence of something? You have to keep track of N yourself. For example, let's say you want to change the fifth occurrence of "whoever" or "whomever" into "whosoever" or "whomsoever", case insensitively. These all assume that \$_ contains the string to be altered. \$count = 0; s{((whom?)ever)}{ ++\$count == 5 # is it the 5th? ? "\${2}soever" # yes, swap : \$1 # renege and leave it there }ige; In the more general case, you can use the "/g" modifier in a "while" loop, keeping count of matches. \$WANT = 3; \$count = 0; \$_ = "One fish two fish red fish blue fish"; while (/(\w+)\s+fish\b/gi) { if (++\$count == \$WANT) { print "The third fish is a \$1 one.\n"; } } That prints out: "The third fish is a red one." You can also use a repetition count and repeated pattern like this: /(?:\w+\s+fish\s+){2}(\w+)\s+fish/i; How can I count the number of occurrences of a substring within a string? There are a number of ways, with varying efficiency. If you want a count of a certain single character (X) within a string, you can use the "tr///" function like so: \$string = "ThisXlineXhasXsomeXx'sXinXit"; \$count = (\$string =~ tr/X//); print "There are \$count X characters in the string"; This is fine if you are just looking for a single character. However, if you are trying to count multiple character substrings within a larger string, "tr///" won't work. What you can do is wrap a while() loop around a global pattern match. For example, let's count negative integers: \$string = "-9 55 48 -2 23 -76 4 14 -44"; while (\$string =~ /-\d+/g) { \$count++ } print "There are \$count negative numbers in the string"; Another version uses a global match in list context, then assigns the result to a scalar, producing a count of the number of matches. \$count = () = \$string =~ /-\d+/g; How do I capitalize all the words on one line? (contributed by brian d foy) Damian Conway's Text::Autoformat handles all of the thinking for you. use Text::Autoformat; my \$x = "Dr. Strangelove or: How I Learned to Stop ". "Worrying and Love the Bomb"; print \$x, "\n"; for my \$style (qw( sentence title highlight )) { print autoformat(\$x, { case => \$style }), "\n"; } How do you want to capitalize those words? FRED AND BARNEY'S LODGE # all uppercase Fred And Barney's Lodge # title case Fred and Barney's Lodge # highlight case It's not as easy a problem as it looks. How many words do you think are in there? Wait for it... wait for it.... If you answered 5 you're right. Perl words are groups of "\w+", but that's not what you want to capitalize. How is Perl supposed to know not to capitalize that "s" after the apostrophe? You could try a regular expression: \$string =~ s/ ( (^\w) #at the beginning of the line | # or (\s\w) #preceded by whitespace ) /\U\$1/xg; \$string =~ s/([\w']+)/\u\L\$1/g; Now, what if you don't want to capitalize that "and"? Just use Text::Autoformat and get on with the next problem. :) How can I split a [character] delimited string except when inside [character]? Several modules can handle this sort of parsingO-"Text::Balanced", "Text::CSV", "Text::CSV_XS", and "Text::ParseWords", among others. Take the example case of trying to split a string that is comma-separated into its different fields. You can't use "split(/,/)" because you shouldn't split if the comma is inside quotes. For example, take a data line like this: SAR001,"","Cimetrix, Inc","Bob Smith","CAM",N,8,1,0,7,"Error, Core Dumped" Due to the restriction of the quotes, this is a fairly complex problem. Thankfully, we have Jeffrey Friedl, author of Mastering Regular 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, "like \"this\"". Alternatively, the "Text::ParseWords" module (part of the standard Perl distribution) lets you say: use Text::ParseWords; @new = quotewords(",", 0, \$text); How do I strip blank space from the beginning/end of a string? (contributed by brian d foy) A substitution can do this for you. For a single line, you want to replace all the leading or trailing whitespace with nothing. You can do that with a pair of substitutions. s/^\s+//; s/\s+\$//; You can also write that as a single substitution, although it turns out the combined statement is slower than the separate ones. That might not matter to you, though. s/^\s+|\s+\$//g; In this regular expression, the alternation matches either at the beginning or the end of the string since the anchors have a lower precedence than the alternation. With the "/g" flag, the substitution makes all possible matches, so it gets both. Remember, the trailing newline matches the "\s+", and the "\$" anchor can match to the physical end of the string, so the newline disappears too. Just add the newline to the output, which has the added benefit of preserving "blank" (consisting entirely of whitespace) lines which the "^\s+" would remove all by itself. while( <> ) { s/^\s+|\s+\$//g; print "\$_\n"; } For a multi-line string, you can apply the regular expression to each logical line in the string by adding the "/m" flag (for "multi-line"). With the "/m" flag, the "\$" matches before an embedded newline, so it doesn't remove it. It still removes the newline at the end of the string. \$string =~ s/^\s+|\s+\$//gm; Remember that lines consisting entirely of whitespace will disappear, since the first part of the alternation can match the entire string and replace it with nothing. If need to keep embedded blank lines, you have to do a little more work. Instead of matching any whitespace (since that includes a newline), just match the other whitespace. \$string =~ s/^[\t\f ]+|[\t\f ]+\$//mg; How do I pad a string with blanks or pad a number with zeroes? In the following examples, \$pad_len is the length to which you wish to pad the string, \$text or \$num contains the string to be padded, and \$pad_char contains the padding character. You can use a single character string constant instead of the \$pad_char variable if you know what it is in advance. And in the same way you can use an integer in place of \$pad_len if you know the pad length in advance. The simplest method uses the "sprintf" function. It can pad on the left or right with blanks and on the left with zeroes and it will not truncate the result. The "pack" function can only pad strings on the right with blanks and it will truncate the result to a maximum length of \$pad_len. # Left padding a string with blanks (no truncation): \$padded = sprintf("%\${pad_len}s", \$text); \$padded = sprintf("%*s", \$pad_len, \$text); # same thing # Right padding a string with blanks (no truncation): \$padded = sprintf("%-\${pad_len}s", \$text); \$padded = sprintf("%-*s", \$pad_len, \$text); # same thing # Left padding a number with 0 (no truncation): \$padded = sprintf("%0\${pad_len}d", \$num); \$padded = sprintf("%0*d", \$pad_len, \$num); # same thing # Right padding a string with blanks using pack (will truncate): \$padded = pack("A\$pad_len",\$text); If you need to pad with a character other than blank or zero you can use one of the following methods. They all generate a pad string with the "x" operator and combine that with \$text. These methods do not truncate \$text. Left and right padding with any character, creating a new string: \$padded = \$pad_char x ( \$pad_len - length( \$text ) ) . \$text; \$padded = \$text . \$pad_char x ( \$pad_len - length( \$text ) ); Left and right padding with any character, modifying \$text directly: substr( \$text, 0, 0 ) = \$pad_char x ( \$pad_len - length( \$text ) ); \$text .= \$pad_char x ( \$pad_len - length( \$text ) ); How do I extract selected columns from a string? (contributed by brian d foy) If you know the columns that contain the data, you can use "substr" to extract a single column. my \$column = substr( \$line, \$start_column, \$length ); You can use "split" if the columns are separated by whitespace or some other delimiter, as long as whitespace or the delimiter cannot appear as part of the data. my \$line = ' fred barney betty '; my @columns = split /\s+/, \$line; # ( '', 'fred', 'barney', 'betty' ); my \$line = 'fred||barney||betty'; my @columns = split /\|/, \$line; # ( 'fred', '', 'barney', '', 'betty' ); If you want to work with comma-separated values, don't do this since that format is a bit more complicated. Use one of the modules that handle that format, such as "Text::CSV", "Text::CSV_XS", or "Text::CSV_PP". If you want to break apart an entire line of fixed columns, you can use "unpack" with the A (ASCII) format. By using a number after the format specifier, you can denote the column width. See the "pack" and "unpack" entries in perlfunc for more details. my @fields = unpack( \$line, "A8 A8 A8 A16 A4" ); Note that spaces in the format argument to "unpack" do not denote literal spaces. If you have space separated data, you may want "split" instead. How do I find the soundex value of a string? (contributed by brian d foy) You can use the Text::Soundex module. If you want to do fuzzy or close matching, you might also try the "String::Approx", and "Text::Metaphone", and "Text::DoubleMetaphone" modules. How can I expand variables in text strings? (contributed by brian d foy) If you can avoid it, don't, or if you can use a templating system, such as "Text::Template" or "Template" Toolkit, do that instead. You might even be able to get the job done with "sprintf" or "printf": my \$string = sprintf 'Say hello to %s and %s', \$foo, \$bar; However, for the one-off simple case where I don't want to pull out a full templating system, I'll use a string that has two Perl scalar variables in it. In this example, I want to expand \$foo and \$bar to their variable's values: my \$foo = 'Fred'; my \$bar = 'Barney'; \$string = 'Say hello to \$foo and \$bar'; One way I can do this involves the substitution operator and a double "/e" flag. The first "/e" evaluates \$1 on the replacement side and turns it into \$foo. The second /e starts with \$foo and replaces it with its value. \$foo, then, turns into 'Fred', and that's finally what's left in the string: \$string =~ s/(\\$\w+)/\$1/eeg; # 'Say hello to Fred and Barney' The "/e" will also silently ignore violations of strict, replacing undefined variable names with the empty string. Since I'm using the "/e" flag (twice even!), I have all of the same security problems I have with "eval" in its string form. If there's something odd in \$foo, perhaps something like "@{[ system "rm -rf /" ]}", then I could get myself in trouble. To get around the security problem, I could also pull the values from a hash instead of evaluating variable names. Using a single "/e", I can check the hash to ensure the value exists, and if it doesn't, I can replace the missing value with a marker, in this case "???" to signal that I missed something: my \$string = 'This has \$foo and \$bar'; my %Replacements = ( foo => 'Fred', ); # \$string =~ s/\\$(\w+)/\$Replacements{\$1}/g; \$string =~ s/\\$(\w+)/ exists \$Replacements{\$1} ? \$Replacements{\$1} : '???' /eg; print \$string; What's wrong with always quoting "\$vars"? The problem is that those double-quotes force stringificationO-coercing numbers and references into stringsO-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 "++" autoincrement operator or the syscall() function. Stringification also destroys arrays. @lines = `command`; print "@lines"; # WRONG - extra blanks print @lines; # right 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<; However, in list context, the line input operator returns all of the lines as a list. The first line goes into @array[1] and the rest of the lines mysteriously disappear: @array[1] = ; # most likely not what you want Either the "use warnings" pragma or the -w flag will warn you when you use an array slice with a single index. How can I remove duplicate elements from a list or array? (contributed by brian d foy) Use a hash. When you think the words "unique" or "duplicated", think "hash keys". If you don't care about the order of the elements, you could just create the hash then extract the keys. It's not important how you create that hash: just that you use "keys" to get the unique elements. my %hash = map { \$_, 1 } @array; # or a hash slice: @hash{ @array } = (); # or a foreach: \$hash{\$_} = 1 foreach ( @array ); my @unique = keys %hash; If you want to use a module, try the "uniq" function from "List::MoreUtils". In list context it returns the unique elements, preserving their order in the list. In scalar context, it returns the number of unique elements. use List::MoreUtils qw(uniq); my @unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 1,2,3,4,5,6,7 my \$unique = uniq( 1, 2, 3, 4, 4, 5, 6, 5, 7 ); # 7 You can also go through each element and skip the ones you've seen before. Use a hash to keep track. The first time the loop sees an element, that element has no key in %Seen. The "next" statement creates the key and immediately uses its value, which is "undef", so the loop continues to the "push" and increments the value for that key. The next time the loop sees that same element, its key exists in the hash and the value for that key is true (since it's not 0 or "undef"), so the next skips that iteration and the loop goes to the next element. my @unique = (); my %seen = (); foreach my \$elem ( @array ) { next if \$seen{ \$elem }++; push @unique, \$elem; } You can write this more briefly using a grep, which does the same thing. my %seen = (); my @unique = grep { ! \$seen{ \$_ }++ } @array; How can I tell whether a certain element is contained in a list or array? (portions of this answer contributed by Anno Siegel and brian d foy) Hearing the word "in" is an indication that you probably should have used a hash, not a list or array, to store your data. Hashes are designed to answer this question quickly and efficiently. Arrays aren't. That being said, there are several ways to approach this. In Perl 5.10 and later, you can use the smart match operator to check that an item is contained in an array or a hash: use 5.010; if( \$item ~~ @array ) { say "The array contains \$item" } if( \$item ~~ %hash ) { say "The hash contains \$item" } With earlier versions of Perl, you have to do a bit more work. If you are going to make this query many times over arbitrary string values, the fastest way is probably to invert the original array and maintain a hash whose keys are the first array's values: @blues = qw/azure cerulean teal turquoise lapis-lazuli/; %is_blue = (); for (@blues) { \$is_blue{\$_} = 1 } Now you can check whether \$is_blue{\$some_color}. It might have been a good idea to keep the blues all in a hash in the first place. If the values are all small integers, you could use a simple indexed array. This kind of an array will take up less space: @primes = (2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31); @is_tiny_prime = (); for (@primes) { \$is_tiny_prime[\$_] = 1 } # or simply @istiny_prime[@primes] = (1) x @primes; Now you check whether \$is_tiny_prime[\$some_number]. If the values in question are integers instead of strings, you can save quite a lot of space by using bit strings instead: @articles = ( 1..10, 150..2000, 2017 ); undef \$read; for (@articles) { vec(\$read,\$_,1) = 1 } Now check whether "vec(\$read,\$n,1)" is true for some \$n. These methods guarantee fast individual tests but require a re-organization of the original list or array. They only pay off if you have to test multiple values against the same array. If you are testing only once, the standard module "List::Util" exports the function "first" for this purpose. It works by stopping once it finds the element. It's written in C for speed, and its Perl equivalent looks like this subroutine: sub first (&@) { my \$code = shift; foreach (@_) { return \$_ if &{\$code}(); } undef; } If speed is of little concern, the common idiom uses grep in scalar context (which returns the number of items that passed its condition) to traverse the entire list. This does have the benefit of telling you how many matches it found, though. my \$is_there = grep \$_ eq \$whatever, @array; If you want to actually extract the matching elements, simply use grep in list context. my @matches = grep \$_ eq \$whatever, @array; How do I compute the difference of two arrays? How do I compute the intersection of two arrays? Use a hash. Here's code to do both and more. It assumes that each element is unique in a given array: @union = @intersection = @difference = (); %count = (); foreach \$element (@array1, @array2) { \$count{\$element}++ } foreach \$element (keys %count) { push @union, \$element; push @{ \$count{\$element} > 1 ? \@intersection : \@difference }, \$element; } Note that this is the symmetric difference, that is, all elements in either A or in B but not in both. Think of it as an xor operation. How do I test whether two arrays or hashes are equal? With Perl 5.10 and later, the smart match operator can give you the answer with the least amount of work: use 5.010; if( @array1 ~~ @array2 ) { say "The arrays are the same"; } if( %hash1 ~~ %hash2 ) # doesn't check values! { say "The hash keys are the same"; } The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs. \$are_equal = compare_arrays(\@frogs, \@toads); sub compare_arrays { my (\$first, \$second) = @_; no warnings; # silence spurious -w undef complaints return 0 unless @\$first == @\$second; for (my \$i = 0; \$i < @\$first; \$i++) { return 0 if \$first->[\$i] ne \$second->[\$i]; } return 1; } For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module "FreezeThaw": use FreezeThaw qw(cmpStr); @a = @b = ( "this", "that", [ "more", "stuff" ] ); printf "a and b contain %s arrays\n", cmpStr(\@a, \@b) == 0 ? "the same" : "different"; This approach also works for comparing hashes. Here we'll demonstrate two different answers: use FreezeThaw qw(cmpStr cmpStrHard); %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); \$a{EXTRA} = \%b; \$b{EXTRA} = \%a; printf "a and b contain %s hashes\n", cmpStr(\%a, \%b) == 0 ? "the same" : "different"; printf "a and b contain %s hashes\n", cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; The first reports that both those the hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader. How do I find the first array element for which a condition is true? To find the first array element which satisfies a condition, you can use the "first()" function in the "List::Util" module, which comes with Perl 5.8. This example finds the first element that contains "Perl". use List::Util qw(first); my \$element = first { /Perl/ } @array; If you cannot use "List::Util", you can make your own loop to do the same thing. Once you find the element, you stop the loop with last. my \$found; foreach ( @array ) { if( /Perl/ ) { \$found = \$_; last } } If you want the array index, you can iterate through the indices and check the array element at each index until you find one that satisfies the condition. my( \$found, \$index ) = ( undef, -1 ); for( \$i = 0; \$i < @array; \$i++ ) { if( \$array[\$i] =~ /Perl/ ) { \$found = \$array[\$i]; \$index = \$i; last; } } How do I handle linked lists? In general, you usually don't need a linked list in Perl, since with regular arrays, you can push and pop or shift and unshift at either end, or you can use splice to add and/or remove arbitrary number of elements at arbitrary points. Both pop and shift are O(1) operations on Perl's dynamic arrays. In the absence of shifts and pops, push in general needs to reallocate on the order every log(N) times, and unshift will need to copy pointers each time. If you really, really wanted, you could use structures as described in perldsc or perltoot and do just what the algorithm book tells you to do. For example, imagine a list node like this: \$node = { VALUE => 42, LINK => undef, }; You could walk the list this way: print "List: "; for (\$node = \$head; \$node; \$node = \$node->{LINK}) { print \$node->{VALUE}, " "; } print "\n"; You could add to the list this way: my (\$head, \$tail); \$tail = append(\$head, 1); # grow a new head for \$value ( 2 .. 10 ) { \$tail = append(\$tail, \$value); } sub append { my(\$list, \$value) = @_; my \$node = { VALUE => \$value }; if (\$list) { \$node->{LINK} = \$list->{LINK}; \$list->{LINK} = \$node; } else { \$_[0] = \$node; # replace caller's version } return \$node; } But again, Perl's built-in are virtually always good enough. How do I handle circular lists? (contributed by brian d foy) If you want to cycle through an array endlessly, you can increment the index modulo the number of elements in the array: my @array = qw( a b c ); my \$i = 0; while( 1 ) { print \$array[ \$i++ % @array ], "\n"; last if \$i > 20; } You can also use "Tie::Cycle" to use a scalar that always has the next element of the circular array: use Tie::Cycle; tie my \$cycle, 'Tie::Cycle', [ qw( FFFFFF 000000 FFFF00 ) ]; print \$cycle; # FFFFFF print \$cycle; # 000000 print \$cycle; # FFFF00 The "Array::Iterator::Circular" creates an iterator object for circular arrays: use Array::Iterator::Circular; my \$color_iterator = Array::Iterator::Circular->new( qw(red green blue orange) ); foreach ( 1 .. 20 ) { print \$color_iterator->next, "\n"; } How do I shuffle an array randomly? If you either have Perl 5.8.0 or later installed, or if you have Scalar-List-Utils 1.03 or later installed, you can say: use List::Util 'shuffle'; @shuffled = shuffle(@list); If not, you can use a Fisher-Yates shuffle. sub fisher_yates_shuffle { my \$deck = shift; # \$deck is a reference to an array return unless @\$deck; # must not be empty! my \$i = @\$deck; while (--\$i) { my \$j = int rand (\$i+1); @\$deck[\$i,\$j] = @\$deck[\$j,\$i]; } } # shuffle my mpeg collection # my @mpeg =
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