Course 1 · Zero to codingLesson 6 of 10Free to read

Loops inside loops · Robot Quest

What if the repeating part itself contains a repeat?

Why you need it

Many tasks repeat at two levels: three rows, and in each row, four seats; four sides of a square, and on each side, several steps. A nested loop (a loop inside another loop) describes exactly this.

The intuition

A clock: the minute hand goes round 60 times for every one turn of the hour hand. The inner loop finishes completely before the outer loop moves on.

for side in range(4):       # outer: 4 sides
    for step in range(3):   # inner: 3 steps per side
        move()
    turn_right()            # once per side

move() runs 4 × 3 = 12 times. turn_right() belongs to the outer loop (it is indented once), so it runs 4 times. Indentation tells you which loop a line belongs to.

Definition

Nested loop

A nested loop is a loop placed inside the body of another loop; the inner loop runs completely for each round of the outer loop.

Examples

Three groups of three

Run it in the lesson

The outer loop counts groups; the inner loop handles the gems in one group.

for group in range(3):
    for gem in range(3):
        move()
        pick()
    move()

Step by step

L1Outer round 1 (group = 0).group = 0
L3The inner loop runs 3 full rounds: move, pick, three times.gem = 0, 1, 2
L5Back in the outer body: one move over the empty square.group = 0
L1Outer round 2 starts the inner loop again from gem = 0.group = 1
•After 3 outer rounds: 9 gems picked and Bit is on the flag.

Around the pillar

Run it in the lesson

Three sides, two steps each, a right turn after every side.

for side in range(3):
    for step in range(2):
        move()
    turn_right()

Step by step

L3Side 1: two moves east.
L4Turn right once, at the end of the side.
L3Side 2: two moves south.
L3Side 3: two moves west, onto the flag.

Common mistakes

Wrong indentation level

A line indented 8 spaces is in the inner loop; 4 spaces is in the outer loop. Moving a line one level changes how often it runs.

Reusing the same counter name

Give the two loops different names (side, step). Using i for both is confusing and causes bugs later when you use the counters.

Quick quiz

1. How many times does move() run?

1
2
3

for a in range(2):
    for b in range(3):
        move()

Show the answer

Answer: B

The inner loop runs 3 times for each of the 2 outer rounds: 2 × 3 = 6.

2. Where does Bit end up?

for side in range(3):
    for step in range(2):
        move()
        turn_right()

AOn the flag
BIt crashes on line 3
CTop-right corner

Show the answer

Answer: B

turn_right() is inside the inner loop, so Bit turns after every step and walks into the pillar.

Practice

3 exercises in three steps: predict what a program prints, change a program to meet a new requirement, then write one from scratch. Your code runs in the browser and is checked the moment you submit.

PredictTwo sides
Predict where Bit ends up.
ModifyBigger groups
The new map has 2 groups of 4 gems. Change the numbers so Bit collects them all and reaches the flag.
WriteGem ring
Collect all 9 gems around the block, then step onto the flag. Use a nested loop, at most 6 lines.

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Terms in this lesson

Term	Chinese	Definition
Nested loop	嵌套循环	A loop inside the body of another loop.
Inner loop	内层循环	The loop inside; it runs completely for each round of the outer loop.
Outer loop	外层循环	The loop that contains another loop.