From: Logan Capaldo <logancapaldo@...>
Date: 2005-05-26T01:24:28+09:00
Subject: Re: loop over array with indices

On 5/24/05, Brian Schr�der <ruby.brian@gmail.com> wrote:
> On 23/05/05, Berger, Daniel <Daniel.Berger@qwest.com> wrote:
> > > -----Original Message-----
> > > From: Johannes Ahlmann [mailto:softpro@gmx.net]
> > > Sent: Monday, May 23, 2005 3:15 PM
> > > To: ruby-talk ML
> > > Subject: loop over array with indices
> > >
> > >
> > > hi,
> > >
> > > i have the recurring problem that i want to loop over an
> > > array with each/collect/... but (as an afterthought) need the
> > > index of the current element.
> >
> > See Enumerable#each_with_index.
> >
> > Regards,
> >
> > Dan
> >
> >
> 
> And define yourself map_with_index, inject_with_index etc. where
> needed. I always miss them.
> 
> best regards,
> 
> Brian
> 
> 
> --
> http://ruby.brian-schroeder.de/
> 
> Stringed instrument chords: http://chordlist.brian-schroeder.de/
> 
> 

require 'enumerator'

arr = [ "a", "b", "c", "d" ]

arr_iter = Enumerable::Enumerator.new(arr, :each_with_index)

arr_iter.each do |x, i|
     puts "arr[#{i}] => #{x}"
end


#=> 
arr[0] => a
arr[1] => b
arr[2] => c
arr[3] => d

arr_iter.collect { |x, i| x[0] + i } #=> [97, 99, 101, 103]

etc. Any methods defined in enumerable should work including each,
map, inject, etc.
inject's args may look a little weird depending on how you call it
ie:

arr_iter.inject("q") { |a, b| a << b[0] << b[1].to_s }
#=> "qa0b1c2d3"

arr_iter.inject { |a, b| (a.kind_of?(Array) ?  a[0] + a[1].to_s : a) +
b[0] + b[1].to_s  }

#=> "a0b1c2d3"

You may want to write your own inject for this.

require 'enumerator'
module Enumerable
        def inject_with_index(*args)
              i = Enumerable::Enumerator.new(self, :each_with_index)
              if args.empty?
                i.inject do |a, b|
                       yield(a[0], a[1], b[0], b[1])
                end
             else
                 i.inject(args[0]) do |a, b|
                        yield(a, b[0], b[1])
                 end
             end
          end
end

Then you can call it less weirdly:

["a", "b", "c"].inject_with_index("") { |a, b, i| a << "#{b}@#{i}" }

#=> "a@0b@1c@2"

["a", "b", "c"].inject_with_index do |a, i1, b, i2|
    [a + b, i1 + i2]
end

#=> ["abc", 3]