Union of hashes

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?

Comment on Union of hashes

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);` [download] I believe that this should work, but test it first: `%a=(%a,%b);` [download] "Livet är hårt" sa bonden. "Grymt" sa grisen...	[reply] [d/l] [select]
Re: Re: Union of hashes by gellyfish (Monsignor) on Feb 07, 2002 at 12:00 UTC
Or, for no particular reason that I can think of : `@a{keys %b} = values %b;` [download] Assuming that `keys` and `values` are guaranteed to return in the same order :) /J\	[reply] [d/l] [select]
Re: Re: Re: Union of hashes by AidanLee (Chaplain) on Feb 07, 2002 at 13:18 UTC
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.	[reply]
Re: Re: Re: Re: Union of hashes by Masem (Monsignor) on Feb 07, 2002 at 14:37 UTC
Re: Union of hashes by trs80 (Priest) on Feb 07, 2002 at 20:50 UTC
This was covered on this node recently.	[reply]
Re: Union of hashes by ropey (Hermit) on Feb 07, 2002 at 16:24 UTC
`map {$b{$_} = $a{$_}} keys %a;` [download] Does a similar thing	[reply] [d/l]
Re: Re: Union of hashes by merlyn (Sage) on Feb 07, 2002 at 16:43 UTC
Only in the sense that `print "hello world";` [download] and `print $_ for split //, "hello world";` [download] do "similar things". The void map solution is to be avoided, especially when there are so many better solutions. -- Randal L. Schwartz, Perl hacker	[reply] [d/l] [select]
(jcwren) Re: (3) Union of hashes by jcwren (Prior) on Feb 07, 2002 at 17:50 UTC
Like masem's most excellent Hash::Merge. It does deep hashes, and optionally does true copies instead of references.	[reply]


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