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Nested for Loops in Python: A Loop Inside a Loop

A loop inside a loop, for checking every cell of a grid — and the order that surprises everyone.

  • Lesson 5 of 5
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  • Ages 10+
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The dragon explains

Loops inside loops

  1. The dragon: So far every loop you've written stands alone. Now let's put one loop INSIDE another.

  2. The dragon: This checks every cell of a 2x2 grid — row by row, and within each row, column by column.

    example.pyPython 3
    for row in range(2):    for col in range(2):        print(row, col)
    Output0 0 0 1 1 0 1 1

  3. The dragon: Look at that order closely. row stays 0 for TWO full lines — col runs 0, then 1 — before row ever becomes 1.

    example.pyPython 3
    for row in range(2):    for col in range(2):        print(row, col)

  4. The dragon: That's the whole trick: the INNER loop (col) runs all the way to the end, every single time, before the OUTER loop (row) moves on even once. They never alternate.

  5. The dragon: This works the same way to read real grid contents — grid[row][col] picks row first, then the item at that column inside it.

    example.pyPython 3
    grid = [["A", "B"], ["C", "D"]]for row in range(2):    for col in range(2):        print(grid[row][col])
    OutputA B C D

  6. The dragon: Careful with which variable goes where. grid[row][row] would use row twice by accident — col has to be the one that changes the fastest, inside the brackets on the right.

    example.pyPython 3
    grid = [["A", "B"], ["C", "D"]]for row in range(2):    for col in range(2):        print(grid[row][col])

  7. The dragon: One more thing worth counting: how many times does that innermost print() actually run?

    example.pyPython 3
    for row in range(3):    for col in range(2):        print(row, col)
    Output0 0 0 1 1 0 1 1 2 0 2 1

  8. The dragon: 3 outer passes, 2 inner passes each — that's 3 times 2, not 3 plus 2. Six lines. Nested loops multiply, they don't just add.

Quick check

Check your understanding

Say your answer out loud before reading the marked one. Every wrong option is a mistake real beginners make.

  1. Question 1 of 3

    for row in range(2): for col in range(2): print(row, col) — what's the FIRST value row becomes AFTER col has printed both 0 and 1?

    question.pyPython 3
    for row in range(2):    for col in range(2):        print(row, col)
    • 1 — row only changes once col has finished ALL its values for the current rowCorrect
    • col and row take turns, one step each — so row becomes 1 right after row 0, col 0
    • row never changes — only the innermost variable actually moves

    Why: The inner loop finishes ENTIRELY — every value col can take — before the outer loop moves to its next value. That's why the output is 0 0, 0 1, then 1 0, 1 1: two full inner passes under row=0, then two more under row=1. Nothing alternates.

  2. Question 2 of 3

    grid = [["A", "B"], ["C", "D"]]. Inside the nested loop, a line reads print(grid[row][row]). What actually prints?

    question.pyPython 3
    grid = [["A", "B"], ["C", "D"]]for row in range(2):    for col in range(2):        print(grid[row][row])
    • A, A, D, D — because row is used in both bracket positions, col is never actually usedCorrect
    • A, B, C, D — the same as if col had been used correctly
    • This crashes, because col is never used anywhere

    Why: grid[row][row] locks the second position to whatever row is, so col changing does nothing to this line's result. row=0: grid[0][0] twice -> A, A. row=1: grid[1][1] twice -> D, D. The fix is grid[row][col], so the SECOND position actually follows the inner loop.

  3. Question 3 of 3

    An outer loop runs 4 times. Its inner loop runs 3 times on EACH of those passes. How many times does the innermost print() run in total?

    • 12 — 4 times 3Correct
    • 7 — 4 plus 3
    • 4 — only the outer loop's count matters

    Why: Every one of the outer loop's 4 passes triggers a COMPLETE run of the inner loop's 3 passes — 4 groups of 3, which is 12, not 4+3. This is why nested loops can produce output much longer than either number alone would suggest.

In the game

What you'll practice

Three levels in the game: one guided first try with the shape of the answer already there, then two to solve on your own.

  1. 1

    GuidedYour first try

    This code visits every cell of a 2x2 grid using a loop inside a loop. Before you change anything, just RUN it and watch the order the coordinates come out in.

  2. +2

    PracticeTwo more, on your own

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