[Wellington-pm] Hash slices with missing keys

Grant McLean grant at mclean.net.nz
Wed Jul 19 19:32:07 PDT 2006

On Thu, 2006-07-20 at 14:19 +1200, Ewen McNeill wrote:
> Hi,
> If I have a hash with some fields in it, and I use slices to pick out
> and swap over some of the values in the hash, and some of the keys
> referenced in the slice do not exist, am I guarenteed to get undef for
> the keys that don't exist?  Also is this warning/error free with "use
> strict".  Experimentation says yes, but I'm wary that I'm relying on
> some undocumented implementation quirk.
> Eg,
> @KEYS_A = qw(A B C D);
> @KEYS_B = qw(E F G H);
> %hash = (
>   'A' => 'aaaaaa',
>   'B' => 'bbbbbb',
>   'C' => 'cccccc',
>   'D' => 'dddddd',
>   'G' => 'gggggg',
>   'H' => 'hhhhhh'
> );
> @hash{@KEYS_A, @KEYS_B} = @hash{@KEYS_B, @KEYS_A};    # swap values over

Yes, for every element in the union of @KEYS_A and @KEYS_B, the
corresponding key in %hash will exist after the assignment, regardless
of whether it existed before and regardless of whether it ends up with a
value of undef.

> Failing this I'll need to copy over fields individually with some third
> variable to hold the values in transit.

Nope, you're safe to rely on Perl doing the intermediate storage thing
for you.


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