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 using a do–while loop?

C++

int i = 1;
do {
    std::cout << i << " ";
    i++;
} while (i <= 3);

A 1 2

B 1 2 3

C 2 3

D No output

💡 Hint

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

The loop prints i starting from 1, then increments i. It repeats while i is less or equal to 3, so it prints 1 2 3.

❓ Predict Output

intermediate

Do–while loop with condition false initially

What will this C++ program print?

C++

int x = 5;
do {
    std::cout << x << " ";
    x++;
} while (x < 5);

A 5

B No output

C 5 6 7 ... (infinite loop)

D 6

💡 Hint

The loop runs once before checking the condition.

Even though x < 5 is false at start, do–while runs once, printing 5, then stops.

🔧 Debug

advanced

Identify the error in this do–while loop

What error will this code cause?

C++

int count = 0;
do {
    std::cout << count << " ";
    count++;
} while (count < 3);

A SyntaxError: missing semicolon

B Runtime error: infinite loop

C No error, prints 0 1 2

D Compilation error: undeclared variable

💡 Hint

Check punctuation carefully in C++.

The line 'count++' is missing a semicolon, causing a syntax error.

❓ Predict Output

advanced

Counting down with do–while loop

What does this code print?

C++

int n = 3;
do {
    std::cout << n << " ";
    n--;
} while (n > 0);

A 3 2 1 0

B 1 2 3

C 3 2 1

D No output

💡 Hint

The loop runs while n is greater than zero.

Starts at 3, prints 3, then 2, then 1, stops before printing 0.

🧠 Conceptual

expert

Behavior difference between while and do–while loops

Which statement best describes the difference between a while loop and a do–while loop in C++?

A A while loop always executes its body at least once, while a do–while loop may not execute at all.

B Neither loop executes if the condition is false initially.

C Both loops always execute their bodies at least once.

D A do–while loop always executes its body at least once, while a while loop may not execute at all.

💡 Hint

Think about when the condition is checked in each loop.

A do–while loop checks the condition after running the body, so it runs at least once. A while loop checks before running, so it may skip the body.

Practice

1.

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

easy

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

B. while loop always runs the code block at least once.

C. do-while loop checks the condition before running the code block.

D. There is no difference; both behave the same.

Solution

1. Step 1: Understand do-while loop behavior

   A do-while loop executes the code block first, then checks the condition to decide if it repeats.

2. Step 2: Compare with while loop

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

3. Final Answer:

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

4. Quick Check:

   do-while runs once before condition check [OK]

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

Common Mistakes:

• Thinking while loop always runs code once
• Confusing order of condition check
• Assuming do-while and while behave the same

2.

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

?

easy

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

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

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

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

Solution

1. Step 1: Recall correct do-while syntax

   The correct syntax uses do { ... } while (condition); with braces and a semicolon after the while condition.

2. Step 2: Identify syntax errors in other options

   do ( /* code */ ) while condition; misses braces and semicolon; C uses incorrect keyword 'until'; A reverses order incorrectly.

3. Final Answer:

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

4. Quick Check:

   Syntax ends with semicolon after while(condition) [OK]

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

Common Mistakes:

• Omitting semicolon after while(condition)
• Using 'until' instead of 'while'
• Incorrect order of do and while keywords

3.

What will be the output of the following C++ code?

int i = 3;
do {
    std::cout << i << " ";
    i--;
} while (i > 0);

medium

A. 3 2 1

B. 3 2 1 0

C. 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   Initially, i=3. The loop prints 3, then decrements i to 2. Condition i>0 is true, so repeats printing 2, then 1.

2. Step 2: Check when loop stops

   After printing 1, i becomes 0. Condition i>0 is false, loop stops. So output is "3 2 1 "

3. Final Answer:

   3 2 1 -> Option A

4. Quick Check:

   Loop runs while i>0, prints 3,2,1 [OK]

Hint: Count printed values before condition fails [OK]

Common Mistakes:

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

4.

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

int x = 5;
do {
    std::cout << x << " ";
    x++;
} while x < 10;

medium

A. The loop should end with a comma, not a semicolon.

B. The variable x should be decremented, not incremented.

C. The do keyword is misspelled.

D. Missing parentheses around the condition in while statement.

Solution

1. Step 1: Check syntax of while condition

   The condition after while must be enclosed in parentheses: while (x < 10);.

2. Step 2: Verify other parts

   Incrementing x is valid; do is spelled correctly; loop ends with semicolon, which is correct.

3. Final Answer:

   Missing parentheses around the condition in while statement. -> Option D

4. Quick Check:

   while condition needs parentheses [OK]

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

Common Mistakes:

• Forgetting parentheses in while condition
• Confusing semicolon with comma at loop end
• Thinking increment is always wrong

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 number greater than zero. Which code snippet correctly implements this?

?

hard

A. int num;
while (num <= 0) {
std::cout << "Enter positive number: ";
std::cin >> num;
}

B. int num;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num <= 0);

C. int num = 0;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num > 0);

D. int num;
do {
std::cout << "Enter positive number: ";
std::cin >> num;
} while (num < 0);

Solution

1. Step 1: Understand the condition to repeat

   The loop should continue while the number is less than or equal to zero, so condition is num <= 0.

2. Step 2: Check each option's condition

   int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num <= 0); uses do-while (num <= 0); which repeats until user enters positive number. int num = 0; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num > 0); repeats while number is positive (wrong). int num; while (num <= 0) { std::cout << "Enter positive number: "; std::cin >> num; } uses while loop, not do-while. int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num < 0); repeats while number is less than zero, so zero input stops loop incorrectly.

3. Final Answer:

   int num; do { std::cout << "Enter positive number: "; std::cin >> num; } while (num <= 0); -> Option B

4. Quick Check:

   Repeat while input ≤ 0 to ensure positive input [OK]

Hint: Repeat while input is not positive (≤ 0) [OK]

Common Mistakes:

• Using wrong condition sign (> 0 instead of ≤ 0)
• Using while loop instead of do-while
• Stopping loop too early on zero input