How can you find the indices of all elements in an array that has a particular value in ruby?
I.e. if you have an array [2,3,52,2,4,1,2], is there an easier way than to use loops, to get the indices of all 2 in the array? So the answer would be something like [0,3,6], if I am looking for 2.
The answer in Get index of array element faster than O(n) gives the solution if I want to find just one instance of the given element.
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5 Answers
Maybe you can use this:
a = [2, 3, 52, 2, 4, 1, 2] b = a.map.with_index{|k, i| i if k == 2}.compact b # => [0,3,6] Or if you wanna to modify a variable so modify version.
a = [2, 3, 52, 2, 4, 1, 2] a.map!.with_index{|k, i| i if k == 2}.compact! a # => [0,3,6] 
2 Comments
Cary Swoveland
Since it is
b you are building, why use map? You are mapping a to an array that you don't use. Isn't a.each_with_index more direct?
Lukas Baliak
@CarySwoveland I modify usage of map in this case. You have right, i use it "uncorrect" in this case.
Try this one,
arr = [2,3,52,2,4,1,2] output = [] arr.each_with_index do |v,i| if v == 2 output << i end end puts output #=> [0, 3, 6] 
1 Comment
Cyriac Domini Thundathil
Thank you for this. I got a shortened version of this which worked perfectly!
a # => [2, 3, 52, 2, 4, 1, 2] b = [] # => [] a.each_with_index{|i, ind| b << ind if i == 2} # => [2, 3, 52, 2, 4, 1, 2] 
2 Comments
Santhosh
This wont get you the output as per your requirements. Luka's or Rick's answers are what you might be looking for.
Vrushali Pawar
Have you checked array b...? I forgot to paste that output.. check once by your own
a = [2,3,52,2,4,1,2] a.each_index.select { |i| a[i]== 2 } #=> [0, 3, 6] Comments
a.each_with_object([]).find_all {|i, index| i == 2}.map {|i, index| index } I feel like there's still a shortcut though.
Another option might be :
a.each_with_object([]).with_index {|(i, result), index| result << index if i == 2 } 