IvorW has asked for the wisdom of the Perl Monks concerning the following question:

Is there an easy way of getting the union of 2 hashes? I know this can be done longhand with a loop, but I wondered if there was a better way. 

foreach (keys %a)
{
    $b{$_} = $a{$_};
}

While this works, I wondered if there was a better way. I know the equivalent with arrays is easy. 


@b = (@b,@a);

This might become significant for performance if one or other hash is not a real hash, but is tied to something.

Any thoughts?

Replies are listed 'Best First'.
Re: Union of hashes
by Biker (Priest) on Feb 07, 2002 at 11:07 UTC

    If you're happy to create a new hash from the two existing ones, the following should work: 

    %new_hash=(%a,%b);

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    I believe that this should work, but test it first: 
    %a=(%a,%b);

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    "Livet är hårt" sa bonden.
    "Grymt" sa grisen...

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      Or, for no particular reason that I can think of :

      
   @a{keys %b} = values %b;

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      Assuming that keys and values are guaranteed to return in the same order :)

      /J\

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        They are guaranteed to be in the same order. Note though that in both your solution and Biker's solution this is a union of keys. Entries in %b will overwrite entries in %a with their values when they share the same key. There's really no way around this, and it's probably what IvorW wants, but I thought I'd mention it for safety's sake.
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Re: Union of hashes
by trs80 (Priest) on Feb 07, 2002 at 20:50 UTC
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Re: Union of hashes
by ropey (Hermit) on Feb 07, 2002 at 16:24 UTC 
    map {$b{$_} = $a{$_}} keys %a;

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    Does a similar thing
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      Only in the sense that 
      print "hello world";

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      and 
      print $_ for split //, "hello world";

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      do "similar things". The void map solution is to be avoided, especially when there are so many better solutions.

      -- Randal L. Schwartz, Perl hacker

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        Like masem's most excellent Hash::Merge. It does deep hashes, and optionally does true copies instead of references.

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