How to apply a function to each row

Use rank 1 to apply a function to each row (1-cell) of a matrix:

      M ← 3 4⍴⍳12
      (⌽⍤1) M
 4  3  2  1
 8  7  6  5
12 11 10  9

      (+/⍤1) M
10 26 42

      ({⍵[⍋⍵]}⍤1) M
1  2  3  4
5  6  7  8
9 10 11 12

Warning

Always parenthesise the derived function: (f⍤1) M, not f⍤1 M. Without parentheses, 1 M forms a strand that becomes the rank operand.

See also: Apply to columns, Replace bracket-axis