Find indices of elements that match a given condition
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?
Solution 1:
Ruby 1.9:
arr = ['x', 'o', 'x', '.', '.', 'o', 'x']
p arr.each_index.select{|i| arr[i] == 'x'} # =>[0, 2, 6]
Code
Solution 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)
Solution 3:
A slight improvement over your each_with_index.map
line
arr.map.with_index {|a, i| a == 'x' ? i : nil}.compact # => [0, 2, 6]
Solution 4:
This methods is a bit longer but double as fast
class Array
def find_each_index find
found, index, q = -1, -1, []
while found
found = self[index+1..-1].index(find)
if found
index = index + found + 1
q << index
end
end
q
end
end
arr = ['x', 'o', 'x', '.', '.', 'o', 'x']
p arr.find_each_index 'x'
# [0, 2, 6]
Here the benchmark of AGS campared with this solution
arr = 10000000.times.map{rand(1000)};
puts Benchmark.measure{arr.each_with_index.map { |a, i| a == 50 ? i : nil }.compact}
puts Benchmark.measure{(0..arr.size-1).select { |i| arr[i] == 50 }}
puts Benchmark.measure{arr.map.with_index {|a, i| a == 50 ? i : nil}.compact}
puts Benchmark.measure{arr.each_index.select{|i| arr[i] == 50}}
puts Benchmark.measure{arr.size.times.select {|i| arr[i] == 50}}
puts Benchmark.measure{arr.find_each_index 50}
# 1.263000 0.031000 1.294000 ( 1.267073)
# 0.843000 0.000000 0.843000 ( 0.846048)
# 0.936000 0.015000 0.951000 ( 0.962055)
# 0.842000 0.000000 0.842000 ( 0.839048)
# 0.843000 0.000000 0.843000 ( 0.843048)
# 0.405000 0.000000 0.405000 ( 0.410024)