Nested loops and patterns

A loop inside another loop is a nested loop. For every round of the outer loop, the inner loop runs completely.

for row in range(1, 4):
    for col in range(1, 4):
        print(row * col, end=" ")
    print()            # new line after each row

Output:

1 2 3
2 4 6
3 6 9

Pattern questions (stars, number triangles) are very common in coding tests. The trick: the outer loop is the row, the inner loop prints what goes in that row.

for i in range(1, 4):
    print("*" * i)