matrix

a = [
    [1, 2, 3, 4],
    [5, 6, 7, 8],
    [9, 10, 11, 12],
    [13, 14, 15, 16],
]

for r in range(len(a)): # 0
    for c in range(len(a)): # 0 1 2 3
        print(a[r][c])