Nested loops in C - Step-by-Step Execution

Concept Flow - Nested loops

Start outer loop i=0

↓

Check outer condition i < 2?

No→EXIT

Yes↓

Start inner loop j=0

↓

Check inner condition j < 3?

No→End inner loop

Yes↓

Execute inner loop body

↓

Update inner loop j = j + 1

Back to inner condition↓

End inner loop

↓

Update outer loop i = i + 1

↩Back to inner condition

The outer loop runs first. For each outer loop iteration, the inner loop runs completely. Then the outer loop moves to the next iteration.

Execution Sample

C

for (int i = 0; i < 2; i++) {
    for (int j = 0; j < 3; j++) {
        printf("%d %d\n", i, j);
    }
}

Print pairs of i and j where i runs 0 to 1 and j runs 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	Exit inner loop
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	Exit inner loop
9	2	-	False	-	Exit outer loop

💡 Outer loop i=2 fails condition i<2, so loops end.

Variable Tracker

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

Key Moments - 3 Insights

Why does the inner loop restart j from 0 for each i?

Because the inner loop variable j is initialized inside the outer loop. Each time the outer loop runs, j starts fresh at 0 (see execution_table rows 1 and 5).

Why does the outer loop variable i only increase after the inner loop finishes?

The outer loop waits for the inner loop to complete all its iterations before increasing i (see execution_table rows 4 and 5). This is how nested loops work.

What happens when j reaches 3?

The inner loop condition j<3 becomes false, so the inner loop stops and control goes back to the outer loop to increase i (see execution_table rows 4 and 8).

Visual Quiz - 3 Questions

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

A 0

B 2

C 1

D 3

💡 Hint

Check the 'j' column in execution_table row 6.

At which step does the outer loop condition become false?

A Step 8

B Step 9

C Step 4

D Step 1

💡 Hint

Look at the 'Condition i<2' column and find where it is False.

If the inner loop condition changed to j < 2, how many total prints would occur?

A 4

B 6

C 3

D 2

💡 Hint

Multiply outer loop iterations (2) by inner loop iterations (2).

Concept Snapshot

Nested loops run one loop inside another.
Outer loop runs first.
Inner loop runs fully for each outer iteration.
Inner loop variable resets each time.
Useful for grids, tables, repeated pairs.

Full Transcript

This example shows nested loops in C. The outer loop variable i runs from 0 to 1. For each i, the inner loop variable j runs from 0 to 2. Each pair i,j is printed. The inner loop resets j to 0 each time the outer loop runs. When j reaches 3, the inner loop stops and the outer loop increases i. When i reaches 2, the outer loop stops. This pattern is common to repeat actions in a grid or matrix style.