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.

Practice

1. What does a nested loop in C do?
for (int i = 0; i < 3; i++) {
for (int j = 0; j < 2; j++) {
// code
}
}

easy

A. Runs one loop inside another, repeating inner loop fully each outer loop

B. Runs two loops side by side without interaction

C. Runs only the outer loop multiple times

D. Runs only the inner loop once

Solution

1. Step 1: Understand nested loops structure

   The outer loop runs 3 times, and for each iteration, the inner loop runs fully 2 times.

2. Step 2: Explain the repetition

   Each time the outer loop runs once, the inner loop runs completely, repeating the inner code multiple times.

3. Final Answer:

   Runs one loop inside another, repeating inner loop fully each outer loop -> Option A

4. Quick Check:

   Nested loops repeat inner loop per outer loop [OK]

Hint: Inner loop runs fully for each outer loop iteration [OK]

Common Mistakes:

• Thinking loops run side by side without nesting
• Assuming inner loop runs only once
• Confusing outer and inner loop roles

2. Which of the following is the correct syntax for a nested for loop in C?

easy

A. for int i = 0; i < 3; i++ { for int j = 0; j < 2; j++ { /* code */ } }

B. for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) /* code */ }

C. for (int i = 0; i < 3; i++) for (int j = 0; j < 2; j++) { /* code */ }

D. for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) { /* code */ }

Solution

1. Step 1: Check syntax correctness

   for (int i = 0; i < 3; i++) for (int j = 0; j < 2; j++) { /* code */ } uses correct C syntax with parentheses and braces properly placed.

2. Step 2: Identify syntax errors in other options

   for int i = 0; i < 3; i++ { for int j = 0; j < 2; j++ { /* code */ } } misses parentheses and braces; for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) /* code */ } misses braces for inner loop; for (int i = 0; i < 3; i++) { for (int j = 0; j < 2; j++) { /* code */ } misses closing brace for outer loop.

3. Final Answer:

   for (int i = 0; i < 3; i++) for (int j = 0; j < 2; j++) { /* code */ } -> Option C

4. Quick Check:

   Correct for loop syntax uses parentheses and braces [OK]

Hint: Remember for loops need parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around loop conditions
• Missing braces for loop bodies
• Incorrect variable declaration syntax

3. What is the output of this C code?

for (int i = 1; i <= 2; i++) {
  for (int j = 1; j <= 3; j++) {
    printf("%d%d ", i, j);
  }
}

medium

A. 12 13 21 23

B. 11 12 13 21 22 23

C. 123 123

D. 11 21 12 22 13 23

Solution

1. Step 1: Trace outer loop iterations

   Outer loop runs i=1 and i=2.

2. Step 2: Trace inner loop for each outer iteration

   For i=1, inner loop prints 11 12 13; for i=2, inner loop prints 21 22 23.

3. Final Answer:

   11 12 13 21 22 23 -> Option B

4. Quick Check:

   Nested loops print pairs in order [OK]

Hint: Outer loop controls first digit, inner loop second digit [OK]

Common Mistakes:

• Mixing order of printed pairs
• Printing numbers without spaces
• Confusing loop limits

4. Find the error in this nested loop code:

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

medium

A. Extra closing brace '}' after inner loop

B. Missing semicolon after printf

C. Inner loop missing braces causing syntax error

D. Outer loop condition is incorrect

Solution

1. Step 1: Check braces matching

   There is an extra closing brace after the inner loop which causes a syntax error.

2. Step 2: Verify other syntax parts

   Printf has semicolon; inner loop without braces is allowed for single statement; outer loop condition is correct.

3. Final Answer:

   Extra closing brace '}' after inner loop -> Option A

4. Quick Check:

   Braces must match exactly [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Adding extra braces accidentally
• Assuming missing braces always cause error
• Ignoring semicolon after statements

5. How many times will the innermost statement run in this nested loop?

for (int i = 1; i <= 4; i++) {
  for (int j = i; j <= 4; j++) {
    for (int k = 1; k <= j; k++) {
      printf("*");
    }
  }
}

hard

A. 70

B. 50

C. 40

D. 30

Solution

1. Step 1: Analyze loop ranges

   i runs 1 to 4; for each i, j runs from i to 4; for each j, k runs 1 to j.

2. Step 2: Calculate total iterations

   Calculate sum over i=1 to 4 of sum over j=i to 4 of j:
   i=1: j=1..4 sum=1+2+3+4=10
   i=2: j=2..4 sum=2+3+4=9
   i=3: j=3..4 sum=3+4=7
   i=4: j=4..4 sum=4
   Total = 10+9+7+4=30

3. Final Answer:

   30 -> Option D

4. Quick Check:

   Total iterations = 10+9+7+4=30 [OK]

Hint: Sum inner loop counts carefully per outer loops [OK]

Common Mistakes:

• Adding loop counts incorrectly
• Ignoring changing start of inner loops
• Confusing loop variables