Extract disjoint columns from Matrix?

Question

Consider a matrix

(mat = Array[a, {5, 5}]) // MatrixForm

If I want to extract columns 2 through 4 which would return a 5x3 matrix, I can do that with:

mat[[;; , 2 ;; 4]] // MatrixForm

But what if I want to extract only column 2 and 4 (without column 3), so that I get a 5x2 matrix? Is there a short syntax to do that?

Answer 1

Rows 2 and 4 can be extracted with mat[[{2, 4}]].

Columns 2 and 4 can be extracted with mat[[All, {2, 4}]] or equivalently mat[[;;, {2, 4}]], or even Part[mat, All, {2, 4}].