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5.11. Merging Hashes

5.11.1. Problem

You
need to make a new hash with the entries of two existing hashes.

5.11.2. Solution

Treat them as lists, and join them as you would lists.

%merged = (%A, %B);

To save memory, loop over the hashes' elements and build a new hash
that way:

%merged = ( );
while ( ($k,$v) = each(%A) ) {
$merged{$k} = $v;
}
while ( ($k,$v) = each(%B) ) {
$merged{$k} = $v;
}

5.11.3. Discussion

The first method, like the earlier recipe on inverting a hash, uses
the hash-list equivalence explained in the introduction.
(%A, %B) evaluates to a list of
paired keys and values. When we assign it to
%merged, Perl turns that list of pairs back into a
hash.

Here's an example of that technique:

# %food_color as per the introduction
%drink_color = ( Galliano  => "yellow",
"Mai Tai" => "blue" );
%ingested_color = (%drink_color, %food_color);

Keys in both input hashes appear just once in the output hash. If a
food and a drink shared the same name, for instance, then the last
one seen by the first merging technique would be the one that showed
up in the resultant hash.

This style of direct assignment, as in the first example, is easier
to read and write, but requires a lot of memory if the hashes are
large. That's because Perl has to unroll both hashes into a temporary
list before the assignment to the merged hash is done. Step-by-step
merging using each, as in the second technique,
spares you that cost and lets you decide what to do with duplicate
keys.

The
first example could be rewritten to use the each
technique:

# %food_color per the introduction, then
%drink_color = ( Galliano  => "yellow",
"Mai Tai" => "blue" );
%substance_color = ( );
while (($k, $v) = each %food_color) {
$substance_color{$k} = $v;
}
while (($k, $v) = each %drink_color) {
$substance_color{$k} = $v;
}

That technique duplicated the while and assignment
code. Here's a sneaky way to get around that:

foreach $substanceref ( \%food_color, \%drink_color ) {
while (($k, $v) = each %$substanceref) {
$substance_color{$k} = $v;
}
}

If we're merging hashes with duplicates, we can insert our own code
to decide what to do with those duplicates:

foreach $substanceref ( \%food_color, \%drink_color ) {
while (($k, $v) = each %$substanceref) {
if (exists $substance_color{$k}) {
print "Warning: $k seen twice.  Using the first definition.\n";
next;
}
$substance_color{$k} = $v;
}
}

In the special case of appending one hash to another, we can use the
hash slice notation to give an elegant shorthand:

@all_colors{keys %new_colors} = values %new_colors;

This requires enough memory for lists of the keys and values of
%new_colors. As with the first technique, the
memory requirement might make this technique infeasible when such
lists would be
large.

5.11.4. See Also

This is a variation on Recipe 4.10; the
each function in perlfunc(1)
and in Chapter 29 of Programming Perl