perltutorial planetscape hash tutorial Data::Dumper data_structure reference FakeHash last updated: <a href="#updates">2008-12-11</a> N o t   E x a c t l y   a   H a s h   T u t o r i a l Quick links: <a href="#code">Code Illustrating Hashes and Related Syntax</a> <blockquote> <a href="#assignment">Hash Assignment</a> <a href="#access">Hash Element Access</a> <a href="#exists">exists ( $hash{$key} )</a> <a href="#keys">keys( %hash )</a> <a href="#delete">delete ( $hash{key} )</a> <a href="#values">values( %hash )</a> <a href="#each">each( %hash )</a> <a href="#inverse">Make an "Inverse" Hash</a> <a href="#expand">Let's Expand Our Real-World Example...</a> </blockquote> <a href="#fakehash">Visualizing a Hash Structure with FakeHash</a> <a href="#resources">A List of Hash Resources</a> <a href="#updates">Updates</a> <hr /> <a name="code"> <h4>Some Very Simple Code Illustrating Hashes and Related Syntax</h4> </a> <readmore><code> #! /usr/local/bin/perl -w # myHash.pl # these are very simplistic syntax examples; # there's much more to know, so see the # resources below! use strict; use warnings; use Data::Dumper; </code> <a name="assignment"></a> <code> # Hash Assignment # In this case, a # 1 : 1 relationship # key => value my %StateName = ( AK => 'Alaska', AL => 'Alabama', AR => 'Arkansas', AZ => 'Arizona', CA => 'California', CO => 'Colorado', CT => 'Connecticut', DC => 'District Of Columbia', DE => 'Delaware', FL => 'Florida', GA => 'Georgia', HI => 'Hawaii', IA => 'Iowa', ID => 'Idaho', IL => 'Illinois', IN => 'Indiana', KS => 'Kansas', KY => 'Kentucky', LA => 'Louisiana', MA => 'Massachusetts', MD => 'Maryland', ME => 'Maine', MI => 'Michigan', MN => 'Minnesota', MO => 'Missouri', MS => 'Mississippi', MT => 'Montana', NC => 'North Carolina', ND => 'North Dakota', NE => 'Nebraska', NH => 'New Hampshire', NJ => 'New Jersey', NM => 'New Mexico', NV => 'Nevada', NY => 'New York', OH => 'Ohio', OK => 'Oklahoma', OR => 'Oregon', PA => 'Pennsylvania', RI => 'Rhode Island', SC => 'South Carolina', SD => 'South Dakota', TN => 'Tennessee', TX => 'Texas', UT => 'Utah', VA => 'Virginia', VT => 'Vermont', WA => 'Washington', WI => 'Wisconsin', WV => 'West Virginia', WY => 'Wyoming' ); my $href = \%StateName; # reference to the hash %StateName print Dumper $href; # note the order will differ from that above print "\n"; </code> <a name="access"></a> <code> # hash element access my $State = 'SD'; # planetscape's home state my $Name = $StateName{$State}; print "planetscape lives in " . $Name . "\.\n\n"; </code> <a name="exists"></a> <code> # exists ( $hash{$key} ) $State = 'QC'; # Quebec, Canada if (exists($StateName{$State})) { print "The abbreviation for $State is " . $StateName{$State} . "\.\n\n"; } else { print $State . " is not a state! (yet)\n\n"; } </code> <a name="keys"></a> <code> # keys( %hash ) my $count = keys %StateName; print "There are $count elements in the hash.\n\n"; foreach $State (keys(%StateName)) { print "State abbreviation is '$State'\n"; } print "\n"; </code> <a name="delete"></a> <code> # delete ( $hash{key} ) delete($StateName{DC}); # DC is not actually a state, # it's a postal abbreviation </code> <a name="values"></a> <code> # values( %hash ) $count = values %StateName; # $count is one less since we deleted DC print "There are $count elements in the hash.\n\n"; foreach $State (values(%StateName)) { print "State name is '$State'\n"; } print "\n"; </code> <a name="each"></a> <code> # each( %hash ) while (my($key,$value)=each(%StateName)) { # there's lots more to know print "key='$key', value='$value'\n"; # about "each" - see the # resources below! } print "\n"; </code> <a name="inverse"></a> <code> # make an inverse hash my %StateAbbreviation = reverse %StateName; # exists ( $hash{$key} ) $State = 'Quebec'; # QC if (exists($StateAbbreviation{$State})) { print "The abbreviation for $State is " . $StateAbbreviation{$State} . "\.\n\n"; } else { print $State . " is not a state! (yet)\n\n"; } print "\n"; </code> <a name="expand"></a> <code> # let's expand our real-world example from a hash of States # to a hash of postal abbreviations (not exhaustive) my %PostalCode = %StateName; # copy %StateName to %PostalCode $PostalCode{AS} = 'American Samoa'; $PostalCode{DC} = 'District of Columbia'; $PostalCode{FM} = 'Federated States of Micronesia'; $PostalCode{GU} = 'Guam'; $PostalCode{MH} = 'Marshall Islands'; $PostalCode{MP} = 'Northern Mariana Islands'; $PostalCode{PW} = 'Palau'; $PostalCode{PR} = 'Puerto Rico'; $PostalCode{VI} = 'Virgin Islands'; $PostalCode{PW} = 'Palau'; # many more postal codes and abbreviations for the US and Canada # may be found here: http://www.usps.com/ncsc/lookups/usps_abbreviations.html # and here: http://canadaonline.about.com/library/bl/blpabb.htm foreach my $pc (keys(%PostalCode)) { print "PostalCode for '$PostalCode{$pc}' is '$pc'\n"; } </code></readmore> <hr /> <a name="fakehash"> <h4>Visualizing a Hash Structure with FakeHash</h4> </a> [cpan://FakeHash|FakeHash] is a module by Mark-Jason [Dominus] which was first announced [id://47536|here]. I have elaborated slightly on the [id://47536|code presented] in order to demonstrate some additional details such as setting drawing parameters. (More info on [cpan://FakeHash|FakeHash] can be found [http://perl.plover.com/FakeHash/FakeHash.pm|here].)   <code> use strict; use warnings; use FakeHash; my %h = ( When => 1, in => 2, the => 3, course => 4, of => 5, human => 6, events => 7, it => 8, becomes => 9, necessary => 10, for => 11, one => 12, people => 13, ); open( my $FILEHANDLE, ">FakeHashDrawing.txt" ); my $fake = FakeHash::DrawHash->new; $fake->draw_param( 'BUCKET', [1.5, 0.75] ); $fake->draw_param( 'KVP' => [2.0, 0.75] ); while ( my ( $key, $value ) = each(%h) ) { $fake->store( $key, $value ); } $fake->draw($FILEHANDLE); close $FILEHANDLE; </code> This code produces the following output: <readmore> <code> .PS boxwid:=1.5; boxht:=0.75 B00: box boxwid:=1.5; boxht:=0.75 B01: box with .n at B00.s circle at B01.c rad 0.1 filled arrow from B01.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N0100: box "becomes" "9" "134720492145(17)" boxwid:=1.5; boxht:=0.75 B02: box with .n at B01.s circle at B02.c rad 0.1 filled arrow from B02.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N0200: box "necessary" "10" "164114526854434(2)" boxwid:=1.5; boxht:=0.75 B03: box with .n at B02.s circle at B03.c rad 0.1 filled arrow from B03.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N0300: box "human" "6" "131651875(3)" boxwid:=1.5; boxht:=0.75 B04: box with .n at B03.s boxwid:=1.5; boxht:=0.75 B05: box with .n at B04.s boxwid:=1.5; boxht:=0.75 B06: box with .n at B05.s circle at B06.c rad 0.1 filled arrow from B06.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N0600: box "in" "2" "3686(6)" boxwid:=2; boxht:=0.75 N0601: box "people" "13" "4647902198(22)" boxwid:=2; boxht:=0.75 N0602: box "course" "4" "4135697990(6)" boxwid:=1.5; boxht:=0.75 B07: box with .n at B06.s boxwid:=1.5; boxht:=0.75 B08: box with .n at B07.s circle at B08.c rad 0.1 filled arrow from B08.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N0800: box "When" "1" "3344568(24)" boxwid:=2; boxht:=0.75 N0801: box "one" "12" "128504(24)" boxwid:=1.5; boxht:=0.75 B09: box with .n at B08.s circle at B09.c rad 0.1 filled arrow from B09.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N0900: box "events" "7" "4224378201(25)" boxwid:=1.5; boxht:=0.75 B10: box with .n at B09.s circle at B10.c rad 0.1 filled arrow from B10.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N1000: box "of" "5" "3882(10)" boxwid:=1.5; boxht:=0.75 B11: box with .n at B10.s circle at B11.c rad 0.1 filled arrow from B11.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N1100: box "the" "3" "133915(27)" boxwid:=1.5; boxht:=0.75 B12: box with .n at B11.s circle at B12.c rad 0.1 filled arrow from B12.c right boxwid/2 + 0.2 boxwid:=2; boxht:=0.75 N1200: box "it" "8" "3692(12)" boxwid:=2; boxht:=0.75 N1201: box "for" "11" "118444(12)" boxwid:=1.5; boxht:=0.75 B13: box with .n at B12.s boxwid:=1.5; boxht:=0.75 B14: box with .n at B13.s boxwid:=1.5; boxht:=0.75 B15: box with .n at B14.s .PE </code> </readmore> Once you have generated <code>FakeHashDrawing.txt</code> with the above Perl code, you can convert it to PostScript with the following (under [http://www.cygwin.com/|Cygwin]/*nix): <code>pic FakeHashDrawing.txt | groff -c -Tps > FakeHashDrawing.ps</code> <a href="http://planetscape.perlmonk.org/FakeHash.png">View the Output</a> (I then used [http://www.jasc.com/products/paintshoppro/|Jasc Paint Shop Pro 8] to convert PostScript to [wp://PNG] for display purposes.) If you prefer a more "modern" standard such as [wp://SVG], you can use a utility such as [http://www.gnu.org/software/plotutils/plotutils.html|plotutils]' <code>pic2plot</code> to convert from Unix <code>pic</code> format to [wp://SVG] or any other of a wide variety of formats. For example: <code>pic2plot -T svg FakeHashDrawing.txt > FakeHashDrawing.svg</code> Produces (on [http://www.cygwin.com/|Cygwin]): <readmore> <code> <?xml version="1.0" encoding="ISO-8859-1" standalone="no"?> <!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 20000303 Stylable//EN" "http://www.w3.org/TR/2000/03/WD-SVG-20000303/DTD/svg-20000303-stylable.dtd"> <svg width="8in" height="8in" viewBox="0 0 1 1" preserveAspectRatio="none"> <title>SVG drawing</title> <desc>This was produced by version 4.1 of GNU libplot, a free library for exporting 2-D vector graphics.</desc> <rect x="0" y="0" width="1" height="1" style="stroke:none;fill:white;"/> <g transform="translate(0.058854,-0.14453) scale(1,-1) scale(0.11458) " xml:space="preserve" style="stroke:black;stroke-linecap:butt;stroke-linejoin:miter;stroke-miterlimit:10.433;stroke-dasharray:none;stroke-dashoffset:0;stroke-opacity:1;fill:none;fill-rule:even-odd;fill-opacity:1;font-style:normal;font-variant:normal;font-weight:normal;font-stretch:normal;font-size-adjust:none;letter-spacing:normal;word-spacing:normal;text-anchor:start;"> <rect x="0" y="-0.375" width="1.5" height="0.75" style="stroke-width:0.010267;"/> <rect x="0" y="-1.125" width="1.5" height="0.75" style="stroke-width:0.010267;"/> <circle cx="0.75" cy="-0.75" r="0.1" style="stroke-width:0.010267;fill:gray;"/> <line x1="0.75" y1="-0.75" x2="1.7" y2="-0.75" style="stroke-width:0.010267;"/> <polygon points="1.6,-0.725 1.7,-0.75 1.6,-0.775 " style="stroke-width:0.010267;fill:black;"/> <rect x="1.7" y="-1.125" width="2" height="0.75" style="stroke-width:0.010267;"/> <text transform="translate(2.7,-0.56818) scale(1,-1) " style="font-family:'Helvetica',sans-serif;font-size:0.15152;text-anchor:middle;baseline-identifier:centerline;stroke:none;fill:black;">becomes</text> <text transform="translate(2.7,-0.75) scale(1,-1) " style="font-family:'Helvetica',sans-serif;font-size:0.15152;text-anchor:middle;baseline-identifier:centerline;stroke:none;fill:black;">9</text> <text transform="translate(2.7,-0.93182) scale(1,-1) " style="font-family:'Helvetica',sans-serif;font-size:0.15152;text-anchor:middle;baseline-identifier:centerline;stroke:none;fill:black;">134720492145(17)</text> <rect x="0" y="-1.875" width="1.5" height="0.75" style="stroke-width:0.010267;"/> <circle cx="0.75" cy="-1.5" r="0.1" style="stroke-width:0.010267;fill:gray;"/> <line x1="0.75" y1="-1.5" x2="1.7" y2="-1.5" style="stroke-width:0.010267;"/> <polygon points="1.6,-1.475 1.7,-1.5 1.6,-1.525 " style="stroke-width:0.010267;fill:black;"/> <rect x="1.7" y="-1.875" width="2" height="0.75" style="stroke-width:0.010267;"/> <text 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by Simon Cozens (<a href="http://www.adobe.com/products/acrobat/readstep2.html/">PDF</a>) Chapter 5 of <a href="http://ebb.org/PickingUpPerl/pickingUpPerl.pdf"> Picking Up Perl</a> by <a href="http://www.ebb.org/bkuhn">Bradley M. Kuhn</a> (<a href="http://www.adobe.com/products/acrobat/readstep2.html/">PDF</a>) Section 2.3 in <a href="http://www.greglondon.com/iperl/pdf/iperl.pdf"> Impatient Perl</a> by <a href="http://www.greglondon.com/">Greg London</a> (<a href="http://www.adobe.com/products/acrobat/readstep2.html/">PDF</a>) <a href="http://www.steve.gb.com/perl/tutorial.html">Steve's place - Perl Tutorial</a>, by Steve Cook (<a href="http://www.steve.gb.com/perl/downloads/tutorial.tar.gz">archive</a>) <blockquote> <ul> <li>Lesson 2: <a href="http://www.steve.gb.com/perl/lesson02.html">No-one likes being type cast</a> </li> <li>Lesson 4: <a href="http://www.steve.gb.com/perl/lesson04.html">Anyone got some hash? Sorted</a> </li> <li>Lesson 7: <a href="http://www.steve.gb.com/perl/lesson07.html">Arrays of hashes of arrays of hashrefs</a> </li> <li>Lesson 15: <a href="http://www.steve.gb.com/perl/lesson15.html">Tied up in knots</a> </li> </ul> </blockquote> <a href="http://www.perltraining.com.au">Perl Training Australia</a>'s superlative materials: <blockquote> <ul> <li><a href="http://perltraining.com.au/notes.html"> Introduction to Perl</a> (<a href="http://www.adobe.com/products/acrobat/readstep2.html/">PDF</a>)</li> <li><a href="http://perltraining.com.au/notes.html"> Intermediate Perl</a> (<a href="http://www.adobe.com/products/acrobat/readstep2.html/">PDF</a>)</li> </ul> </blockquote> <a href="http://www.plover.com/~mjd/perl/context.html">What is Scalar Context?</a> by [Dominus] <a href="http://www.plover.com/~mjd/perl/FAQs/references.html">Understand References Today</a> by [Dominus] <a href="http://web.archive.org/web/20070218230223rn_1/gisle.aas.no/perl/illguts/#hv">Hashes</a>, as explained in <a href="http://web.archive.org/web/20070218230223rn_1/gisle.aas.no/perl/illguts/">PerlGuts Illustrated</a>, by <a href="http://gisle.aas.no/">Gisle Aas</a> <a href="http://www.perl.com">Perl.com</a>: <a href="http://www.perl.com/lpt/a/2002/10/01/hashes.html">How Hashes Really Work</a>, by Abhijit Menon-Sen <a href="http://web.archive.org/web/20060826041456/http://www.5sigma.com/perl/effective_perl.pdf">Effective Perl: Intermediate and Advanced Topics</a> contains a wealth of information, not limited to hashes, but also including several pages on <a href="http://web.archive.org/web/20031011161713/http://www.effectiveperl.com/pegs/"> Joseph N. Hall's PErl Graphical Structures (PEGS)</a>. <a href="http://www.perlarchive.com/___TLC/7026.shtml">Uri Guttman's tutorial on AUTOVIVIFICATION</a> </blockquote> <h6>PerlMonks Tutorials:</h6> <blockquote> [id://861|the basic datatypes, three], by [root] [id://125289|Hash Keys (strings or numbers?)], by [robot_tourist] [id://494836|The Uniqueness of hashes], by [injunjoel] [id://90647|Multidimensional Arrays], by [CharlesClarkson] [id://137108|references], by [busunsl] [id://69927|References quick reference], by [tye] [id://8070|Object Serialization Basics], by [chromatic] </blockquote> <h6>PerlMonks Categorized Questions and Answers:</h6> <blockquote> [id://1825|Hashes] </blockquote> <h6>Other PerlMonks Nodes:</h6> <blockquote> [id://87227|Extracting array of hashes from data] by [nysus] [id://224434|how to avoid mis-spelling hash keys?], by [Gorilla] [id://345200|Perl Internals: Hashes], by [Kozz] [id://439591|help with hashes], by [Anonymous Monk] [id://445927|The Bad, the Ugly, and the Good of autovivification], by [tlm] the third item on [pad://tye] is a hash intro [id://567020|Autovivification trick], by [blazar] </blockquote> <h6>perldocs:</h6> <blockquote> [doc://perlfaq4]: Data Manipulation - Manipulating numbers, dates, strings, arrays, hashes, and miscellaneous data issues [doc://perldata] [doc://perlref] [doc://perlreftut] } [doc://perldsc]    }Thanks, [wfsp]! [doc://perllol]     } </blockquote> <h6>Google:</h6> <blockquote> [google://perl hash tutorial|Obligatory Google] </blockquote>   </blockquote> <h5>Print:</h5> <blockquote> Chapters 5 and 15 of <a href="http://www.oreilly.com/catalog/lperl3/index.html"> Learning Perl</a> by Randal L. Schwartz and Tom Phoenix Pages 10-12 and 76-78 of <a href="http://www.oreilly.com/catalog/pperl3/index.html">Programming Perl</a>, 3rd Ed., by Larry Wall, Tom Christiansen, Jon Orwant Chapter 5 of the <a href="http://www.oreilly.com/catalog/cookbook/index.html">Perl Cookbook</a>, 1st Ed., by Tom Christiansen, Nathan Torkington </blockquote> <hr /> <a name="updates"></a> Update: This is only a beginning. I noticed a lack of material on hashes in the [Tutorials] section, and thought I could contribute something, even though it might be one step at a time... Update(s): Thanks to [wfsp] for pointing out... I forgot the perldocs! <blockquote> 2005-07-02: added [doc://perlfaq4], [doc://perlref], [id://125289|125289], [id://137108|137108], [id://69927|69927], [id://224434|224434] 2005-07-06: several links per section; too lazy to enumerate 2005-07-07: added [id://1825], [pad://tye], and some <code>code</code> 2005-08-07: added info on [cpan://FakeHash|FakeHash], including code and [http://planetscape.perlmonk.org/FakeHash.png|graphics] 2005-09-12: fixed several broken links, brought to my attention by [Hue-Bond] (Thanks!) 2005-12-18: added link to [id://494836|The Uniqueness of hashes], by [injunjoel] 2006-03-25: re-arranged code and references in more logical order; added anchors; added <code>pic</code> and [wp://SVG] output and info on <code>pic2plot</code> 2006-03-26: added link to [id://8070|Object Serialization Basics], by [chromatic] 2006-08-13: added link to [id://567020|Autovivification trick], by [blazar] 2008-12-11: fixed several broken links, brought to my attention by [id://637526] (Thanks!); also fixed a few b0rked tags </blockquote>     p l a n e t s c a p e