RCR 284: Let Array#index take a block

I think it is a frequent task to find the index of an element of an Array that matches a specific condition. Currently this can only be done easily in two passes.

Array.index() can currently only compare Objects directly, but finding the index where a condition that is not only a simple object equality check has currently to be done manually. This is for example a problem when trying to find the index of a Float:

  items = [1.5, 4.9, 1.3, 8.4]
  items.index(2.2 - 0.7) # => nil
  items.find_all { |item| (item - (2.2 - 0.7)).abs < 1e-10 }.map { |item| items.index(item) } # => [0]

I think it should be possible to express this like this:

  items.index { |item| (item - (2.2 - 0.7)).abs < 1e-10 } # => [0]

It can be argued over whether this should return an Array of indices or only the first matching index.

The direct object comparisation form of Array#index currently only returns the first result, but with blocks I think it is more useful to return all of them.

In Ruby: (Note that this doesn't give the correct indices if multiple matching elements are equal and that it doesn't give you a performance advantage)

  class Array
    alias :old_index :index
  def index(*args, &block)
      if not args.empty?
        old_index(*args)
      else
        result = find_all(&block).map { |item| old_index(item) }
        return nil if result.empty?
        return result
      end
    end
  end