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5.16. Representing Relationships Between Data

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).

5.16.2. Solution

Use a hash to
represent the relationship.

5.16.3. Discussion

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.

5.16.4. See Also

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

5.15. Finding the Most Common Anything		5.17. Program: dutree

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