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

I need to define a hash with keys whose values are undef.

Of course, I can just do 'key' => undef, etc... but this becomes tedious when you have a bunch of keys.

I want to do this when I am declaring the hash, and not afterwards. How can I do this when I first declare the hash without having to do 'key' => undef, for each key/value pair?

Thank you Monks.

  • Comment on How to create a hash whose values are undef?

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Re: How to create a hash whose values are undef?
by GrandFather (Saint) on Jul 06, 2009 at 03:13 UTC

    You may like:

    my %hash = map {$_ => undef} @keys;

    [download]

    or:

    my $hashRef = {foo => {map {$_ => undef} @keys}};

    [download]

    for the nested key hash ref version.

    True laziness is hard work
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Re: How to create a hash whose values are undef?
by roubi (Hermit) on Jul 06, 2009 at 02:35 UTC
    Assuming your keys are in @keys: 
    my %hash;
@hash{@keys} = ();

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      But what if I want @keys to be the keys in a second-level hash, like so: 

      $hash = {'foo' => { 'key1 => undef, 'key2' => undef, ...

      [download]
      I am using a hash ref here, but I don't think that should make too much of a difference.
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Re: How to create a hash whose values are undef?
by missingthepoint (Friar) on Jul 06, 2009 at 05:25 UTC

    Thanks GrandFather :)

    Ca3n w2e ple6ase st4op doi5ng th4is?
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Re: How to create a hash whose values are undef?
by Anonymous Monk on Jul 06, 2009 at 03:17 UTC
    Thank you everyone!!
[reply]
      Great that you have figured out "how" to do it. My question is "why"?.

      Perl has the ability to automatically bring a new key into existence when you assign something to it without the need for that key to have been created beforehand - this is true in n-dimensional hash structures also. So in many situations, there is no need to make an "undef" value for a key or even to create that key. Sometimes there is a need ("in many situations" does definitely not mean "in all situations").

      I am just curious as to why you want to do this?

      Update: I am not saying that anything in the other posts is wrong - quite the contrary - there are some excellent formulations of how to do this. Grandfather's stuff looks fantastic to me. I'm just saying that often it is not necessary to set a whole bunch of keys to "undef" in the first place. Maybe in this application it is warranted. But if its not needed, then there can be some simplifications.

[reply]
        If you want to initialise the values to some other value, say zero, for example:
        my %hash;
@hash{@keys} = (0) x @keys;

        [download]
        The LHS is a slice, on the RHS the value, 0, is in parentheses which generates a list, the @keys is used in scalar context to get the correct number of values.
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