Nested loops in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a nested loop in C?

A nested loop is a loop inside another loop. The inner loop runs completely every time the outer loop runs once.

beginner

How many times will the inner loop run in this code?

for(int i=0; i<3; i++) {
  for(int j=0; j<2; j++) {
    // inner loop body
  }
}

The inner loop runs 2 times for each of the 3 outer loop runs, so 3 * 2 = 6 times total.

beginner

Why use nested loops? Give a real-life example.

Nested loops help when you need to repeat actions in layers. For example, printing a multiplication table: outer loop for rows, inner loop for columns.

intermediate

What happens if the inner loop depends on the outer loop variable?

The inner loop can change how many times it runs based on the outer loop variable, allowing patterns like printing a triangle of stars.

beginner

What is the output of this code?

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

The output is:
1,1
1,2
1,3
2,1
2,2
2,3
This shows the inner loop runs fully for each outer loop value.

In nested loops, which loop runs first?

A The outer loop runs first, then the inner loop runs completely each time.

B The inner loop runs first, then the outer loop runs completely each time.

C Both loops run at the same time.

D Only the outer loop runs.

The outer loop starts first, and for each iteration of the outer loop, the inner loop runs fully.

How many times will the inner loop run in this code?

for(int i=0; i<4; i++) {
  for(int j=0; j<5; j++) {
    // code
  }
}

A 20 times

B 5 times

C 4 times

D 9 times

The inner loop runs 5 times for each of the 4 outer loop runs, so 4 * 5 = 20 times.

What is a common use of nested loops?

A To repeat a task once

B To handle multi-dimensional data like grids or tables

C To avoid repetition

D To run code only once

Nested loops are often used to work with multi-dimensional data such as rows and columns in tables.

What will this code print?

for(int i=1; i<=3; i++) {
  for(int j=1; j<=i; j++) {
    printf("*");
  }
  printf("\n");
}

A No output

B A square of stars

C A triangle of stars

D An error

The inner loop runs up to the current outer loop count, printing a triangle shape.

If the outer loop runs 3 times and the inner loop runs 4 times, how many total iterations happen?

A 3

B 4

C 7

D 12

Total iterations = outer loop count * inner loop count = 3 * 4 = 12.

Explain what nested loops are and how they work in C.

Think about loops inside loops and how many times each runs.

Describe a real-life scenario where nested loops would be useful.

Imagine doing something for each item in a list, and for each item, doing something else multiple times.

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