From: Fredrik <fredjoha@...>
Date: 2009-04-30T12:25:06+09:00
Subject: Re: Hash#map

On 29 Apr, 21:39, James Gray <ja...@grayproductions.net> wrote:
> On Apr 29, 2009, at 1:00 AM, Fredrik wrote:
>
> > Is there any good reason why Hash#map does not give back a Hash (Ruby
> > 1.8 but same in 1.9 as far as I know)?
>
> It wouldn't make sense for Hash#map to return a Hash since you are not  
> required to transform it into key-value pairs:
>
>  >> {1 => "one", 2 => "two"}.map { |k, v| "#{k} is #{v}" }
> => ["1 is one", "2 is two"]
>
> > I often find myself writing
> > these kind of things:
>
> > newhash = oldhash.inject({}) { |h,(k,v)| h[k] = some_operation(v); h }
>
> 1.9 adds each_with_object() for this very usage.  You don't want the  
> return value of the block to carry forward, but instead the object you  
> started with.  That's what each_with_object() does.  So your code  
> becomes the following in Ruby 1.9:
>
> newhash = oldhash.each_with_object({}) { |(k, v), h|
>    h[k] = some_operation(v)
>
> }
>
> Hope that helps.
>
> James Edward Gray II

Thanks! It's not a very big improvement over inject though. I don't
see the use of Hash#map -> Array so if it was up to me, Hash#map would
give a Hash. If one really wants an Array it seems more reasonable to
use something like
 hash.to_a{|k,v| "#{k} is #{v}" }

Maybe it's just me, but when I read "map" I am thinking "map this
block to each element and give back the same structure" (i.e.
Array#map gives an Array and Hash#map gives a Hash).

(I know map is just an alias for collect, but I never understood why
"collect" is a good name for this method.)