From: Brian Candler <B.Candler@...>
Date: 2005-07-13T19:35:05+09:00
Subject: Re: accessing index inside map

> [1];  obj.enum(:each_with_index).inject(0) {|sum,(a,i)| sum + a*i}
> 
> [2];  i=-1; obj.inject(0) {|sum, a| sum + a*(i+=1)}
> 
> Can anyone shade me in on what, if any, advantage [1] has over [2]:
> -- in terms of:
> 
>  a) Readability
>  b) Ease of use and understanding
>  c) Performance
>  d) Flexibility
>  e) Accuracy
>  f) Memory usage
>  g) Anything else you care to throw in  ;-)

1. It retains the enapsulation of "iterating with an index"; you are
reimplementing it in your inner block. (Violation of DRY principle).

2. It's more general; it works with other iteration methods. Consider
Hash#each_value and Hash#each_key, or String#each_byte for example.

    require 'enumerator'
    "abc".to_enum(:each_byte).collect    #=> [97, 98, 99]

Of course you could rewrite this too, but then you're reimplementing
'collect':

    res = []
    "abc".each_byte { |x| res << x }
    res

3. If you were using the index in more than one place in the inner block, it
would become more verbose.

    i=0; obj.inject(0) { ... use i ... use i ...; i+=1 }

Regards,

Brian.