Nested loops

Rows and columns

Remember end from lesson 0.3? With end="", print doesn't start a new line after what it prints. Predict what this draws.

for row in range(3):
    for col in range(4):
        print("*", end="")
    print()

****
****
****

The outer loop makes rows; the inner loop fills each row. The empty print() belongs to the outer loop, not the inner one: it's indented under for row, so it runs once per row, after the inner loop has finished. Indentation decides which loop a line belongs to, and Python is strict about it:

The inner loop starts again

The key idea: for every round of the outer loop, the inner loop runs completely, from its start. Step through it and watch col go back to 0 when row changes.

0 0
0 1
0 2
1 0
1 1
1 2

row changes slowly, col quickly, like the hour and minute hands of a clock. Every pair of values appears exactly once.

A times table

Use the row and column numbers themselves. With :>3 from lesson 1.4, every number takes three characters, so the columns line up:

for row in range(1, 10):
    for col in range(1, 10):
        print(f"{row * col:>3}", end="")
    print()

Try :>2 to see why the width matters, then put it back.

break leaves one loop

break leaves only the loop it's directly inside. Predict:

for row in range(3):
    for col in range(3):
        if col == 1:
            break
        print(row, col)

0 0
1 0
2 0

If you expected only 0 0, you pictured break leaving both loops. It doesn't: it ends the inner loop, and the outer one goes on.

Why nested loops get slow

The rounds multiply. A 10 × 10 grid is 100 rounds, which is nothing. But comparing each of 1,000 students with every student is 1,000 × 1,000 = a million rounds, and 10,000 students is a hundred million. When a program gets slow, a loop inside a loop is the first place to look.