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Do-While Loop in C Programming

Learn the do-while loop in C with syntax, flowchart, difference from while loop, menu-driven programs, and practical examples. Includes interview questions.

The do-while loop is the exit-controlled loop in C — it guarantees that the loop body executes at least once, regardless of whether the condition is true or false. The condition is checked after the body executes, not before. This makes it fundamentally different from the while loop.

Think of it like this: a while loop is like checking if the pool is warm before jumping in. A do-while loop is jumping in first, and then deciding whether to stay for another lap. The "do first, ask questions later" approach is exactly what makes this loop special.

Syntax of Do-While Loop

c example

do {
    // loop body - executes at least once
    // statements
} while (condition);  // Note the semicolon!

Critical Note: The semicolon after while(condition) is mandatory. Forgetting it is a compilation error.

Component	Description
`do`	Keyword starting the loop
`{ }`	Loop body (executes first)
`while (condition)`	Condition checked after each execution
`;`	Mandatory semicolon ending the statement

Flowchart of Do-While Loop

(execute)
Condition?

Basic Example

Example

#include <stdio.h> int main() { int count = 1; do { printf("Count: %d\n", count); count++; } while (count <= 5); printf("Loop ended.\n"); return 0; }

Output

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Loop ended.

The Key Difference: Executes At Least Once

This is the most important feature of do-while. Even if the condition is false from the start, the body runs once:

Example

#include <stdio.h> int main() { int x = 100; // While loop - body never executes printf("While loop:\n"); while (x < 5) { printf(" x = %d\n", x); x++; } printf(" (nothing printed)\n"); // Do-while loop - body executes once x = 100; printf("\nDo-while loop:\n"); do { printf(" x = %d\n", x); x++; } while (x < 5); return 0; }

Output

While loop:
  (nothing printed)

Do-while loop:
  x = 100

Practical Examples

The do-while loop is perfect for menu-driven programs because you always want to show the menu at least once:

Example

#include <stdio.h> int main() { int choice; float num1, num2; do { printf("\n===== Calculator Menu =====\n"); printf("1. Addition\n"); printf("2. Subtraction\n"); printf("3. Multiplication\n"); printf("4. Division\n"); printf("5. Exit\n"); printf("Enter your choice (1-5): "); scanf("%d", &choice); if (choice >= 1 && choice <= 4) { printf("Enter two numbers: "); scanf("%f %f", &num1, &num2); } switch (choice) { case 1: printf("Result: %.2f + %.2f = %.2f\n", num1, num2, num1 + num2); break; case 2: printf("Result: %.2f - %.2f = %.2f\n", num1, num2, num1 - num2); break; case 3: printf("Result: %.2f * %.2f = %.2f\n", num1, num2, num1 * num2); break; case 4: if (num2 != 0) printf("Result: %.2f / %.2f = %.2f\n", num1, num2, num1 / num2); else printf("Error: Cannot divide by zero!\n"); break; case 5: printf("Thank you! Goodbye.\n"); break; default: printf("Invalid choice! Try again.\n"); } } while (choice != 5); return 0; }

Output

===== Calculator Menu =====
1. Addition
2. Subtraction
3. Multiplication
4. Division
5. Exit
Enter your choice (1-5): 1
Enter two numbers: 15 8
Result: 15.00 + 8.00 = 23.00

===== Calculator Menu =====
1. Addition
2. Subtraction
3. Multiplication
4. Division
5. Exit
Enter your choice (1-5): 5
Thank you! Goodbye.

Example 2: Input Validation

Another classic use case — keep asking until valid input is provided:

Example

#include <stdio.h> int main() { int age; do { printf("Enter your age (1-120): "); scanf("%d", &age); if (age < 1 || age > 120) { printf("Invalid! Age must be between 1 and 120.\n"); } } while (age < 1 || age > 120); printf("Your age is: %d\n", age); if (age >= 18) { printf("You are an adult.\n"); } else { printf("You are a minor.\n"); } return 0; }

Output

Enter your age (1-120): -5
Invalid! Age must be between 1 and 120.
Enter your age (1-120): 200
Invalid! Age must be between 1 and 120.
Enter your age (1-120): 25
Your age is: 25
You are an adult.

Example 3: Sum of Digits

c example

#include <stdio.h>

int main() {
    int num, sum = 0;

    printf("Enter a positive number: ");
    scanf("%d", &num);

    int original = num;

    do {
        sum += num % 10;
        num /= 10;
    } while (num > 0);

    printf("Sum of digits of %d = %d\n", original, sum);
    return 0;
}

Output

Enter a positive number: 9876
Sum of digits of 9876 = 30

Example 4: Password Verification with Attempts

Example

#include <stdio.h> int main() { int password = 1234; int input; int attempts = 0; int maxAttempts = 3; do { printf("Enter password: "); scanf("%d", &input); attempts++; if (input == password) { printf("Access granted! Welcome.\n"); break; } else { printf("Wrong password! %d attempt(s) remaining.\n", maxAttempts - attempts); } } while (attempts < maxAttempts); if (attempts >= maxAttempts && input != password) { printf("Account locked! Too many failed attempts.\n"); } return 0; }

Output

Enter password: 1111
Wrong password! 2 attempt(s) remaining.
Enter password: 2222
Wrong password! 1 attempt(s) remaining.
Enter password: 1234
Access granted! Welcome.

While vs Do-While Comparison

Feature	while	do-while
Type	Entry-controlled	Exit-controlled
Condition check	Before loop body	After loop body
Minimum executions	0 (may never run)	1 (always runs once)
Semicolon	No semicolon after while()	Requires `;` after while()
Best for	Unknown iterations, may skip	Menu programs, input validation
Syntax	`while(cond) { }`	`do { } while(cond);`

When to Use Do-While

Use do-while when:

You need to execute the body at least once
Input validation (ask, check, repeat if invalid)
Menu-driven programs (show menu, process, ask to continue)
Games (play at least one round)
Reading data where you process before checking

Use while when:

The loop might not need to execute at all
The condition depends on something set before the loop
File reading (might be empty)

Common Mistakes

Mistake 1: Forgetting the Semicolon

c example

do {
    printf("Hello\n");
} while (condition)  // ERROR: Missing semicolon!

Mistake 2: Wrong Condition Logic

c example

// Want to repeat while input is invalid
do {
    scanf("%d", &num);
} while (num > 0);  // Wrong! This continues for VALID input

// Correct
do {
    scanf("%d", &num);
} while (num <= 0);  // Repeats for invalid (non-positive) input

Interview Questions

Q1: What is the minimum number of times a do-while loop executes?

A do-while loop always executes at least once because the condition is evaluated after the loop body runs. Even if the condition is false, the body has already executed once.

Q2: When would you choose do-while over while?

Choose do-while when the loop body must execute at least once — typical examples are menu-driven programs, input validation loops, and "play again?" game loops. If there's a possibility the loop should not run at all, use while instead.

Q3: What happens if you forget the semicolon after do-while?

It causes a compilation error. The semicolon after while(condition) in a do-while is syntactically required — it terminates the do-while statement.

Q4: Can we nest a do-while loop inside a while loop and vice versa?

Yes. Any combination of loop nesting is valid in C. You can nest do-while inside while, for inside do-while, or any other combination. Each loop maintains its own control flow independently.

Q5: Is do { } while(0); useful? When?

Yes! do { } while(0) is a common C idiom used in macros to create a safe multi-statement block that works correctly with if-else. It executes exactly once and the compiler optimizes away the loop overhead entirely.

Summary

The do-while loop is C's exit-controlled loop that guarantees at least one execution of the loop body. It checks the condition after running the body, making it ideal for input validation, menu-driven programs, and any situation where you need to "do something first, then decide whether to repeat." Don't forget the mandatory semicolon after the while condition, and always make sure your condition logic correctly expresses when to continue (not when to stop).

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Do-While Loop in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

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c-programming, c programming, programming, control, statements, while, loop, do-while loop in c programming

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C Notes

Do-While Loop in C Programming

Learn the do-while loop in C with syntax, flowchart, difference from while loop, menu-driven programs, and practical examples. Includes interview questions.

Syntax of Do-While Loop

c example

do {
    // loop body - executes at least once
    // statements
} while (condition);  // Note the semicolon!

Critical Note: The semicolon after while(condition) is mandatory. Forgetting it is a compilation error.

Component	Description
`do`	Keyword starting the loop
`{ }`	Loop body (executes first)
`while (condition)`	Condition checked after each execution
`;`	Mandatory semicolon ending the statement

Flowchart of Do-While Loop

(execute)
Condition?

Basic Example

Example

#include <stdio.h> int main() { int count = 1; do { printf("Count: %d\n", count); count++; } while (count <= 5); printf("Loop ended.\n"); return 0; }

Output

Count: 1
Count: 2
Count: 3
Count: 4
Count: 5
Loop ended.

The Key Difference: Executes At Least Once

This is the most important feature of do-while. Even if the condition is false from the start, the body runs once:

Example

Output

While loop:
  (nothing printed)

Do-while loop:
  x = 100

Practical Examples

The do-while loop is perfect for menu-driven programs because you always want to show the menu at least once:

Example

Output

===== Calculator Menu =====
1. Addition
2. Subtraction
3. Multiplication
4. Division
5. Exit
Enter your choice (1-5): 1
Enter two numbers: 15 8
Result: 15.00 + 8.00 = 23.00

===== Calculator Menu =====
1. Addition
2. Subtraction
3. Multiplication
4. Division
5. Exit
Enter your choice (1-5): 5
Thank you! Goodbye.

Example 2: Input Validation

Another classic use case — keep asking until valid input is provided:

Example

Output

Enter your age (1-120): -5
Invalid! Age must be between 1 and 120.
Enter your age (1-120): 200
Invalid! Age must be between 1 and 120.
Enter your age (1-120): 25
Your age is: 25
You are an adult.

Example 3: Sum of Digits

c example

#include <stdio.h>

int main() {
    int num, sum = 0;

    printf("Enter a positive number: ");
    scanf("%d", &num);

    int original = num;

    do {
        sum += num % 10;
        num /= 10;
    } while (num > 0);

    printf("Sum of digits of %d = %d\n", original, sum);
    return 0;
}

Output

Enter a positive number: 9876
Sum of digits of 9876 = 30

Example 4: Password Verification with Attempts

Example

Output

Enter password: 1111
Wrong password! 2 attempt(s) remaining.
Enter password: 2222
Wrong password! 1 attempt(s) remaining.
Enter password: 1234
Access granted! Welcome.

While vs Do-While Comparison

Feature	while	do-while
Type	Entry-controlled	Exit-controlled
Condition check	Before loop body	After loop body
Minimum executions	0 (may never run)	1 (always runs once)
Semicolon	No semicolon after while()	Requires `;` after while()
Best for	Unknown iterations, may skip	Menu programs, input validation
Syntax	`while(cond) { }`	`do { } while(cond);`

When to Use Do-While

Use do-while when:

You need to execute the body at least once
Input validation (ask, check, repeat if invalid)
Menu-driven programs (show menu, process, ask to continue)
Games (play at least one round)
Reading data where you process before checking

Use while when:

The loop might not need to execute at all
The condition depends on something set before the loop
File reading (might be empty)

Common Mistakes

Mistake 1: Forgetting the Semicolon

c example

do {
    printf("Hello\n");
} while (condition)  // ERROR: Missing semicolon!

Mistake 2: Wrong Condition Logic

c example

// Want to repeat while input is invalid
do {
    scanf("%d", &num);
} while (num > 0);  // Wrong! This continues for VALID input

// Correct
do {
    scanf("%d", &num);
} while (num <= 0);  // Repeats for invalid (non-positive) input

Interview Questions

Q1: What is the minimum number of times a do-while loop executes?

A do-while loop always executes at least once because the condition is evaluated after the loop body runs. Even if the condition is false, the body has already executed once.

Q2: When would you choose do-while over while?

Q3: What happens if you forget the semicolon after do-while?

It causes a compilation error. The semicolon after while(condition) in a do-while is syntactically required — it terminates the do-while statement.

Q4: Can we nest a do-while loop inside a while loop and vice versa?

Yes. Any combination of loop nesting is valid in C. You can nest do-while inside while, for inside do-while, or any other combination. Each loop maintains its own control flow independently.

Q5: Is do { } while(0); useful? When?

Summary

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for Do-While Loop in C Programming.

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this C Programming topic.

Search Terms

c-programming, c programming, programming, control, statements, while, loop, do-while loop in c programming

Continue learning this concept

PreviousContinue Statement in C Programming NextFor Loop in C Programming

Do-While Loop in C Programming

Syntax of Do-While Loop

Flowchart of Do-While Loop

Basic Example

The Key Difference: Executes At Least Once

Practical Examples

Example 1: Menu-Driven Program

Example 2: Input Validation

Example 3: Sum of Digits

Example 4: Password Verification with Attempts

While vs Do-While Comparison

When to Use Do-While

Common Mistakes

Mistake 1: Forgetting the Semicolon

Mistake 2: Wrong Condition Logic

Interview Questions

Summary

Continue learning this concept

Do-While Loop in C Programming

Syntax of Do-While Loop

Flowchart of Do-While Loop

Basic Example

The Key Difference: Executes At Least Once

Practical Examples

Example 1: Menu-Driven Program

Example 2: Input Validation

Example 3: Sum of Digits

Example 4: Password Verification with Attempts

While vs Do-While Comparison

When to Use Do-While

Common Mistakes

Mistake 1: Forgetting the Semicolon

Mistake 2: Wrong Condition Logic

Interview Questions

Summary

Continue learning this concept