FInd location of element in a vector

Question

I'm new to APL and I would like to find the position of an element(s) within a vector. For example, if I create a vector of 50 random numbers:

lst ←  50 ? 100

How can I find the positions of 91 assuming it occurs 3 times in the vector?

Thanks.

Answer 1

I'm not an expert, but a simple way is to just select the elements from ⍳ 100 where the corresponding element in lst is 91

(lst=91)/⍳100

Answer 2

With Dyalog 16.0, you can use the new monadic function ⍸ "Where".

⍸lst=91

lst=91 gives a vector of 0s and 1s. Applying ⍸ on this gives the locations of all the 1s. This also works if lst is a matrix.