Nested for loop in Java - Step-by-Step Execution

Concept Flow - Nested for loop

Initialize outer i=0

↓

Check outer: i < 3?

No→EXIT

Yes↓

Initialize inner j=0

↓

Check inner: j < 2?

No→Increment outer i

Yes↓

Execute inner loop body

↓

Increment inner j

↩Back to inner check

The outer loop runs from i=0 to 2. For each i, the inner loop runs from j=0 to 1, executing its body each time.

Execution Sample

Java

for (int i = 0; i < 3; i++) {
    for (int j = 0; j < 2; j++) {
        System.out.println("i=" + i + ", j=" + j);
    }
}

Prints pairs of i and j values for nested loops where i goes 0 to 2 and j goes 0 to 1.

Execution Table

Step	Outer i	Inner j	Condition Outer (i<3)	Condition Inner (j<2)	Action	Output
1	0	0	true	true	Print i=0, j=0	i=0, j=0
2	0	1	true	true	Print i=0, j=1	i=0, j=1
3	0	2	true	false	Inner loop ends, increment i
4	1	0	true	true	Print i=1, j=0	i=1, j=0
5	1	1	true	true	Print i=1, j=1	i=1, j=1
6	1	2	true	false	Inner loop ends, increment i
7	2	0	true	true	Print i=2, j=0	i=2, j=0
8	2	1	true	true	Print i=2, j=1	i=2, j=1
9	2	2	true	false	Inner loop ends, increment i
10	3	-	false	-	Outer loop ends

💡 Outer loop stops when i=3 because condition i<3 is false.

Variable Tracker

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

Key Moments - 3 Insights

Why does the inner loop reset j to 0 each time the outer loop increments i?

Because the inner loop initializes j=0 at the start of each outer loop iteration, as shown in execution_table steps 3 to 4 and 6 to 7.

Why does the outer loop increment only after the inner loop finishes all its iterations?

The outer loop waits for the inner loop to complete (j reaches 2 and condition j<2 is false) before incrementing i, as seen in steps 3, 6, and 9.

What causes the entire nested loop to stop running?

When the outer loop variable i reaches 3, the condition i<3 becomes false (step 10), so the loops stop.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of j at Step 5?

A 0

B 1

C 2

D -

💡 Hint

Check the 'Inner j' column at Step 5 in the execution_table.

At which step does the outer loop variable i first become 2?

A Step 7

B Step 6

C Step 3

D Step 9

💡 Hint

Look at the 'Outer i' column in execution_table to find when i changes to 2.

If the inner loop condition changed to j < 3, how many total print outputs would there be?

A 12

B 6

C 9

D 3

💡 Hint

Currently, inner loop runs 2 times per outer loop iteration; increasing to 3 means 3 * 3 = 9 prints.

Concept Snapshot

Nested for loop syntax:
for (int i = 0; i < N; i++) {
  for (int j = 0; j < M; j++) {
    // code using i and j
  }
}
Outer loop runs fully for each i, inner loop runs fully for each j per i.
Inner loop resets each outer iteration.

Full Transcript

This example shows a nested for loop in Java where the outer loop variable i runs from 0 to 2, and for each i, the inner loop variable j runs from 0 to 1. Each step prints the current values of i and j. The inner loop resets j to 0 every time the outer loop increments i. The loops stop when i reaches 3, making the outer loop condition false. The execution table traces each step, showing variable values and actions. Key moments clarify why inner loop resets and when loops end. The quiz tests understanding of variable values at specific steps and effects of changing loop conditions.