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\', \'.\', \'.\         


        

Answer 1

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)