5.15. Representing Relationships Between DataProblemYou want to represent relationships between elements of data - for instance, the mother of relationship in a family tree or parent process for a process table. This is closely related to representing tables in relational databases (tables represent relationships between information) and to representing computer science graph structures (edges represent relationships between nodes). SolutionUse a hash to represent the relationship. DiscussionHere's part of the family tree from the Bible: %father = ( 'Cain' => 'Adam', 'Abel' => 'Adam', 'Seth' => 'Adam', 'Enoch' => 'Cain', 'Irad' => 'Enoch', 'Mehujael' => 'Irad', 'Methusael' => 'Mehujael', 'Lamech' => 'Methusael', 'Jabal' => 'Lamech', 'Jubal' => 'Lamech', 'Tubalcain' => 'Lamech', 'Enos' => 'Seth' ); This lets us, for instance, easily trace a person's lineage: while (<>) { chomp; do { print "$_ "; # print the current name $_ = $father{$_}; # set $_ to $_'s father } while defined; # until we run out of fathers print "\n"; }
We can already ask questions like "Who begat Seth?" by checking the while ( ($k,$v) = each %father ) { push( @{ $children{$v} }, $k ); } $" = ', '; # separate output with commas while (<>) { chomp; if ($children{$_}) { @children = @{$children{$_}}; } else { @children = "nobody"; } print "$_ begat @children.\n"; }
Hashes can also represent relationships such as the C language foreach $file (@files) { local *F; # just in case we want a local FH unless (open (F, "<$file")) { warn "Couldn't read $file: $!; skipping.\n"; next; } while (<F>) { next unless /^\s*#\s*include\s*<([^>]+)>/; push(@{$includes{$1}}, $file); } close F; } This shows which files don't include any others: @include_free = (); # list of files that don't include others @uniq{map { @$_ } values %includes} = undef; foreach $file (sort keys %uniq) { push( @include_free , $file ) unless $includes{$file}; }
The values of See AlsoRecipe 4.6 ; the more complex data structures in Recipe 11.9 through Recipe 11.14 Copyright © 2001 O'Reilly & Associates. All rights reserved. |
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