From: Christopher Dicely <cmdicely@...>
Date: 2008-08-25T11:14:16+09:00
Subject: Re: indices of non nil elements in an array

On Sun, Aug 24, 2008 at 6:20 PM, Patrick Doyle <wpdster@gmail.com> wrote:
> a = []
> a[10] = "foo"
>
> a.nitems returns a count of the non-nil, items. What is the simplest
> way to return the indices of "a" that are not nil?
>
> I'm sure there is some simple elegant way to do this, but I can't figure it out.

If you need this frequently, you can monkeypatch Array to offer this
functionality; like Array#detect but gathers the indexes instead of
the values, and gathers indexes of non-nil values where no block is
given:

class Array
  def detect_indices
    result = []
    each_with_index { |val, idx| (block_given? ? yield(val) : (not
val.nil?)) && result << idx }
    result
  end
end