Is there a way to multiply or add a scalar to the same row or column in each array of a nested array without a loop? For example I would like to multiply all the values in the 2nd column (or row) of each array by 2 for the sample nested array below.
(4 3)(5 3)(3 3)⍴¨⊂⍳15
┌────────┬────────┬─────┐
│ 1 2 3│ 1 2 3│1 2 3│
│ 4 5 6│ 4 5 6│4 5 6│
│ 7 8 9│ 7 8 9│7 8 9│
│10 11 12│10 11 12│ │
│ │13 14 15│ │
└────────┴────────┴─────┘
You can do it without using a :For control structure, but there'll still be an explicit loop in the form of the Each operator (¨):
a ← (4 3)(5 3)(3 3)⍴¨⊂⍳15 ⍝ naming your array
2×@2¨ a ⍝ 2 multiplies at [row] 2 of each
┌────────┬────────┬───────┐
│ 1 2 3│ 1 2 3│1 2 3│
│ 8 10 12│ 8 10 12│8 10 12│
│ 7 8 9│ 7 8 9│7 8 9│
│10 11 12│10 11 12│ │
│ │13 14 15│ │
└────────┴────────┴───────┘
2+@2⍤1¨ a ⍝ 2 adds at [element] 2 row-wise of each
┌────────┬────────┬──────┐
│ 1 4 3│ 1 4 3│1 4 3│
│ 4 7 6│ 4 7 6│4 7 6│
│ 7 10 9│ 7 10 9│7 10 9│
│10 13 12│10 13 12│ │
│ │13 16 15│ │
└────────┴────────┴──────┘
The @ operator work on leading axes, the leading axis of a matrix being containing the rows, so the 2nd cell is the 2nd row. The ⍤1 operator restricts the "view" of +@2 to only ever see rows, which are subarrays of rank 1..