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14.4. Merging DBM Files
14.4.1. Problem
You want to
combine two DBM files into a single DBM file with original key-value
pairs.
14.4.2. Solution
Either merge the databases by treating their hashes as lists:%OUTPUT = (%INPUT1, %INPUT2);
or, more wisely, by iterating over each key-value pair:%OUTPUT = ( );
foreach $href ( \%INPUT1, \%INPUT2 ) {
while (my($key, $value) = each(%$href)) {
if (exists $OUTPUT{$key}) {
# decide which value to use and set $OUTPUT{$key} if necessary
} else {
$OUTPUT{$key} = $value;
}
}
}
14.4.3. Discussion
This straightforward application of Recipe 5.11 comes with the same caveats. Merging hashes by
treating them as lists requires that the hashes be preloaded into
memory, creating a potentially humongous temporary list. If you're
dealing with large hashes, have little virtual memory, or both, then
you want to iterate over the keys with each to
save memory.Another difference between these merging techniques is what to do if
the same key exists in both input databases. The blind assignment
merely overwrites the first value with the second value. The
iterative merging technique lets you decide what to do. Possibilities
include issuing a warning or error, choosing the first over the
second, choosing the second over the first, or concatenating the new
value to the old one. If you're using the MLDBM module, you can even
store them both, using an array reference to the two values.
14.4.4. See Also
Recipe 5.11; Recipe 14.6
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| 14.3. Converting Between DBM Files |  | 14.5. Sorting Large DBM Files |
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