Find indices of elements that match a given condition

Question

Given an array, how can I find all indices of elements those match a given condition?

For example, if I have:

arr = ['x', 'o', 'x', '.', '.', 'o', 'x'] 

To find all indices where the item is x, I could do:

arr.each_with_index.map { |a, i| a == 'x' ? i : nil }.compact   # => [0, 2, 6] 

or

(0..arr.size-1).select { |i| arr[i] == 'x' }   # => [0, 2, 6] 

Is there a nicer way to achieve this?

Answer 1

Ruby 1.9:

arr = ['x', 'o', 'x', '.', '.', 'o', 'x'] p arr.each_index.select{|i| arr[i] == 'x'} # =>[0, 2, 6] 

Code

Answer 2

Another way:

arr.size.times.select {|i| arr[i] == 'x'} # => [0, 2, 6] 

EDIT:

Not sure if this is even needed, but here they are.

Benchmarks:

arr = 10000000.times.map{rand(1000)};  Benchmark.measure{arr.each_with_index.map { |a, i| a == 50 ? i : nil }.compact} 2.090000   0.120000   2.210000 (  2.205431)  Benchmark.measure{(0..arr.size-1).select { |i| arr[i] == 50 }} 1.600000   0.000000   1.600000 (  1.604543)  Benchmark.measure{arr.map.with_index {|a, i| a == 50 ? i : nil}.compact} 1.810000   0.020000   1.830000 (  1.829151)  Benchmark.measure{arr.each_index.select{|i| arr[i] == 50}} 1.590000   0.000000   1.590000 (  1.584074)  Benchmark.measure{arr.size.times.select {|i| arr[i] == 50}} 1.570000   0.000000   1.570000 (  1.574474)