Question

If I have an array like

ary = [0, 0, 3, 0, 0, 0, 2, 0, 1, 0, 1, 1, 0] 

What is the most performant way to get a list of how many indexes were in the array?

inverted = [2,2,2,6,6,8,10,11] 

This is what I’ve come up with, but it seems like there is a more efficient way:

a = []
ary.each_with_index{|v,i| a << Array.new(v, i) if v != 0}
a.flatten
 => [2, 2, 2, 6, 6, 8, 10, 11] 

Answer 1

Unless profiling proves this to be a bottleneck, the cleaner is a functional approach:

>> ary.each_with_index.map { |x, idx| [idx]*x }.flatten(1)
=> [2, 2, 2, 6, 6, 8, 10, 11]

If you use Ruby 1.9, I’d recommend this (thanks to sawa for pointing out Enumerable#flat_map):

>> ary.flat_map.with_index { |x, idx| [idx]*x }
=> [2, 2, 2, 6, 6, 8, 10, 11]

[edit: removed examples using inject and each_with_object, it’s unlikely they are faster than flat_map + with_index]