Get indices for sorted permutation of an array in Ruby?

Question

Let's say I have an Array ary = [0.0, 1.0, 5.0, 1.0, -2.0, 3.5], and I want as output another array of the same size containing ary's indices in sorted-by-value-order. In other words, the output should be [4,0,1,3,5,2]. Is there an efficient way to do this with Enumerable or Array?

The most trivial solution I can imagine is as follows:

class Array
  def sorted_indices
    self.map.with_index{ |v,i| [v,i] }.sort{ |a,b| a[0] <=> b[0] }.map { |v| v[1] }
  end
end

but I feel like there has to be something simpler already built in.

It's important to note that the values in the array are not unique. No index should appear in the result array more than once (in other words, [4,0,1,1,5,2] is not correct).

Answer 1

ary.each_index.sort_by{|i| ary[i]}
# => [4, 0, 1, 3, 5, 2]