Do–while loop in C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple do–while loop

What is the output of this C code?

C

int main() {
    int i = 1;
    do {
        printf("%d ", i);
        i++;
    } while (i <= 3);
    return 0;
}

A 1 2 3

B 1 2

C No output

D 2 3

💡 Hint

Remember, do–while executes the loop body before checking the condition.

The loop starts with i=1, prints it, then increments i. It repeats while i <= 3, so it prints 1, 2, and 3.

❓ Predict Output

intermediate

Do–while loop with initial false condition

What will this program print?

C

int main() {
    int i = 5;
    do {
        printf("%d ", i);
        i++;
    } while (i < 5);
    return 0;
}

A 5 6

B No output

C Runtime error

D 5

💡 Hint

The loop body runs once before the condition is checked.

Even though i < 5 is false initially, the do–while loop runs the body once, printing 5.

❓ Predict Output

advanced

Nested do–while loops output

What is the output of this nested do–while loop code?

C

int main() {
    int i = 1, j;
    do {
        j = 1;
        do {
            printf("%d%d ", i, j);
            j++;
        } while (j <= 2);
        i++;
    } while (i <= 2);
    return 0;
}

A 11 21 12 22

B 11 12 21 22

C 12 11 22 21

D 11 12 22 21

💡 Hint

The inner loop runs fully for each iteration of the outer loop.

For i=1, j runs 1 to 2 printing 11 and 12. Then i=2, j runs 1 to 2 printing 21 and 22.

❓ Predict Output

advanced

Do–while loop with break statement

What will this program print?

C

int main() {
    int i = 1;
    do {
        if (i == 3) break;
        printf("%d ", i);
        i++;
    } while (i <= 5);
    return 0;
}

A 1 2 3

B 1 2 3 4 5

C 1 2

D No output

💡 Hint

The break stops the loop when i equals 3 before printing.

When i is 3, the loop breaks before printing, so only 1 and 2 are printed.

❓ Predict Output

expert

Value of variable after do–while loop with complex condition

What is the value of variable x after this code runs?

C

int main() {
    int x = 0, y = 5;
    do {
        x += y;
        y--;
    } while (y > 0);
    printf("%d", x);
    return 0;
}

A 15

B 10

C 5

D 0

💡 Hint

Sum y from 5 down to 1 added to x.

The loop adds 5 + 4 + 3 + 2 + 1 = 15 to x before y becomes 0 and loop stops.

Practice

1. What is the main difference between a do-while loop and a while loop in C?

easy

A. A do-while loop cannot be used with integers.

B. A do-while loop checks the condition before executing the code block.

C. A do-while loop executes the code block at least once before checking the condition.

D. A do-while loop does not require a semicolon after the while condition.

Solution

1. Step 1: Understand the execution order of loops

   A while loop checks the condition first, so it may not run the code block if the condition is false initially.

2. Step 2: Identify the behavior of do-while loop

   A do-while loop runs the code block once before checking the condition, ensuring at least one execution.

3. Final Answer:

   A do-while loop executes the code block at least once before checking the condition. -> Option C

4. Quick Check:

   do-while runs once first = A [OK]

Hint: Remember: do-while runs code first, then checks condition [OK]

Common Mistakes:

• Thinking do-while checks condition first
• Believing semicolon after while is optional
• Confusing do-while with while loop behavior

2. Which of the following is the correct syntax for a do-while loop in C?

easy

A. do { /* code */ } while (condition);

B. do ( /* code */ ) while condition;

C. do { /* code */ } while condition

D. do while (condition) { /* code */ };

Solution

1. Step 1: Recall the correct syntax structure

   The do-while loop requires braces around the code block and a semicolon after the while condition in parentheses.

2. Step 2: Compare options to syntax

   do { /* code */ } while (condition); matches the correct syntax: do { /* code */ } while (condition); with braces and semicolon.

3. Final Answer:

   do { /* code */ } while (condition); -> Option A

4. Quick Check:

   Semicolon after while(condition) = B [OK]

Hint: Look for braces and semicolon after while(condition) [OK]

Common Mistakes:

• Omitting semicolon after while condition
• Using parentheses instead of braces for code block
• Placing while before do

3. What will be the output of the following C code?

int i = 3;
do {
    printf("%d ", i);
    i--;
} while (i > 0);

medium

A. 1 2 3

B. 2 1 0

C. 3 2 1 0

D. 3 2 1

Solution

1. Step 1: Trace the loop iterations

   Initially, i = 3. The loop prints i, then decrements i by 1. It repeats while i > 0.

2. Step 2: List printed values

   First iteration: prints 3, i becomes 2
   Second iteration: prints 2, i becomes 1
   Third iteration: prints 1, i becomes 0
   Loop stops because i is not > 0.

3. Final Answer:

   3 2 1 -> Option D

4. Quick Check:

   Prints 3 down to 1 = A [OK]

Hint: Count printed values before condition fails [OK]

Common Mistakes:

• Including 0 in output
• Starting from 2 instead of 3
• Printing in ascending order

4. Identify the error in this do-while loop code snippet:

int x = 5;
do {
    printf("%d", x);
    x--;
} while x > 0;

medium

A. The loop condition should be inside braces.

B. Missing parentheses around the while condition.

C. Variable x is not initialized.

D. Missing semicolon after the do block.

Solution

1. Step 1: Check syntax of the while condition

   The while condition must be enclosed in parentheses, like while (x > 0);.

2. Step 2: Identify the error in the code

   The code has while x > 0; missing parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the while condition. -> Option B

4. Quick Check:

   while(condition) needs parentheses = D [OK]

Hint: Always put condition in parentheses after while [OK]

Common Mistakes:

• Omitting parentheses in while condition
• Forgetting semicolon after while statement
• Misplacing braces around condition

5. You want to write a do-while loop that asks the user to enter a positive number and repeats until the user enters a positive number. Which code snippet correctly implements this?

hard

A. int num;
do {
printf("Enter a positive number: ");
scanf("%d", &num);
} while (num <= 0);

B. int num = 0;
do {
scanf("%d", &num);
printf("Enter a positive number: ");
} while (num > 0);

C. int num;
do {
scanf("%d", &num);
} while (num < 0);

D. int num;
do {
printf("Enter a positive number: ");
scanf("%d", &num);
} while (num < 0);

Solution

1. Step 1: Understand the requirement

   The loop must ask for input at least once and repeat while the number is not positive (<= 0).

2. Step 2: Analyze each option

   int num; do { printf("Enter a positive number: "); scanf("%d", &num); } while (num <= 0); prompts before input and repeats while number is <= 0, which matches the requirement.
   int num = 0; do { scanf("%d", &num); printf("Enter a positive number: "); } while (num > 0); prints prompt after input and repeats while number > 0, which is incorrect.
   int num; do { scanf("%d", &num); } while (num < 0); repeats while number < 0, missing zero case.
   int num; do { printf("Enter a positive number: "); scanf("%d", &num); } while (num < 0); repeats while number < 0, missing zero case.

3. Final Answer:

   int num; do { printf("Enter a positive number: "); scanf("%d", &num); } while (num <= 0); -> Option A

4. Quick Check:

   Repeat while number not positive = C [OK]

Hint: Repeat while input ≤ 0, prompt before input [OK]

Common Mistakes:

• Checking wrong condition (e.g., num < 0 instead of <= 0)
• Prompting after input instead of before
• Not handling zero as invalid input