[Omaha.pm] map: make a hash from an array

[Jay Hannah]

On Sep 14, 2005, at 1:55 PM, Andy Lester wrote:
> On Wed, Sep 14, 2005 at 09:46:28AM -0500, Jay Hannah 
> (jhannah at omnihotels.com) wrote:
>> Here's a faster way:
>>
>> my %keepthese = map { $_, 1 } @keepthese;
>
> If you only care about the existence of a given element, and don't care
> if it gets a value, you can assign to a hash slice:
>
>     my %hash;
>     @hash{@keepthese} = ();
>
> If you need them to have a value, you can do this:
>
>     my %hash;
>     @hash{@keepthese} = (1) x @keepthese;

I've never used hash slices... I'll have to do some reading.

> Finally, if you're just checking for existence, and don't really need
> to worry about speed, you can do
>
>     my $exists = grep { $_ eq $searching_for }, @keepthese;

Usually when I'm building a hash from an array I'm doing it for the 
sake of speed. My theory is that when I have unique keys building and 
then performing multiple lookups against a large hash is faster than 
performing multiple greps against a large array.

I use grep when I'm only doing 1 or 2 lookups against an array. My 
theory there being that its more efficient to do that than to build a 
hash that's only going to be used once.

(You need to remove the "," from your grep statement, btw. It's a 
syntax error.)

Thanks!

j