Nested while loops in Python - Step-by-Step Execution

Concept Flow - Nested while loops

Initialize outer variable i=0

↓

Check outer condition i < 2?

No→EXIT

Yes↓

Initialize inner variable j=0

↓

Check inner condition j < 3?

No→Increment i

Yes↓

Execute inner loop body

↓

Increment j

Back to inner condition↓

Increment i

↩Back to inner condition

The outer loop runs while i < 2. For each i, the inner loop runs while j < 3. Inner loop resets j each time.

Execution Sample

Python

i = 0
while i < 2:
    j = 0
    while j < 3:
        print(i, j)
        j += 1
    i += 1

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

Execution Table

Step	i	j	Outer condition (i<2)	Inner condition (j<3)	Action	Output
1	0	-	True	-	Initialize j=0
2	0	0	True	True	Print (0,0), j+=1	(0, 0)
3	0	1	True	True	Print (0,1), j+=1	(0, 1)
4	0	2	True	True	Print (0,2), j+=1	(0, 2)
5	0	3	True	False	Inner loop ends, i+=1
6	1	-	True	-	Initialize j=0
7	1	0	True	True	Print (1,0), j+=1	(1, 0)
8	1	1	True	True	Print (1,1), j+=1	(1, 1)
9	1	2	True	True	Print (1,2), j+=1	(1, 2)
10	1	3	True	False	Inner loop ends, i+=1
11	2	-	False	-	Outer loop ends

💡 At step 11, i=2, outer condition i<2 is False, so loop ends.

Variable Tracker

Variable	Start	After 1	After 2	After 3	After 4	After 5	After 6	After 7	After 8	After 9	After 10	Final
i	0	0	0	0	0	1	1	1	1	1	2	2
j	-	0	1	2	3	-	0	1	2	3	-	-

Key Moments - 3 Insights

Why does j reset to 0 each time the outer loop runs?

Because at each outer loop iteration (see steps 1 and 6), j is set to 0 before the inner loop starts. This is shown in the execution_table where j is '-' before initialization and then 0 after.

When does the inner loop stop running?

The inner loop stops when j reaches 3, making the condition j < 3 false (see steps 5 and 10). Then control goes back to the outer loop.

Why does the outer loop run only twice?

Because the outer loop condition is i < 2. When i becomes 2 at step 11, the condition is false and the loop ends.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What are the values of i and j?

A i=0, j=2

B i=1, j=2

C i=0, j=3

D i=1, j=3

💡 Hint

Check the 'i' and 'j' columns in execution_table row with Step 4.

At which step does the outer loop condition become false?

A Step 10

B Step 11

C Step 5

D Step 6

💡 Hint

Look at the 'Outer condition' column in execution_table and find where it is False.

If we change the outer loop condition to i < 3, how many times will the inner loop run in total?

A 6 times

B 3 times

C 9 times

D 2 times

💡 Hint

Each outer loop runs inner loop 3 times; with i < 3, outer runs 3 times, so 3*3=9.

Concept Snapshot

Nested while loops:
- Outer loop runs while condition true
- Inner loop runs fully for each outer iteration
- Inner loop variable resets each outer iteration
- Use indentation to show nesting
- Loop ends when outer condition false

Full Transcript

This example shows nested while loops in Python. The outer loop variable i starts at 0 and runs while i < 2. For each i, the inner loop variable j starts at 0 and runs while j < 3. Each inner loop prints the pair (i, j). After inner loop ends, i increments. The loops stop when i reaches 2. Variables i and j change as shown in the variable tracker. Key points: j resets each outer loop, inner loop runs fully each time, outer loop runs twice total.