Do–while loop in C - Mini Project: Build & Apply

Do-while loop in C

📖 Scenario: You are creating a simple program that repeatedly asks a user to enter a number until they enter zero. This is like asking a friend to keep giving you numbers until they say stop by entering zero.

🎯 Goal: Build a C program that uses a do-while loop to keep asking for numbers and stops when the user enters zero.

📋 What You'll Learn

Create an integer variable called number

Use a do-while loop to ask the user to enter a number

Inside the loop, read the number from the user using scanf

The loop should continue as long as number is not zero

After the loop ends, print "Done"

💡 Why This Matters

🌍 Real World

Do-while loops are useful when you want to run a block of code at least once and then repeat it based on a condition, like menus or repeated user input.

💼 Career

Understanding loops like do-while is essential for programming jobs where you handle user input, data processing, or repetitive tasks.

1

Create the variable to store the number

Create an integer variable called number and set it to 0.

C

#include 

int main() {
    int number = 0;  // Your code here

    return 0;
}

Hint

Think about how to create a variable to hold a number in C.

2

Set up the do-while loop structure

Write a do block followed by a while condition that checks if number is not equal to 0. Leave the do block empty for now.

C

#include 

int main() {
    int number = 0;

    do {
        // Your code here
    } while (number != 0);

    return 0;
}

Hint

Remember the syntax of a do-while loop in C: do { ... } while(condition);

3

Add input reading inside the loop

Inside the do block, add a printf statement to ask the user to "Enter a number (0 to stop): ", then use scanf to read the integer into number.

C

#include 

int main() {
    int number = 0;

    do {
        // Your code here
    } while (number != 0);

    return 0;
}

Hint

Use printf to show the message and scanf to read the number into the variable.

4

Print "Done" after the loop ends

After the do-while loop, write a printf statement to print "Done\n".

C

#include 

int main() {
    int number = 0;

    do {
        printf("Enter a number (0 to stop): ");
        scanf("%d", &number);
    } while (number != 0);

    // Your code here

    return 0;
}

Hint

Use printf("Done\n"); to show the message after the loop.

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