Return inside loops in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of return inside a for loop

What is the output of this C program?

C

#include <stdio.h>

int test() {
    for (int i = 0; i < 5; i++) {
        if (i == 2) {
            return i;
        }
    }
    return -1;
}

int main() {
    printf("%d\n", test());
    return 0;
}

A -1

B 0

C 4

D 2

💡 Hint

The function returns as soon as i equals 2 inside the loop.

The loop runs from i=0 to i=4. When i reaches 2, the return statement exits the function immediately with value 2.

❓ Predict Output

intermediate

Return inside while loop behavior

What will this program print?

C

#include <stdio.h>

int count_down(int n) {
    while (n > 0) {
        if (n == 3) {
            return n;
        }
        n--;
    }
    return 0;
}

int main() {
    printf("%d\n", count_down(5));
    return 0;
}

A 0

B 3

C 1

D 5

💡 Hint

The function returns when n equals 3 during the countdown.

The while loop decreases n from 5 down to 1. When n is 3, the function returns 3 immediately.

🔧 Debug

advanced

Why does this function return early?

Consider this function. Why does it return 0 instead of 10?

C

#include <stdio.h>

int sum_until_five() {
    int sum = 0;
    for (int i = 1; i <= 10; i++) {
        if (i == 5) {
            return 0;
        }
        sum += i;
    }
    return sum;
}

int main() {
    printf("%d\n", sum_until_five());
    return 0;
}

A Because the loop never runs

B Because sum is reset to 0 when i is 5

C Because the return 0 inside the loop stops the function when i is 5

D Because sum is returned before the loop starts

💡 Hint

Look at the return statement inside the loop and when it triggers.

The function returns 0 immediately when i reaches 5, so it never sums beyond 4.

❓ Predict Output

advanced

Return inside nested loops

What does this program print?

C

#include <stdio.h>

int nested_loop() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            if (i * j == 4) {
                return i + j;
            }
        }
    }
    return 0;
}

int main() {
    printf("%d\n", nested_loop());
    return 0;
}

A 5

B 6

C 4

D 0

💡 Hint

Find the first pair (i,j) where i*j equals 4 and return their sum.

The nested loops check i from 1 to 3 and j from 1 to 3. The first time i*j == 4 is when i=2 and j=2, so it returns 2+2=4.

❓ Predict Output

expert

Return inside loop with break and continue

What is the output of this program?

C

#include <stdio.h>

int tricky() {
    for (int i = 0; i < 5; i++) {
        if (i == 2) {
            continue;
        }
        if (i == 3) {
            break;
        }
        if (i == 1) {
            return i;
        }
    }
    return -1;
}

int main() {
    printf("%d\n", tricky());
    return 0;
}

A 1

B -1

C 0

D 3

💡 Hint

Remember that return exits the function immediately, continue skips to next loop iteration, and break exits the loop.

When i=0, no condition triggers. When i=1, return i exits function with 1. So output is 1.

Practice

1. What happens when a return statement is executed inside a loop in a C function?

easy

A. The program crashes due to an error.

B. The loop skips the current iteration and continues.

C. The loop finishes all iterations before returning.

D. The function immediately stops and returns the specified value.

Solution

1. Step 1: Understand the role of return inside a function

   The return statement immediately ends the function and sends back a value.

2. Step 2: Effect of return inside a loop

   When return is inside a loop, the function stops as soon as return is executed, even if the loop is not finished.

3. Final Answer:

   The function immediately stops and returns the specified value. -> Option D

4. Quick Check:

   Return inside loop = immediate function exit [OK]

Hint: Return inside loop stops function immediately [OK]

Common Mistakes:

• Thinking return only exits the loop, not the function
• Assuming loop completes before return
• Confusing return with continue or break

2. Which of the following is the correct syntax to return the value 5 inside a for loop in C?

easy

A. for(int i=0; i<3; i++) { return 5; }

B. for(int i=0; i<3; i++) return(5)

C. for(int i=0; i<3; i++) { return; 5; }

D. for(int i=0; i<3; i++) { return 5 }

Solution

1. Step 1: Check syntax of return statement

   Return must be followed by a value and a semicolon inside braces if used.

2. Step 2: Analyze each option

   for(int i=0; i<3; i++) { return 5; } uses correct syntax with braces and semicolon. for(int i=0; i<3; i++) return(5) misses semicolon, invalid. for(int i=0; i<3; i++) { return; 5; } has return without value then 5; which is invalid. for(int i=0; i<3; i++) { return 5 } misses semicolon after return 5.

3. Final Answer:

   for(int i=0; i<3; i++) { return 5; } -> Option A

4. Quick Check:

   Return syntax needs value and semicolon [OK]

Hint: Return must have value and semicolon inside braces [OK]

Common Mistakes:

• Omitting semicolon after return
• Placing code after return on same line
• Using return without value in non-void function

3. What is the output of this C function?

int findFirstEven(int arr[], int size) {
  for(int i = 0; i < size; i++) {
    if(arr[i] % 2 == 0) {
      return arr[i];
    }
  }
  return -1;
}

int main() {
  int nums[] = {3, 7, 8, 5};
  printf("%d", findFirstEven(nums, 4));
  return 0;
}

medium

A. 3

B. 7

C. 8

D. -1

Solution

1. Step 1: Trace the loop to find first even number

   The array is {3, 7, 8, 5}. Loop checks each element: 3 (odd), 7 (odd), 8 (even).

2. Step 2: Return when first even found

   At element 8, condition is true, so function returns 8 immediately, skipping rest.

3. Final Answer:

   8 -> Option C

4. Quick Check:

   Return stops loop at first even number 8 [OK]

Hint: Return inside loop returns first matching value [OK]

Common Mistakes:

• Assuming loop continues after return
• Returning wrong element index
• Confusing return value with loop index

4. Identify the error in this C function that tries to return the first negative number from an array:

int firstNegative(int arr[], int n) {
  for(int i=0; i

medium

A. The function has no return statement.

B. The function returns 0 too early, skipping rest of array.

C. The loop syntax is incorrect.

D. The function returns negative numbers incorrectly.

Solution

1. Step 1: Analyze the loop and return logic

   The else clause returns 0 immediately if the first element is not negative, so the loop never checks further elements.

2. Step 2: Understand why this is a problem

   This causes the function to stop early and miss negative numbers later in the array.

3. Final Answer:

   The function returns 0 too early, skipping rest of array. -> Option B

4. Quick Check:

   Return in else exits function too soon [OK]

Hint: Avoid return in else inside loop to check all elements [OK]

Common Mistakes:

• Returning inside else causes early exit
• Not using braces causing confusion
• Assuming loop checks all elements

5. You want to write a C function that returns the index of the first prime number in an array, or -1 if none found. Which code snippet correctly uses return inside a loop to achieve this?

hard

A. for(int i=0; i

B. for(int i=0; i

C. for(int i=0; i

D. for(int i=0; i

Solution

1. Step 1: Understand the goal

   We want to return the index of the first prime number found, or -1 if none.

2. Step 2: Analyze each option's logic

   for(int i=0; i

3. Final Answer:

   for(int i=0; i -> Option A

4. Quick Check:

   Return index on first prime, else -1 after loop [OK]

Hint: Return inside loop on condition, return default after loop [OK]

Common Mistakes:

• Returning wrong value inside loop
• Using break without return
• Returning default inside loop