Lessons · Python · loop inside a loop

A loop inside a loop multiplies

If the outer loop runs n times and the inner runs n times, the body runs n times n times. 1,000 becomes a million.

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What it is for

This is how a feature that works on the demo data times out on real data. Recognising the shape lets you fix it before it ships.

How to think about it

When you write a loop inside a loop over the same data, ask: is the inner loop searching? If so, replace it with a dict or set lookup. If both loops are needed (a grid), accept it and keep the body tiny.

Worked example

for a in names:

n names.

    for b in names:

n again: n times n comparisons.

        if a == b: ...

Searching for a match is what dicts are for.

seen = set(names)  # instead

One pass to build, one step per lookup: n, not n times n.

Your turn

Every cell of a 3x3 grid needs visiting. Which loops?

for row in range(3):
    for col in range():
        visit(row, col)

The trap

Calling something inside the inner loop that itself loops (like list.index or 'in list'). Three loops hiding as two.

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