From: "David A. Black" <dblack@...>
Date: 2008-07-01T21:48:47+09:00
Subject: Re: Hash#select returns an array but Hash#reject returns a hash...

Hi --

On Tue, 1 Jul 2008, Srijayanth Sridhar wrote:

>>
>> that will change in newer version of ruby, eg ruby1.9
>>
>> irb(main):015:0> RUBY_VERSION
>> => "1.9.0"
>> irb(main):016:0> a=Hash.new
>> => {}
>> irb(main):017:0> a[1]=2
>> => 2
>> irb(main):018:0> a[2]=2
>> => 2
>> irb(main):019:0> a[3]=4
>> => 4
>> irb(main):020:0> a.select { |key,value| value > 2 }
>> => {3=>4}
>> irb(main):021:0> a.reject { |key,value| value <= 2 }
>> => {3=>4}
>>
>> kind regards -botp
>>
>>
>
> Thanks.
>
> So this is just some sort of artefact/legacy code that never got changed?

You can actually make a case that select and reject are not exactly
symmetrical operations. Imagine a line of people:


     Joe John Joan David Jim Jenny Jeff Matz

If I tell everyone in the line whose name does not begin with J to
step backwards (reject), the original line is smaller but it's still
the same line.

If I tell everyone whose name *does* begin with J to step forwards
(select), I've got a new line of J people.

"Same line" and "new line" don't necessarily map to "same object" and
"new object" in Ruby (since the post-reject hash is a different hash).
But it suggests that there's a difference, arguably, between select
and reject, in terms of the formal disturbance of the object, which in
turn makes it easier to understand why select would return objects in
a different container, while reject would leave the container in the
same form but just contain fewer things.

However, that doesn't mean it's bad for select to return a hash (which
it does, as was mentioned, as of 1.9). Just that the behavior of one
doesn't necessarily imply the behavior of the other.


David

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