This is Info file perl.info, produced by Makeinfo version 1.68 from the input file bigperl.texi. settitle perl  File: perl.info, Node: perlfaq3, Next: Top, Prev: Top, Up: Top Programming Tools ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $) ******************************************************************* NAME ==== perlfaq3 - Programming Tools ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $) DESCRIPTION =========== This section of the FAQ answers questions related to programmer tools and programming support. How do I do (anything)? ----------------------- Have you looked at CPAN (see *Note Perlfaq2: perlfaq2,)? The chances are that someone has already written a module that can solve your problem. Have you read the appropriate man pages? Here's a brief index: Basics perldata, perlvar, perlsyn, perlop, perlsub Execution perlrun, perldebug Functions perlfunc Objects perlref, perlmod, perlobj, perltie Data Structures perlref, perllol, perldsc Modules perlmod, perlmodlib, perlsub Regexes perlre, perlfunc, perlop, perllocale Moving to perl5 perltrap, perl Linking w/C perlxstut, perlxs, perlcall, perlguts, perlembed Various http://www.perl.com/CPAN/doc/FMTEYEWTK/index.html (not a man-page but still useful) A crude table of contents for the Perl man page set is found in *Note Perltoc: perltoc,. How can I use Perl interactively? --------------------------------- The typical approach uses the Perl debugger, described in the perldebug(1) man page, on an "empty" program, like this: perl -de 42 Now just type in any legal Perl code, and it will be immediately evaluated. You can also examine the symbol table, get stack backtraces, check variable values, set breakpoints, and other operations typically found in symbolic debuggers. Is there a Perl shell? ---------------------- In general, no. The Shell.pm module (distributed with Perl) makes Perl try commands which aren't part of the Perl language as shell commands. perlsh from the source distribution is simplistic and uninteresting, but may still be what you want. How do I debug my Perl programs? -------------------------------- Have you tried `use warnings' or used -w? They enable warnings for dubious practices. Have you tried `use strict'? It prevents you from using symbolic references, makes you predeclare any subroutines that you call as bare words, and (probably most importantly) forces you to predeclare your variables with my or our or `use vars'. Did you check the returns of each and every system call? The operating system (and thus Perl) tells you whether they worked or not, and if not why. open(FH, "> /etc/cantwrite") or die "Couldn't write to /etc/cantwrite: $!\n"; Did you read *Note Perltrap: perltrap,? It's full of gotchas for old and new Perl programmers, and even has sections for those of you who are upgrading from languages like *awk* and C. Have you tried the Perl debugger, described in *Note Perldebug: perldebug,? You can step through your program and see what it's doing and thus work out why what it's doing isn't what it should be doing. How do I profile my Perl programs? ---------------------------------- You should get the Devel::DProf module from CPAN, and also use Benchmark.pm from the standard distribution. Benchmark lets you time specific portions of your code, while Devel::DProf gives detailed breakdowns of where your code spends its time. Here's a sample use of Benchmark: use Benchmark; @junk = `cat /etc/motd`; $count = 10_000; timethese($count, { 'map' => sub { my @a = @junk; map { s/a/b/ } @a; return @a }, 'for' => sub { my @a = @junk; local $_; for (@a) { s/a/b/ }; return @a }, }); This is what it prints (on one machine-your results will be dependent on your hardware, operating system, and the load on your machine): Benchmark: timing 10000 iterations of for, map... for: 4 secs ( 3.97 usr 0.01 sys = 3.98 cpu) map: 6 secs ( 4.97 usr 0.00 sys = 4.97 cpu) Be aware that a good benchmark is very hard to write. It only tests the data you give it, and really proves little about differing complexities of contrasting algorithms. How do I cross-reference my Perl programs? ------------------------------------------ The B::Xref module, shipped with the new, alpha-release Perl compiler (not the general distribution prior to the 5.005 release), can be used to generate cross-reference reports for Perl programs. perl -MO=Xref[,OPTIONS] scriptname.plx Is there a pretty-printer (formatter) for Perl? ----------------------------------------------- There is no program that will reformat Perl as much as indent(1) does for C. The complex feedback between the scanner and the parser (this feedback is what confuses the vgrind and emacs programs) makes it challenging at best to write a stand-alone Perl parser. Of course, if you simply follow the guidelines in *Note Perlstyle: perlstyle,, you shouldn't need to reformat. The habit of formatting your code as you write it will help prevent bugs. Your editor can and should help you with this. The perl-mode for emacs can provide a remarkable amount of help with most (but not all) code, and even less programmable editors can provide significant assistance. Tom swears by the following settings in vi and its clones: set ai sw=4 map! ^O {^M}^[O^T Now put that in your `.exrc' file (replacing the caret characters with control characters) and away you go. In insert mode, ^T is for indenting, ^D is for undenting, and ^O is for blockdenting - as it were. If you haven't used the last one, you're missing a lot. A more complete example, with comments, can be found at http://www.perl.com/CPAN-local/authors/id/TOMC/scripts/toms.exrc.gz If you are used to using the *vgrind* program for printing out nice code to a laser printer, you can take a stab at this using http://www.perl.com/CPAN/doc/misc/tips/working.vgrind.entry, but the results are not particularly satisfying for sophisticated code. The a2ps at http://www.infres.enst.fr/%7Edemaille/a2ps/ does lots of things related to generating nicely printed output of documents. Is there a ctags for Perl? -------------------------- There's a simple one at http://www.perl.com/CPAN/authors/id/TOMC/scripts/ptags.gz which may do the trick. And if not, it's easy to hack into what you want. Is there an IDE or Windows Perl Editor? --------------------------------------- If you're on Unix, you already have an IDE - Unix itself. This powerful IDE derives from its interoperability, flexibility, and configurability. If you really want to get a feel for Unix-qua-IDE, the best thing to do is to find some high-powered programmer whose native language is Unix. Find someone who has been at this for many years, and just sit back and watch them at work. They have created their own IDE, one that suits their own tastes and aptitudes. Quietly observe them edit files, move them around, compile them, debug them, test them, etc. The entire development *is* integrated, like a top-of-the-line German sports car: functional, powerful, and elegant. You will be absolutely astonished at the speed and ease exhibited by the native speaker of Unix in his home territory. The art and skill of a virtuoso can only be seen to be believed. That is the path to mastery - all these cobbled little IDEs are expensive toys designed to sell a flashy demo using cheap tricks, and being optimized for immediate but shallow understanding rather than enduring use, are but a dim palimpsest of real tools. In short, you just have to learn the toolbox. However, if you're not on Unix, then your vendor probably didn't bother to provide you with a proper toolbox on the so-called complete system that you forked out your hard-earned cash on. PerlBuilder (XXX URL to follow) is an integrated development environment for Windows that supports Perl development. Perl programs are just plain text, though, so you could download emacs for Windows (???) or a vi clone (vim) which runs on for win32 (http://www.cs.vu.nl/%7Etmgil/vi.html). If you're transferring Windows files to Unix, be sure to transfer in ASCII mode so the ends of lines are appropriately mangled. Where can I get Perl macros for vi? ----------------------------------- For a complete version of Tom Christiansen's vi configuration file, see http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/toms.exrc.gz, the standard benchmark file for vi emulators. This runs best with nvi, the current version of vi out of Berkeley, which incidentally can be built with an embedded Perl interpreter - see http://www.perl.com/CPAN/src/misc. Where can I get perl-mode for emacs? ------------------------------------ Since Emacs version 19 patchlevel 22 or so, there have been both a perl-mode.el and support for the Perl debugger built in. These should come with the standard Emacs 19 distribution. In the Perl source directory, you'll find a directory called "emacs", which contains a cperl-mode that color-codes keywords, provides context-sensitive help, and other nifty things. Note that the perl-mode of emacs will have fits with `"main'foo"' (single quote), and mess up the indentation and highlighting. You are probably using `"main::foo"' in new Perl code anyway, so this shouldn't be an issue. How can I use curses with Perl? ------------------------------- The Curses module from CPAN provides a dynamically loadable object module interface to a curses library. A small demo can be found at the directory http://www.perl.com/CPAN/authors/Tom_Christiansen/scripts/rep; this program repeats a command and updates the screen as needed, rendering *rep ps axu* similar to top. How can I use X or Tk with Perl? -------------------------------- Tk is a completely Perl-based, object-oriented interface to the Tk toolkit that doesn't force you to use Tcl just to get at Tk. Sx is an interface to the Athena Widget set. Both are available from CPAN. See the directory http://www.perl.com/CPAN/modules/by-category/08_User_Interfaces/ Invaluable for Perl/Tk programming are: the Perl/Tk FAQ at http://w4.lns.cornell.edu/%7Epvhp/ptk/ptkTOC.html , the Perl/Tk Reference Guide available at http://www.perl.com/CPAN-local/authors/Stephen_O_Lidie/ , and the online manpages at http://www-users.cs.umn.edu/%7Eamundson/perl/perltk/toc.html . How can I generate simple menus without using CGI or Tk? -------------------------------------------------------- The http://www.perl.com/CPAN/authors/id/SKUNZ/perlmenu.v4.0.tar.gz module, which is curses-based, can help with this. What is undump? --------------- See the next questions. How can I make my Perl program run faster? ------------------------------------------ The best way to do this is to come up with a better algorithm. This can often make a dramatic difference. Chapter 8 in the Camel has some efficiency tips in it you might want to look at. Jon Bentley's book "Programming Pearls" (that's not a misspelling!) has some good tips on optimization, too. Advice on benchmarking boils down to: benchmark and profile to make sure you're optimizing the right part, look for better algorithms instead of microtuning your code, and when all else fails consider just buying faster hardware. A different approach is to autoload seldom-used Perl code. See the AutoSplit and AutoLoader modules in the standard distribution for that. Or you could locate the bottleneck and think about writing just that part in C, the way we used to take bottlenecks in C code and write them in assembler. Similar to rewriting in C is the use of modules that have critical sections written in C (for instance, the PDL module from CPAN). In some cases, it may be worth it to use the backend compiler to produce byte code (saving compilation time) or compile into C, which will certainly save compilation time and sometimes a small amount (but not much) execution time. See the question about compiling your Perl programs for more on the compiler-the wins aren't as obvious as you'd hope. If you're currently linking your perl executable to a shared *libc.so*, you can often gain a 10-25% performance benefit by rebuilding it to link with a static libc.a instead. This will make a bigger perl executable, but your Perl programs (and programmers) may thank you for it. See the INSTALL file in the source distribution for more information. Unsubstantiated reports allege that Perl interpreters that use sfio outperform those that don't (for I/O intensive applications). To try this, see the INSTALL file in the source distribution, especially the "Selecting File I/O mechanisms" section. The undump program was an old attempt to speed up your Perl program by storing the already-compiled form to disk. This is no longer a viable option, as it only worked on a few architectures, and wasn't a good solution anyway. How can I make my Perl program take less memory? ------------------------------------------------ When it comes to time-space tradeoffs, Perl nearly always prefers to throw memory at a problem. Scalars in Perl use more memory than strings in C, arrays take more than that, and hashes use even more. While there's still a lot to be done, recent releases have been addressing these issues. For example, as of 5.004, duplicate hash keys are shared amongst all hashes using them, so require no reallocation. In some cases, using substr() or vec() to simulate arrays can be highly beneficial. For example, an array of a thousand booleans will take at least 20,000 bytes of space, but it can be turned into one 125-byte bit vector for a considerable memory savings. The standard Tie::SubstrHash module can also help for certain types of data structure. If you're working with specialist data structures (matrices, for instance) modules that implement these in C may use less memory than equivalent Perl modules. Another thing to try is learning whether your Perl was compiled with the system malloc or with Perl's builtin malloc. Whichever one it is, try using the other one and see whether this makes a difference. Information about malloc is in the INSTALL file in the source distribution. You can find out whether you are using perl's malloc by typing `perl -V:usemymalloc'. Is it unsafe to return a pointer to local data? ----------------------------------------------- No, Perl's garbage collection system takes care of this. sub makeone { my @a = ( 1 .. 10 ); return \@a; } for $i ( 1 .. 10 ) { push @many, makeone(); } print $many[4][5], "\n"; print "@many\n"; How can I free an array or hash so my program shrinks? ------------------------------------------------------ You can't. On most operating systems, memory allocated to a program can never be returned to the system. That's why long-running programs sometimes re-exec themselves. Some operating systems (notably, FreeBSD and Linux) allegedly reclaim large chunks of memory that is no longer used, but it doesn't appear to happen with Perl (yet). The Mac appears to be the only platform that will reliably (albeit, slowly) return memory to the OS. We've had reports that on Linux (Redhat 5.1) on Intel, `undef $scalar' will return memory to the system, while on Solaris 2.6 it won't. In general, try it yourself and see. However, judicious use of my() on your variables will help make sure that they go out of scope so that Perl can free up their storage for use in other parts of your program. A global variable, of course, never goes out of scope, so you can't get its space automatically reclaimed, although undef()ing and/or delete()ing it will achieve the same effect. In general, memory allocation and de-allocation isn't something you can or should be worrying about much in Perl, but even this capability (preallocation of data types) is in the works. How can I make my CGI script more efficient? -------------------------------------------- Beyond the normal measures described to make general Perl programs faster or smaller, a CGI program has additional issues. It may be run several times per second. Given that each time it runs it will need to be re-compiled and will often allocate a megabyte or more of system memory, this can be a killer. Compiling into C *isn't going to help you* because the process start-up overhead is where the bottleneck is. There are two popular ways to avoid this overhead. One solution involves running the Apache HTTP server (available from http://www.apache.org/) with either of the mod_perl or mod_fastcgi plugin modules. With mod_perl and the Apache::Registry module (distributed with mod_perl), httpd will run with an embedded Perl interpreter which pre-compiles your script and then executes it within the same address space without forking. The Apache extension also gives Perl access to the internal server API, so modules written in Perl can do just about anything a module written in C can. For more on mod_perl, see http://perl.apache.org/ With the FCGI module (from CPAN) and the mod_fastcgi module (available from http://www.fastcgi.com/) each of your Perl programs becomes a permanent CGI daemon process. Both of these solutions can have far-reaching effects on your system and on the way you write your CGI programs, so investigate them with care. See http://www.perl.com/CPAN/modules/by-category/15_World_Wide_Web_HTML_HTTP_CGI/ . A non-free, commercial product, "The Velocity Engine for Perl", (http://www.binevolve.com/ or http://www.binevolve.com/velocigen/) might also be worth looking at. It will allow you to increase the performance of your Perl programs, up to 25 times faster than normal CGI Perl by running in persistent Perl mode, or 4 to 5 times faster without any modification to your existing CGI programs. Fully functional evaluation copies are available from the web site. How can I hide the source for my Perl program? ---------------------------------------------- Delete it. :-) Seriously, there are a number of (mostly unsatisfactory) solutions with varying levels of "security". First of all, however, you *can't* take away read permission, because the source code has to be readable in order to be compiled and interpreted. (That doesn't mean that a CGI script's source is readable by people on the web, though, only by people with access to the filesystem) So you have to leave the permissions at the socially friendly 0755 level. Some people regard this as a security problem. If your program does insecure things, and relies on people not knowing how to exploit those insecurities, it is not secure. It is often possible for someone to determine the insecure things and exploit them without viewing the source. Security through obscurity, the name for hiding your bugs instead of fixing them, is little security indeed. You can try using encryption via source filters (Filter::* from CPAN), but any decent programmer will be able to decrypt it. You can try using the byte code compiler and interpreter described below, but the curious might still be able to de-compile it. You can try using the native-code compiler described below, but crackers might be able to disassemble it. These pose varying degrees of difficulty to people wanting to get at your code, but none can definitively conceal it (this is true of every language, not just Perl). If you're concerned about people profiting from your code, then the bottom line is that nothing but a restrictive license will give you legal security. License your software and pepper it with threatening statements like "This is unpublished proprietary software of XYZ Corp. Your access to it does not give you permission to use it blah blah blah." We are not lawyers, of course, so you should see a lawyer if you want to be sure your license's wording will stand up in court. How can I compile my Perl program into byte code or C? ------------------------------------------------------ Malcolm Beattie has written a multifunction backend compiler, available from CPAN, that can do both these things. It is included in the perl5.005 release, but is still considered experimental. This means it's fun to play with if you're a programmer but not really for people looking for turn-key solutions. Merely compiling into C does not in and of itself guarantee that your code will run very much faster. That's because except for lucky cases where a lot of native type inferencing is possible, the normal Perl run time system is still present and so your program will take just as long to run and be just as big. Most programs save little more than compilation time, leaving execution no more than 10-30% faster. A few rare programs actually benefit significantly (like several times faster), but this takes some tweaking of your code. You'll probably be astonished to learn that the current version of the compiler generates a compiled form of your script whose executable is just as big as the original perl executable, and then some. That's because as currently written, all programs are prepared for a full eval() statement. You can tremendously reduce this cost by building a shared *libperl.so* library and linking against that. See the INSTALL podfile in the Perl source distribution for details. If you link your main perl binary with this, it will make it minuscule. For example, on one author's system, `/usr/bin/perl' is only 11k in size! In general, the compiler will do nothing to make a Perl program smaller, faster, more portable, or more secure. In fact, it will usually hurt all of those. The executable will be bigger, your VM system may take longer to load the whole thing, the binary is fragile and hard to fix, and compilation never stopped software piracy in the form of crackers, viruses, or bootleggers. The real advantage of the compiler is merely packaging, and once you see the size of what it makes (well, unless you use a shared *libperl.so*), you'll probably want a complete Perl install anyway. How can I compile Perl into Java? --------------------------------- You can't. Not yet, anyway. You can integrate Java and Perl with the Perl Resource Kit from O'Reilly and Associates. See http://www.oreilly.com/catalog/prkunix/ for more information. The Java interface will be supported in the core 5.6 release of Perl. How can I get `#!perl' to work on [MS-DOS,NT,...]? -------------------------------------------------- For OS/2 just use extproc perl -S -your_switches as the first line in `*.cmd' file (-S due to a bug in cmd.exe's `extproc' handling). For DOS one should first invent a corresponding batch file, and codify it in `ALTERNATIVE_SHEBANG' (see the INSTALL file in the source distribution for more information). The Win95/NT installation, when using the ActiveState port of Perl, will modify the Registry to associate the `.pl' extension with the perl interpreter. If you install another port, perhaps even building your own Win95/NT Perl from the standard sources by using a Windows port of gcc (e.g., with cygwin or mingw32), then you'll have to modify the Registry yourself. In addition to associating `.pl' with the interpreter, NT people can use: `SET PATHEXT=%PATHEXT%;.PL' to let them run the program `install-linux.pl' merely by typing `install-linux'. Macintosh Perl programs will have the appropriate Creator and Type, so that double-clicking them will invoke the Perl application. *IMPORTANT!*: Whatever you do, PLEASE don't get frustrated, and just throw the perl interpreter into your cgi-bin directory, in order to get your programs working for a web server. This is an EXTREMELY big security risk. Take the time to figure out how to do it correctly. Can I write useful Perl programs on the command line? ----------------------------------------------------- Yes. Read *Note Perlrun: perlrun, for more information. Some examples follow. (These assume standard Unix shell quoting rules.) # sum first and last fields perl -lane 'print $F[0] + $F[-1]' * # identify text files perl -le 'for(@ARGV) {print if -f && -T _}' * # remove (most) comments from C program perl -0777 -pe 's{/\*.*?\*/}{}gs' foo.c # make file a month younger than today, defeating reaper daemons perl -e '$X=24*60*60; utime(time(),time() + 30 * $X,@ARGV)' * # find first unused uid perl -le '$i++ while getpwuid($i); print $i' # display reasonable manpath echo $PATH | perl -nl -072 -e ' s![^/+]*$!man!&&-d&&!$s{$_}++&&push@m,$_;END{print"@m"}' OK, the last one was actually an Obfuscated Perl Contest entry. :-) Why don't Perl one-liners work on my DOS/Mac/VMS system? -------------------------------------------------------- The problem is usually that the command interpreters on those systems have rather different ideas about quoting than the Unix shells under which the one-liners were created. On some systems, you may have to change single-quotes to double ones, which you must *NOT* do on Unix or Plan9 systems. You might also have to change a single % to a %%. For example: # Unix perl -e 'print "Hello world\n"' # DOS, etc. perl -e "print \"Hello world\n\"" # Mac print "Hello world\n" (then Run "Myscript" or Shift-Command-R) # VMS perl -e "print ""Hello world\n""" The problem is that none of this is reliable: it depends on the command interpreter. Under Unix, the first two often work. Under DOS, it's entirely possible neither works. If 4DOS was the command shell, you'd probably have better luck like this: perl -e "print "Hello world\n"" Under the Mac, it depends which environment you are using. The MacPerl shell, or MPW, is much like Unix shells in its support for several quoting variants, except that it makes free use of the Mac's non-ASCII characters as control characters. Using qq(), q(), and qx(), instead of "double quotes", 'single quotes', and `backticks`, may make one-liners easier to write. There is no general solution to all of this. It is a mess, pure and simple. Sucks to be away from Unix, huh? :-) [Some of this answer was contributed by Kenneth Albanowski.] Where can I learn about CGI or Web programming in Perl? ------------------------------------------------------- For modules, get the CGI or LWP modules from CPAN. For textbooks, see the two especially dedicated to web stuff in the question on books. For problems and questions related to the web, like "Why do I get 500 Errors" or "Why doesn't it run from the browser right when it runs fine on the command line", see these sources: WWW Security FAQ http://www.w3.org/Security/Faq/ Web FAQ http://www.boutell.com/faq/ CGI FAQ http://www.webthing.com/tutorials/cgifaq.html HTTP Spec http://www.w3.org/pub/WWW/Protocols/HTTP/ HTML Spec http://www.w3.org/TR/REC-html40/ http://www.w3.org/pub/WWW/MarkUp/ CGI Spec http://www.w3.org/CGI/ CGI Security FAQ http://www.go2net.com/people/paulp/cgi-security/safe-cgi.txt Where can I learn about object-oriented Perl programming? --------------------------------------------------------- A good place to start is *Note Perltoot: perltoot,, and you can use *Note Perlobj: perlobj, and *Note Perlbot: perlbot, for reference. Perltoot didn't come out until the 5.004 release, but you can get a copy (in pod, html, or postscript) from http://www.perl.com/CPAN/doc/FMTEYEWTK/ . Where can I learn about linking C with Perl? [h2xs, xsubpp] ----------------------------------------------------------- If you want to call C from Perl, start with *Note Perlxstut: perlxstut,, moving on to *Note Perlxs: perlxs,, `xsubpp' in this node, and *Note Perlguts: perlguts,. If you want to call Perl from C, then read *Note Perlembed: perlembed,, *Note Perlcall: perlcall,, and *Note Perlguts: perlguts,. Don't forget that you can learn a lot from looking at how the authors of existing extension modules wrote their code and solved their problems. I've read perlembed, perlguts, etc., but I can't embed perl in my C program, what am I doing wrong? --------------------------------------------------------------------------------------------------- Download the ExtUtils::Embed kit from CPAN and run `make test'. If the tests pass, read the pods again and again and again. If they fail, see `perlbug' in this node and send a bug report with the output of `make test TEST_VERBOSE=1' along with `perl -V'. When I tried to run my script, I got this message. What does it mean? --------------------------------------------------------------------- A complete list of Perl's error messages and warnings with explanatory text can be found in *Note Perldiag: perldiag,. You can also use the splain program (distributed with Perl) to explain the error messages: perl program 2>diag.out splain [-v] [-p] diag.out or change your program to explain the messages for you: use diagnostics; or use diagnostics -verbose; What's MakeMaker? ----------------- This module (part of the standard Perl distribution) is designed to write a Makefile for an extension module from a Makefile.PL. For more information, see *Note ExtUtils/MakeMaker: (pm.info)ExtUtils/MakeMaker,. AUTHOR AND COPYRIGHT ==================== Copyright (c) 1997-1999 Tom Christiansen and Nathan Torkington. All rights reserved. When included as an integrated part of the Standard Distribution of Perl or of its documentation (printed or otherwise), this works is covered under Perl's Artistic License. For separate distributions of all or part of this FAQ outside of that, see *Note Perlfaq: perlfaq,. Irrespective of its distribution, all code examples here are in the public domain. You are permitted and encouraged to use this code and any derivatives thereof in your own programs for fun or for profit as you see fit. A simple comment in the code giving credit to the FAQ would be courteous but is not required.  File: perl.info, Node: perlfaq4, Next: Top, Prev: Top, Up: Top Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $) ******************************************************************* NAME ==== perlfaq4 - Data Manipulation ($Revision: 1.49 $, $Date: 1999/05/23 20:37:49 $) DESCRIPTION =========== The section of the FAQ answers question related to the manipulation of data as numbers, dates, strings, arrays, hashes, and miscellaneous data issues. Data: Numbers ============= Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)? ------------------------------------------------------------------------------------------------------------- The infinite set that a mathematician thinks of as the real numbers can only be approximate on a computer, since the computer only has a finite number of bits to store an infinite number of, um, numbers. Internally, your computer represents floating-point numbers in binary. Floating-point numbers read in from a file or appearing as literals in your program are converted from their decimal floating-point representation (eg, 19.95) to the internal binary representation. However, 19.95 can't be precisely represented as a binary floating-point number, just like 1/3 can't be exactly represented as a decimal floating-point number. The computer's binary representation of 19.95, therefore, isn't exactly 19.95. When a floating-point number gets printed, the binary floating-point representation is converted back to decimal. These decimal numbers are displayed in either the format you specify with printf(), or the current output format for numbers (see `"$#"', *Note Perlvar: perlvar, if you use print. $# has a different default value in Perl5 than it did in Perl4. Changing $# yourself is deprecated.) This affects 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, `printf("%.2f", 19.95)') to get the required precision. See `"Floating-point Arithmetic"', *Note Perlop: perlop,. 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 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 bits into ints? -------------------------------- To turn a string of 1s and 0s like `10110110' into a scalar containing its binary value, use the pack() and unpack() functions (documented in `"pack"', *Note Perlfunc: perlfunc, and `"unpack"', *Note Perlfunc: perlfunc,): $decimal = unpack('c', pack('B8', '10110110')); This packs the string `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: $binary_string = unpack('B*', "\x29"); Why doesn't & work the way I want it to? ---------------------------------------- The behavior of binary arithmetic operators depends on whether they're used on numbers or strings. The operators treat a string as a series of bits and work with that (the string `"3"' is the bit pattern `00110011'). The operators work with the binary form of a number (the number 3 is treated as the bit pattern `00000011'). So, saying `11 & 3' performs the "and" operation on numbers (yielding 1). Saying `"11" & "3"' performs the "and" operation on strings (yielding `"1"'). Most problems with & and | arise because the programmer thinks they have a number but really it's a string. The rest arise because the programmer says: if ("\020\020" & "\101\101") { # ... } but a string consisting of two null bytes (the result of `"\020\020" & "\101\101"') is not a false value in Perl. You need: if ( ("\020\020" & "\101\101") !~ /[^\000]/) { # ... } How do I multiply matrices? --------------------------- Use the Math::Matrix or Math::MatrixReal modules (available from CPAN) or the PDL extension (also available from CPAN). How do I perform an operation on a series of integers? ------------------------------------------------------ To call a function on each element in an array, and collect the results, use: @results = map { my_func($_) } @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.perl.com/CPAN/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. 5.004 and later automatically call srand at the beginning. Don't call srand more than once-you make your numbers less random, rather than more. Computers are good at being predictable and bad at being random (despite appearances caused by bugs in your programs :-). 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 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 http://www.nr.com/ . Data: Dates =========== 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 `"localtime"', *Note Perlfunc: perlfunc,): $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. 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 format, which they sometimes claim is the "century". It isn't, because on most such systems, this is only the first two digits of the four-digit year, and thus cannot be used to reliably determine the current century or millennium. How can I compare two dates and find the difference? ---------------------------------------------------- If you're storing your dates as epoch seconds then simply subtract one from the other. If you've got a structured date (distinct year, day, month, hour, minute, seconds values), then for reasons of 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. How can I take a string and turn it into epoch seconds? ------------------------------------------------------- If it's a regular enough string that it always has the same format, you can split it up and pass the parts to `timelocal' in the standard Time::Local module. Otherwise, you should look into the Date::Calc and Date::Manip modules from CPAN. How can I find the Julian Day? ------------------------------ Use the Time::JulianDay module (part of the Time-modules bundle available from CPAN.) Before you immerse yourself too deeply in this, be sure to verify that it is the Julian Day you really want. Are they really just interested in a way of getting serial days so that they can do date arithmetic? If you are interested in performing date arithmetic, this can be done using either Date::Manip or Date::Calc, without converting to Julian Day first. There is too much confusion on this issue to cover in this FAQ, but the term is applied (correctly) to a calendar now supplanted by the Gregorian Calendar, with the Julian Calendar failing to adjust properly for leap years on centennial years (among other annoyances). The term is also used (incorrectly) to mean: [1] days in the Gregorian Calendar; and [2] days since a particular starting time or `epoch', usually 1970 in the Unix world and 1980 in the MS-DOS/Windows world. If you find that it is not the first meaning that you really want, then check out the Date::Manip and Date::Calc modules. (Thanks to David Cassell for most of this text.) How do I find yesterday's date? ------------------------------- The `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 `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 # This code is in the public domain Does Perl have a Year 2000 problem? Is Perl Y2K compliant? ----------------------------------------------------------- Short answer: No, Perl does not have a Year 2000 problem. Yes, Perl is Y2K compliant (whatever that means). The programmers you've hired to use it, however, probably are not. Long answer: The question belies a true understanding of the issue. Perl is just as Y2K compliant as your pencil-no more, and no less. Can you use your pencil to write a non-Y2K-compliant memo? Of course you can. Is that the pencil's fault? Of course it isn't. The date and time functions supplied with Perl (gmtime and localtime) supply adequate information to determine the year well beyond 2000 (2038 is when trouble strikes for 32-bit machines). The year returned by these functions when used in an array context is the year minus 1900. For years between 1910 and 1999 this *happens* to be a 2-digit decimal number. To avoid the year 2000 problem simply do not treat the year as a 2-digit number. It isn't. When gmtime() and localtime() are used in scalar context they return a timestamp string that contains a fully-expanded year. For example, `$timestamp = gmtime(1005613200)' sets $timestamp to "Tue Nov 13 01:00:00 2001". There's no year 2000 problem here. That doesn't mean that Perl can't be used to create non-Y2K compliant programs. It can. But so can your pencil. It's the fault of the user, not the language. At the risk of inflaming the NRA: "Perl doesn't break Y2K, people do." See http://language.perl.com/news/y2k.html for a longer exposition. Data: Strings ============= How do I validate input? ------------------------ The answer to this question is usually a regular expression, perhaps with auxiliary logic. See the more specific questions (numbers, mail addresses, etc.) for details. How do I unescape a string? --------------------------- It depends just what you mean by "escape". URL escapes are dealt with in *Note Perlfaq9: perlfaq9,. Shell escapes with the backslash (\) character are removed with: s/\\(.)/$1/g; This won't expand `"\n"' or `"\t"' or any other special escapes. How do I remove consecutive pairs of characters? ------------------------------------------------ To turn `"abbcccd"' into `"abccd"': s/(.)\1/$1/g; # add /s to include newlines Here's a solution that turns "abbcccd" to "abcd": y///cs; # y == tr, but shorter :-) How do I expand function calls in a string? ------------------------------------------- This is documented in *Note Perlref: 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 ${\($n + 5)} widgets\n"; Version 5.004 of Perl had a bug that gave list context to the expression in `${...}', but this is fixed in version 5.005. See also "How can I expand variables in text strings?" in this section of the FAQ. How do I find matching/nesting anything? ---------------------------------------- This isn't something that can be done in one regular expression, no matter how complicated. To find something between two single 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, 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] ); How do I reverse a string? -------------------------- Use reverse() in scalar context, as documented in `reverse', *Note Perlfunc: 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). 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/; 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"; How do I capitalize all the words on one line? ---------------------------------------------- To make the first letter of each word upper case: $line =~ s/\b(\w)/\U$1/g; This has the strange effect of turning "`don't do it'" into "`Don'T Do It'". Sometimes you might want this, instead (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 `use locale' pragma in your program. See *Note Perllocale: perllocale, for endless details on locales. This is sometimes referred to as putting something into "title case", but that's not quite accurate. Consider the proper capitalization of the movie *Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb*, for example. 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 `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, `"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 module on CPAN. How do I strip blank space from the beginning/end of a string? -------------------------------------------------------------- Although the simplest approach would seem to be: $string =~ s/^\s*(.*?)\s*$/$1/; Not only is this unnecessarily slow and destructive, it also fails with embedded newlines. It is much faster to do this operation in two steps: $string =~ s/^\s+//; $string =~ s/\s+$//; Or more nicely written as: for ($string) { s/^\s+//; s/\s+$//; } This idiom takes advantage of the foreach loop's aliasing behavior to factor out common code. You can do this on several strings at once, or arrays, or even the values of a hash if you use a slice: # trim whitespace in the scalar, the array, # and all the values in the hash foreach ($scalar, @array, @hash{keys %hash}) { s/^\s+//; s/\s+$//; } How do I pad a string with blanks or pad a number with zeroes? -------------------------------------------------------------- (This answer contributed by Uri Guttman, with kibitzing from Bart Lateur.) 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); # 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 x operator and combine that with $text. These methods do not truncate $text. Left and right padding with any character, creating a new string: $padded = $pad_char x ( $pad_len - length( $text ) ) . $text; $padded = $text . $pad_char x ( $pad_len - length( $text ) ); Left and right padding with any character, modifying $text directly: substr( $text, 0, 0 ) = $pad_char x ( $pad_len - length( $text ) ); $text .= $pad_char x ( $pad_len - length( $text ) ); How do I extract selected columns from a string? ------------------------------------------------ Use substr() or unpack(), both documented in *Note Perlfunc: perlfunc,. If you prefer thinking in terms of columns instead of widths, you can use this kind of thing: # determine the unpack format needed to split Linux ps output # arguments are cut columns my $fmt = cut2fmt(8, 14, 20, 26, 30, 34, 41, 47, 59, 63, 67, 72); sub cut2fmt { my(@positions) = @_; my $template = ''; my $lastpos = 1; for my $place (@positions) { $template .= "A" . ($place - $lastpos) . " "; $lastpos = $place; } $template .= "A*"; return $template; } How do I find the soundex value of a string? -------------------------------------------- Use the standard Text::Soundex module distributed with Perl. But before you do so, you may want to determine whether `soundex' is in fact what you think it is. Knuth's soundex algorithm compresses words into a small space, and so it does not necessarily distinguish between two words which you might want to appear separately. For example, the last names `Knuth' and `Kant' are both mapped to the soundex code K530. If Text::Soundex does not do what you are looking for, you might want to consider the String::Approx module available at CPAN. 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{$1}/g; See also "How do I expand function calls in a string?" in this section of the FAQ. What's wrong with always quoting "$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. 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 white space, with remaining indentation correctly preserved: $poem = fix< 1 ? \@intersection : \@difference }, $element; } Note that this is the *symmetric difference*, that is, all elements in either A or in B, but not in both. Think of it as an xor operation. How do I test whether two arrays or hashes are equal? ----------------------------------------------------- The following code works for single-level arrays. It uses a stringwise comparison, and does not distinguish defined versus undefined empty strings. Modify if you have other needs. $are_equal = compare_arrays(\@frogs, \@toads); sub compare_arrays { my ($first, $second) = @_; no warnings; # silence spurious -w undef complaints return 0 unless @$first == @$second; for (my $i = 0; $i < @$first; $i++) { return 0 if $first->[$i] ne $second->[$i]; } return 1; } For multilevel structures, you may wish to use an approach more like this one. It uses the CPAN module FreezeThaw: use FreezeThaw qw(cmpStr); @a = @b = ( "this", "that", [ "more", "stuff" ] ); printf "a and b contain %s arrays\n", cmpStr(\@a, \@b) == 0 ? "the same" : "different"; This approach also works for comparing hashes. Here we'll demonstrate two different answers: use FreezeThaw qw(cmpStr cmpStrHard); %a = %b = ( "this" => "that", "extra" => [ "more", "stuff" ] ); $a{EXTRA} = \%b; $b{EXTRA} = \%a; printf "a and b contain %s hashes\n", cmpStr(\%a, \%b) == 0 ? "the same" : "different"; printf "a and b contain %s hashes\n", cmpStrHard(\%a, \%b) == 0 ? "the same" : "different"; The first reports that both those the hashes contain the same data, while the second reports that they do not. Which you prefer is left as an exercise to the reader. How do I find the first array element for which a condition is true? -------------------------------------------------------------------- You can use this if you care about the index: for ($i= 0; $i < @array; $i++) { if ($array[$i] eq "Waldo") { $found_index = $i; last; } } Now `$found_index' has what you want. 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 *Note Perldsc: perldsc, or *Note Perltoot: 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 $node->{VALUE}, " "; } print "\n"; You could grow the list this way: my ($head, $tail); $tail = append($head, 1); # grow a new head for $value ( 2 .. 10 ) { $tail = append($tail, $value); } sub append { my($list, $value) = @_; my $node = { VALUE => $value }; if ($list) { $node->{LINK} = $list->{LINK}; $list->{LINK} = $node; } else { $_[0] = $node; # replace caller's version } return $node; } But again, Perl's built-in are virtually always good enough. How do I handle circular lists? ------------------------------- Circular lists could be handled in the traditional fashion with linked lists, or you could just do something like this with an array: unshift(@array, pop(@array)); # the last shall be first push(@array, shift(@array)); # and vice versa How do I shuffle an array randomly? ----------------------------------- Use this: # fisher_yates_shuffle( \@array ) : # generate a random permutation of @array in place sub fisher_yates_shuffle { my $array = shift; my $i; for ($i = @$array; --$i; ) { my $j = int rand ($i+1); next if $i == $j; @$array[$i,$j] = @$array[$j,$i]; } } fisher_yates_shuffle( \@array ); # permutes @array in place You've probably seen shuffling algorithms that work using splice, randomly picking another element to swap the current element with: srand; @new = (); @old = 1 .. 10; # just a demo while (@old) { push(@new, splice(@old, rand @old, 1)); } This is bad because splice is already O(N), and since you do it N times, you just invented a quadratic algorithm; that is, O(N**2). This does not scale, although Perl is so efficient that you probably won't notice this until you have rather largish arrays. How do I process/modify each element of an array? ------------------------------------------------- Use for/foreach: for (@lines) { s/foo/bar/; # change that word y/XZ/ZX/; # swap those letters } Here's another; let's compute spherical volumes: for (@volumes = @radii) { # @volumes has changed parts $_ **= 3; $_ *= (4/3) * 3.14159; # this will be constant folded } If you want to do the same thing to modify the values of the hash, you may not use the values function, oddly enough. You need a slice: for $orbit ( @orbits{keys %orbits} ) { ($orbit **= 3) *= (4/3) * 3.14159; } How do I select a random element from an array? ----------------------------------------------- Use the rand() function (see `rand', *Note Perlfunc: perlfunc,): # 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 *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. 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]); } } } How do I sort an array by (anything)? ------------------------------------- Supply a comparison function to sort() (described in `sort', *Note Perlfunc: perlfunc,): @list = sort { $a <=> $b } @list; The default sort function is cmp, string comparison, which would sort `(1, 2, 10)' into `(1, 10, 2)'. `< <=' >>, used above, is the numerical comparison operator. If you have a complicated function needed to pull out the part you want to sort on, then don't do it inside the sort function. Pull it out first, because the sort BLOCK can be called many times for the same element. Here's an example of how to pull out the first word after the first number on each item, and then sort those words case-insensitively. @idx = (); for (@data) { ($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. 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), $bits); print "bits are: @bytes\n\n"; } Why does defined() return true on empty arrays and hashes? ---------------------------------------------------------- The short story is that you should probably only use defined on scalars or functions, not on aggregates (arrays and hashes). See `defined', *Note Perlfunc: perlfunc, in the 5.004 release or later of Perl for more detail. Data: Hashes (Associative Arrays) ================================= How do I process an entire hash? -------------------------------- Use the each() function (see `each', *Note Perlfunc: perlfunc,) if you don't care whether it's sorted: while ( ($key, $value) = each %hash) { print "$key = $value\n"; } If you want it sorted, you'll have to use foreach() on the result of sorting the keys as shown in an earlier question. What happens if I add or remove keys from a hash while iterating over it? ------------------------------------------------------------------------- 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. How do I look up a hash element by value? ----------------------------------------- Create a reverse hash: %by_value = reverse %by_key; $key = $by_value{$value}; That's not particularly efficient. It would be more space-efficient to use: while (($key, $value) = each %by_key) { $by_value{$value} = $key; } If your hash could have repeated values, the methods above will only find one of the associated keys. This may or may not worry you. If it does worry you, you can always reverse the hash into a hash of arrays instead: while (($key, $value) = each %by_key) { push @{$key_list_by_value{$value}}, $key; } How can I know how many entries are in a hash? ---------------------------------------------- If you mean how many keys, then all you have to do is take the scalar sense of the keys() function: $num_keys = scalar keys %hash; In void context, the keys() function just resets the iterator, which is faster for tied hashes than would be iterating through the whole hash, one key-value pair at a time. 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, and if that fails, by straight ASCII comparison of the keys (well, possibly modified by your locale - see *Note Perllocale: perllocale,). @keys = sort { $hash{$b} <=> $hash{$a} || length($b) <=> length($a) || $a cmp $b } keys %hash; How can I always keep my hash sorted? ------------------------------------- You can look into using the DB_File module and tie() using the $DB_BTREE hash bindings as documented in `"In Memory Databases"', *Note DB_File: (pm.info)DB_File,. The Tie::IxHash module from CPAN might also be instructive. 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 $key is present in the array, `exists($key)' will return true. The value for a given key can be undef, in which case `$array{$key}' will be undef while `$exists{$key}' will return true. This corresponds to ($key, undef) being in the hash. Pictures help... here's the `%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! 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. How do I reset an each() operation part-way through? ---------------------------------------------------- Using `keys %hash' in scalar context returns the number of keys in the hash and resets the iterator associated with the hash. You may need to do this if you use last to exit a loop early so that when you re-enter it, the hash iterator has been reset. How can I get the unique keys from two hashes? ---------------------------------------------- First you extract the keys from the hashes into lists, then solve the "removing duplicates" problem described above. For example: %seen = (); for $element (keys(%foo), keys(%bar)) { $seen{$element}++; } @uniq = keys %seen; Or more succinctly: @uniq = keys %{{%foo,%bar}}; Or if you really want to save space: %seen = (); while (defined ($key = each %foo)) { $seen{$key}++; } while (defined ($key = each %bar)) { $seen{$key}++; } @uniq = keys %seen; How can I store a multidimensional array in a DBM file? ------------------------------------------------------- Either stringify the structure yourself (no fun), or else get the MLDBM (which uses Data::Dumper) module from CPAN and layer it on top of either DB_File or GDBM_File. How can I make my hash remember the order I put elements into it? ----------------------------------------------------------------- Use the Tie::IxHash from CPAN. use Tie::IxHash; tie(%myhash, Tie::IxHash); for ($i=0; $i<20; $i++) { $myhash{$i} = 2*$i; } @keys = keys %myhash; # @keys = (0,1,2,3,...) Why does passing a subroutine an undefined element in a hash create it? ----------------------------------------------------------------------- If you say something like: somefunc($hash{"nonesuch key here"}); Then that element "autovivifies"; that is, it springs into existence whether you store something there or not. That's because functions get scalars passed in by reference. If somefunc() modifies `$_[0]', it has to be ready to write it back into the caller's version. This has been fixed as of Perl5.004. Normally, merely accessing a key's value for a nonexistent key does not cause that key to be forever there. This is different than awk's behavior. How can I make the Perl equivalent of a C structure/C++ class/hash or array of hashes or arrays? ------------------------------------------------------------------------------------------------ Usually a hash ref, perhaps like this: $record = { NAME => "Jason", EMPNO => 132, TITLE => "deputy peon", AGE => 23, SALARY => 37_000, PALS => [ "Norbert", "Rhys", "Phineas"], }; References are documented in *Note Perlref: perlref, and the upcoming *Note Perlreftut: perlreftut,. Examples of complex data structures are given in *Note Perldsc: perldsc, and *Note Perllol: perllol,. Examples of structures and object-oriented classes are in *Note Perltoot: perltoot,. How can I use a reference as a hash key? ---------------------------------------- You can't do this directly, but you could use the standard Tie::Refhash module distributed with Perl. Data: Misc ========== How do I handle binary data correctly? -------------------------------------- Perl is binary clean, so this shouldn't be a problem. For example, this works fine (assuming the files are found): if (`cat /vmunix` =~ /gzip/) { print "Your kernel is GNU-zip enabled!\n"; } On less elegant (read: Byzantine) systems, however, you have to play tedious games with "text" versus "binary" files. See `"binmode"', *Note Perlfunc: perlfunc, or `"binmode"', *Note Perlopentut: perlopentut,. Most of these ancient-thinking systems are curses out of Microsoft, who seem to be committed to putting the backward into backward compatibility. If you're concerned about 8-bit ASCII data, then see *Note Perllocale: perllocale,. If you want to deal with multibyte characters, however, there are some gotchas. See the section on Regular Expressions. How do I determine whether a scalar is a number/whole/integer/float? -------------------------------------------------------------------- Assuming that you don't care about IEEE notations like "NaN" or "Infinity", you probably just want to use a regular expression. if (/\D/) { print "has nondigits\n" } if (/^\d+$/) { 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 `POSIX::strtod' function. Its semantics are somewhat cumbersome, so here's a `getnum' wrapper function for more convenient access. This function takes a string and returns the number it found, or undef for input that isn't a C float. The `is_numeric' function is a front end to `getnum' if you just want to say, "Is this a float?" sub getnum { use POSIX qw(strtod); my $str = shift; $str =~ s/^\s+//; $str =~ s/\s+$//; $! = 0; my($num, $unparsed) = strtod($str); if (($str eq '') || ($unparsed != 0) || $!) { return undef; } else { return $num; } } sub is_numeric { defined getnum($_[0]) } Or you could check out the String::Scanf module on CPAN instead. The POSIX module (part of the standard Perl distribution) provides the strtol and strtod for converting strings to double and longs, respectively. How do I keep persistent data across program calls? --------------------------------------------------- For some specific applications, you can use one of the DBM modules. See *Note AnyDBM_File: (pm.info)AnyDBM_File,. More generically, you should consult the FreezeThaw, Storable, or Class::Eroot modules from CPAN. Here's one example using Storable's store and retrieve functions: use Storable; store(\%hash, "filename"); # later on... $href = retrieve("filename"); # by ref %hash = %{ retrieve("filename") }; # direct to hash How do I print out or copy a recursive data structure? ------------------------------------------------------ The Data::Dumper module on CPAN (or the 5.005 release of Perl) is great for printing out data structures. The Storable module, found on CPAN, provides a function called `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 `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) }; How do I define methods for every class/object? ----------------------------------------------- Use the UNIVERSAL class (see *Note UNIVERSAL: (pm.info)UNIVERSAL,). How do I verify a credit card checksum? --------------------------------------- Get the Business::CreditCard module from CPAN. How do I pack arrays of doubles or floats for XS code? ------------------------------------------------------ The kgbpack.c code in the PGPLOT module on CPAN does just this. If you're doing a lot of float or double processing, consider using the PDL module from CPAN instead-it makes number-crunching easy. AUTHOR AND COPYRIGHT ==================== Copyright (c) 1997-1999 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 *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.