Nested for loop execution in Python - Step-by-Step Execution

Concept Flow - Nested for loop execution

Start outer loop i=0

↓

Start inner loop j=0

↓

Execute inner loop body

↓

Update inner loop j=j+1

↓

Check inner loop j < limit?

No→End inner loop

Yes↓

Update outer loop i=i+1

↓

Check outer loop i < limit?

No→End outer loop

↩Back to inner loop body

The outer loop starts and for each iteration, the inner loop runs completely before the outer loop moves to the next iteration.

Execution Sample

Python

for i in range(2):
    for j in range(3):
        print(i, j)

Print pairs of i and j where i goes from 0 to 1 and j goes from 0 to 2 for each i.

Execution Table

Step	i	j	Condition i < 2	Condition j < 3	Action	Output
1	0	0	True	True	Print (0,0)	(0, 0)
2	0	1	True	True	Print (0,1)	(0, 1)
3	0	2	True	True	Print (0,2)	(0, 2)
4	0	3	True	False	Inner loop ends
5	1	0	True	True	Print (1,0)	(1, 0)
6	1	1	True	True	Print (1,1)	(1, 1)
7	1	2	True	True	Print (1,2)	(1, 2)
8	1	3	True	False	Inner loop ends
9	2	0	False	N/A	Outer loop ends

💡 Outer loop i=2 is not less than 2, so the loop ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	Final
i	N/A	0	0	0	0	1	1	1	2
j	N/A	0	1	2	3	0	1	2	3

Key Moments - 3 Insights

Why does j reset to 0 after the inner loop ends?

Because the inner loop starts fresh for each new i in the outer loop, as shown in steps 4 to 5 in the execution_table where j goes from 3 (end) back to 0.

Why does the outer loop increment only after the inner loop finishes?

The outer loop waits for the inner loop to complete all its iterations before increasing i, as seen between steps 4 and 5 where inner loop ends before i increments.

What happens when the inner loop condition j < 3 becomes false?

The inner loop stops running and control returns to the outer loop to increment i, as shown in steps 4 and 8 where j=3 causes inner loop to end.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of j at step 6?

A 2

B 0

C 1

D 3

💡 Hint

Check the 'j' column in execution_table row for step 6.

At which step does the outer loop condition become false?

A Step 9

B Step 8

C Step 4

D Step 3

💡 Hint

Look at the 'Condition i < 2' column in execution_table to find when it is False.

If the inner loop range changed to 2, how many total print outputs would there be?

A 3

B 4

C 6

D 2

💡 Hint

Multiply outer loop count (2) by new inner loop count (2) to find total prints.

Concept Snapshot

Nested for loops run one loop inside another.
Outer loop runs first, then inner loop runs fully each time.
Syntax: for i in range(...): for j in range(...):
Inner loop resets each outer loop iteration.
Useful for grids, tables, or combinations.

Full Transcript

This visual execution shows how nested for loops work in Python. The outer loop variable i starts at 0 and runs while less than 2. For each i, the inner loop variable j runs from 0 to 2. Each step prints the current i and j values. When j reaches 3, the inner loop ends and i increments. When i reaches 2, the outer loop ends. Variables i and j change as shown in the variable tracker. Key moments explain why j resets and how loops end. The quiz tests understanding of variable values and loop conditions.