How to apply a function to each column

Select a column with From

Use ⌷ (From) with rank 0 1 to select a column by index:

      M ← 3 4⍴⍳12
      2(⌷⍤0 1)M
2 6 10

This selects column 2 from each row (1-cell) of M.

Transpose, operate, transpose back

For arbitrary functions, transpose the matrix, operate on rows, then transpose back:

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

See also: Apply to rows, Replace bracket-axis