Find indices of array elements that fulfill a condition

Question

I have an array, and I need an array of subscripts of the original array's elements that satisfy a certain condition.

map doesn't do because it yields an array of the same size. select doesn't do because it yields references to the individual array elements, not their indices. I came up with the following solution:

my_array.map.with_index {|elem,i| cond(elem) ? i : nil}.compact

If the array is large and only a few elements fulfill the conditions, another possibility would be

index_array=[] my_array.each_with_index {|elem,i| index_array << i if cond(elem)}

Both work, but isn't there a simpler way?

I guess technically my_array.map.with_index {|elem,i| cond(elem) ? i : nil}.compact could be simplified to my_array.map.with_index {|elem,i| i if cond(elem)}.compact but it is essentially the same — engineersmnky
– engineersmnky, Commented May 30, 2017 at 18:27

Andrey Deineko · Accepted Answer · 2017-05-30 07:06:47Z

4

Nope, there is nothing inbuilt or much simpler that what you already got.

Variation:

my_array.each_with_index.with_object([]) do |(elem, idx), indices| indices << idx if cond(elem) end

edited May 30, 2017 at 7:06

answered May 30, 2017 at 6:55

Andrey Deineko

52.5k11 gold badges119 silver badges151 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Aleksei Matiushkin Over a year ago

1. Consider using with_object rather than inject to use the same array, instead of recreating it on each iteration. 2. Also, do |(elem, idx), indices|.

Jörg W Mittag · Accepted Answer · 2017-05-30 11:37:35Z

3

Another possible alternative:

my_array.select.with_index {|elem, _| cond(elem) }.map(&:last)

answered May 30, 2017 at 11:37

Jörg W Mittag

371k79 gold badges457 silver badges666 bronze badges

Comments

Santhosh · Accepted Answer · 2017-05-30 06:57:19Z

2

You can use Array#each_index with select

arr = [1, 2, 3, 4] arr.each_index.select {|i| arr[i].odd? } # => [0, 2]

answered May 30, 2017 at 6:57

Santhosh

29.3k9 gold badges84 silver badges88 bronze badges

1 Comment

Aleksei Matiushkin Over a year ago

This is way worse than each.with_index.with_object because it performs array index lookup on each iteration.

Collectives™ on Stack Overflow

Find indices of array elements that fulfill a condition

3 Answers 3

1 Comment

Comments

1 Comment

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

1 Comment

Comments

1 Comment

Related