You 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).

Use a hash to represent the relationship.

We can already ask questions like "Who begat Seth?" by checking the %father hash. By inverting this hash, we invert the relationship. This lets us use Recipe 5.9 to answer questions like "Whom did Lamech beget?"

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 #include s. A includes B if A contains #include B. This code builds the hash (it doesn't look for files in /usr/include as it should, but that's a minor change):

foreach $file (@files) {
    local *FH;
    unless (open(FH, " < $file")) {
        warn "Couldn't read $file: $!; skipping.\n";
        next;
    }

    while (<FH>) {
        next unless /^\s*#\s*include\s*<([^>]+)>/;
        push(@{$includes{$1}}, $file);
    }
    close FH;
}

This shows which files with include statements are not included in other files:

@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 %includes are anonymous arrays because a single file can (and often does) include more than one other file. We use map to build up a big list of the included files and remove duplicates using a hash.

Recipe 4.7; the more complex data structures in Recipe 11.9 through Recipe 11.14