Method to turn an array into a hash

[schwern]

I write this a lot:

my %hash = map { $_ => 1 } @array;

Usually because I want to check if there's an element in @array a lot.

So my %hash = @array->as_hash seems sensible. Should it do anything else?

[exodist]

I think that is too ambuguous. Most perl peopel know that %thing = @thing works to turn an array of key, value pairs into a list. Many might see this method and just assume it is that and think it is pointless.

Even more concerning is that some people not as familiar with perl might think it does that not realising it is so easy on it's own. Truth be told %things = @things sounds to me like an @Array->as_hash too.

perhaps:

@Array->as_hash_keys( $value )
@Array->pair_to( $value )
@Array->hash_map()
@Array->truth_hash()
@Array->present( $want ) # Where the object itself keeps a cached copy of the array mapped to a hash, and re-maps it when the array changes?

[jarich]

@exodist:

my %hash = @array;

is completely different from:

my %hash = map { $_ => 1 } @array;

@schwern: I usually write:

my %hash;
@hash{@array} = ():

which is more verbose, but importantly is different again. Perhaps giving a value is a good idea.

[exodist]

@jarich, please re-read my comment in detail. I know the 2 things are different, that was my whole point. The point was that the name could resonably apply to either and is thus ambiguous.

[notbenh]

I feel like I've solved this already with hash_from_list ( https://github.com/notbenh/random_odds_and_ends/blob/master/Data/Manip.pm ) though it's never been posted to CPAN for exactly the reasons stated here, everyone who needs this already built there own or would likely be confused by the idea.

The code is a bit more verbose then is needed but it's the same idea.

[doherty]

The use case schwern used my %hash = map { $_ => 1 } @array; for is better accomplished with the lovely ~~, which is included in 5.10.0, which perl5i requires... though the semantics changed in 5.10.1... is 5.10.0 enough, or should we require 5.10.1? (or maybe by the time perl5i::3 is ready, will we want to require 5.12.something)

[schwern]

I'd forgotten ~~ does that.

The issue I have with ~~ is it does an O(n) search rather than O(1). However, for most uses of the array2hash technique, the array is so small it probably doesn't matter.

[flowchartsman]

I think it's a fairly useful thing to have under certain circumstances. Like you say, smartmatch is O(n) while using a lookup table is O(1). I would much rather generate a hash if I have, say, a huge list I only need to prepare once but might look through hundreds or even thousands of subsequent times. I'd say that's the main usage for such a method anyway, so why not give it a name that's suggestive of same? Like:

@Array->lookuptable
or
@Array->valuehash ?

[qrazhan]

Hello, I am working on this issue for my GCI task. I ran into something I don't quite understand and was hoping someone could explain it to me. So I added this code to the ARRAY.pm file:

method as_hash{
my @Result = map{ $_ => 1 } @$self;
return wantarray ? @Result : @Result;
}

But when I try to do:
my @arr = ("x", "y", "z");
my %hash = @arr->as_hash;
I get a "Can't call method "as_hash" on unblessed reference" error. Could someone explain what I'm doing wrong?

[schwern]

It works for me™.

There's few possibilities.

• You didn't use perl5i::2 (just covering all the bases)
• You didn't point perl at the new version of the library, usually done with perl -Ilib yourprogram or prove -l foo.t when testing.

[schwern]

There was a lot of debate over minor details of this, but on the principle of "the patch wins", I've merged in @qrazhan's work. It implements a straightforward @array->as_hash.

Now we get to see how/if it's used. There's always 3.0 if it turns out to be not quite right.

Congrats and thanks for your first patch, @qrazhan!