From: Robert Klemme <shortcutter@...>
Date: 2008-03-25T21:49:08+09:00
Subject: Re: numbers of a record in an array

2008/3/25, Jesús Gabriel y Galán <jgabrielygalan@gmail.com>:
> On Tue, Mar 25, 2008 at 11:23 AM, Robert Klemme
>  <shortcutter@googlemail.com> wrote:
>  > 2008/3/25, Jesús Gabriel y Galán <jgabrielygalan@gmail.com>:
>  > > On Tue, Mar 25, 2008 at 10:09 AM, Bu Mihai <mihai.bulhac@yahoo.com> wrote:
>  >  >  > a have an array array=["a","b","c","a","b"]
>  >  >  >  how can i find the numbers of "a", "b","c" items in the array?
>
> >  >  irb(main):006:0> a = %w{a b a c b a c c}
>  >  >  => ["a", "b", "a", "c", "b", "a", "c", "c"]
>  >  >  irb(main):007:0> h = Hash.new {|h,k| h[k] = 0}
>  >  >  => {}
>  >  >  irb(main):008:0> a.each {|x| h[x] += 1}
>  >  >  => ["a", "b", "a", "c", "b", "a", "c", "c"]
>  >  >  irb(main):009:0> h
>  >  >  => {"a"=>3, "b"=>2, "c"=>3}
>  >
>  >  Here's another
>  >
>  >  irb(main):001:0> a = %w{a b a c b a c c}
>  >
>  > => ["a", "b", "a", "c", "b", "a", "c", "c"]
>  >  irb(main):002:0> a.inject(Hash.new(0)){|cnt,e| cnt[e]+=1; cnt}
>  >
>  > => {"a"=>3, "b"=>2, "c"=>3}
>  >
>  >  Jesús, note that you do not need the block form of Hash.new because of
>  >  the way + works.  It is sufficient to make 0 the default element.
>
>
> Oh, that's right. I'm so used to that form for defaulting to an array :-).
>  You say the reason is because of how + works, but is it not the reason
>  the fact that 0 is an immediate value
>  of which there's only one instance? So having the same reference to
>  the 0 object works, whereas having the same
>  reference to an array (for example) wouldn't achieve the desired effect?

No, this has nothing to do with immediate values.  The reason why this
works is that an operation is used that creates a new instance
(Fixnum#+ in this case) and uses assignment to place a new value on
every iteration.  This also works with arrays and other non immediate
and even mutable instances:

irb(main):001:0> collector = Hash.new []
=> {}
irb(main):002:0> 10.times {|i| collector[i%3] += [i]}
=> 10
irb(main):003:0> collector
=> {0=>[0, 3, 6, 9], 1=>[1, 4, 7], 2=>[2, 5, 8]}

You can even freeze the default object

irb(main):007:0> collector = Hash.new([].freeze)
=> {}
irb(main):008:0> 10.times {|i| collector[i%3] += [i]}
=> 10
irb(main):009:0> collector
=> {0=>[0, 3, 6, 9], 1=>[1, 4, 7], 2=>[2, 5, 8]}

Kind regards

robert


-- 
use.inject do |as, often| as.you_can - without end