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?<br>

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?<br>

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

The output is:<br>1,1<br>1,2<br>1,3<br>2,1<br>2,2<br>2,3<br>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?<br>

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?<br>

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.