lesson 25 Loops & Iteration · beginner

A loop inside a loop

For each pass of the outer loop, the inner loop runs all the way through. That gives you grids, combinations and every pair.

You'll be able to

Indent a second loop under the first and see it restart on every outer pass
Build one line of a grid per outer pass
Compare every character with every other one

The idea

Some results have two directions: rows and seats in a cinema, sizes and colours of a T-shirt. Put one for inside another. For each pass of the outer loop, the inner loop starts again from its beginning and runs all the way through. 3 rows with 4 seats each means the inner body runs 3 × 4 = 12 times.

Indentation shows which loop a line belongs to. A line under the inner for runs for every seat. A line under the outer for, after the inner loop, runs once per row: the place to finish a row.

Run each example to see what it prints. Running examples doesn't count toward progress.

example-1.py · A seat map

for row in "ABC":
    for seat in range(1, 5):
        print(row + str(seat), end=" ")
    print()

example-2.py · Every size with every colour

variants = 0
for size in "SML":
    for colour in "RB":
        print("TEE-" + size + "-" + colour)
        variants += 1
print(variants, "variants")

When to use it: grids, every combination of two things, or comparing every character with every other one.

Mistakes to avoid:

A value that should carry over between rows (a running count) must be set before the outer loop. Set it inside the outer loop and it restarts for every row.
break inside the inner loop only leaves the inner loop. The outer loop carries on with its next pass.
The work grows fast: 1,000 × 1,000 is a million passes.

Review

The inner loop restarts and runs completely on every pass of the outer loop.
Indentation decides whether a line runs per inner pass, per outer pass, or once at the end.
break only leaves the loop it is in. Next module: strings and text, with indexes, slices and methods.

Practise this