Why Do–while loop in C? - Purpose & Use Cases

The Big Idea

What if you could ask a question just once and still keep repeating it perfectly until you get the right answer?

The Scenario

Imagine you want to ask a friend if they want to play a game, and you want to keep asking until they say yes. Doing this by repeating the question manually over and over is tiring and easy to mess up.

The Problem

Manually repeating the same question or code again and again is slow and boring. It's easy to forget to ask at least once or to stop at the right time. This can cause mistakes or endless asking.

The Solution

The do-while loop makes sure you ask the question at least once, then keep asking only if needed. It saves time and avoids mistakes by handling the repeat automatically.

Before vs After

✗ Before

char answer;
printf("Play game? (y/n): ");
scanf(" %c", &answer);
while(answer != 'y') {
    printf("Play game? (y/n): ");
    scanf(" %c", &answer);
}

✓ After

char answer;
do {
    printf("Play game? (y/n): ");
    scanf(" %c", &answer);
} while(answer != 'y');

What It Enables

It lets your program do something first, then decide if it should repeat, making sure the action always happens at least once.

Real Life Example

When filling out a form, you want to show the form once, then keep asking for corrections until everything is right. The do-while loop fits perfectly here.

Key Takeaways

Ensures code runs at least once before checking a condition.

Makes repeating tasks easier and less error-prone.

Great for user input that must happen at least once.

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