in reply to Extract hash keys for values stoted in array

Based on the little you've told us, you could do:


...
   my @selectedKeys = grep{ $hash{ $_ } eq $scalar } keys %hash;
...

[download]

But doing that, ie. looking up keys in a hash from the values, defeats the purpose of using a hash.

The chances are that you should really be building the hash the other way around.

Instead of 

$hash{ $key } = $value;

[download]
you should probably be doing something like 
push @{ $hash{ $value } }, $key;

[download]
.

Then, when you need the 'keys' associated with a particular 'value' you find them directly:

...
   my @selectKeys = @{ $hash{ $scalar } };

[download]

thus avoiding a potentially costly linear search.

If you were to describe the bigger picture of your code, and post the code you have so far, rather than just this tiny fragment of requirement, then you'd probably get not just a solution to the small problem, but good advice on solving the bigger problem more effectively.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.

Replies are listed 'Best First'.
Re^2: Extract hash keys for values stoted in array
by onlyIDleft (Scribe) on May 01, 2011 at 22:20 UTC

    So my general problem is one in bioinformatics: given the entire proteome of a species, where each gene can be spliced into multiple protein isoforms of different lengths, pick only the longest protein isoform for each gene

    In other words, if there are genes A, B, C etc.

    with respectively 3, 1 and 2 protein isoform(s)

    a.1, a.2, a.3, b.1, c.2, c.5, c.7, and

    of respective lengths 12, 11, 12, 15, 34, 12, 45, and

    and with their corresponding peptide sequences,

    then, I want the PERL script to return ONLY a.1 or a.3 for gene A and its corresponding sequence, b.1 and its sequence for gene B, c.7 and its sequence for gene C... you get the idea?

    Your suggested syntax using grep worked, except that it returned all the matches into an array, so I used shift to gather the first match (which for my purposes is the same as any other matches, if multiple keys are present as matches to my values in the array)

    Thanks for the useful syntax, I had not come across it yet in the Beginning Perl 3rd edition book, and its only been 10 days since I started teaching myself PERL! So your help is much appreciated...

    I can this my "non-redundification" PRL script that removes protein isoform redundancy by selecting ONLY the longest isoform for each gene in a proteome. My script is ~100 lines long, which I think would be a joke for you Monks! But hey, I am just a Padawan learner as of now! :)

    Thanks again to both of you!

[reply]

      If all you want is one, then I'd suggest List::Util::first().

      Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
      "Science is about questioning the status quo. Questioning authority".
      In the absence of evidence, opinion is indistinguishable from prejudice.
[reply]
[d/l]
        Cool, thanks! :)
[reply]

In Section Seekers of Perl Wisdom