1 of 2

APL 29


This is a nice question for the key operator (⌸) which was introduced with Dyalog APL v14. It takes the left argument function ({1↑1↓⍵}) and gives it for each unique argument, the indices in the vector for that argument. Here I'm taking the second index, then i check which of the indices is present in this list ((⍳⍴⍵)∊) and use the resulting boolean for splitting the original vector.

Can be tried online here: