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

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple do–while loop

What is the output of the following Java code?

Java

int count = 1;
do {
    System.out.print(count + " ");
    count++;
} while (count <= 3);

A 1 2

B 1 2 3

C 2 3

D No output

💡 Hint

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

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

❓ Predict Output

intermediate

Value of variable after do–while loop

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

Java

int x = 5;
do {
    x -= 2;
} while (x > 0);

A -1

B 1

C 0

D 3

💡 Hint

Check how x changes each loop and when the loop stops.

x starts at 5, then subtracts 2 each time: 5->3->1->-1. The loop stops when x <= 0, so final x is -1.

🔧 Debug

advanced

Identify the error in this do–while loop

What error does this code produce when compiled?

Java

int i = 0;
do {
    System.out.println(i);
    i++
} while (i < 3);

A SyntaxError: missing parentheses in while

B RuntimeException: infinite loop

C No error, prints 0 1 2

D SyntaxError: missing semicolon

💡 Hint

Look carefully at the line before the while condition.

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

❓ Predict Output

advanced

Output with break inside do–while loop

What is the output of this code?

Java

int n = 1;
do {
    if (n == 3) break;
    System.out.print(n + " ");
    n++;
} while (n <= 5);

A 1 2 3 4 5

B 1 2 3

C 1 2

D No output

💡 Hint

The break stops the loop when n equals 3.

The loop prints 1 and 2, then when n is 3, it breaks before printing.

🧠 Conceptual

expert

Minimum iterations of do–while vs while loop

Which statement is true about do–while loops compared to while loops?

A A do–while loop always executes its body at least once, even if the condition is false initially.

B A do–while loop may never execute its body if the condition is false initially.

C A while loop always executes its body at least once, even if the condition is false initially.

D Both do–while and while loops execute their body the same number of times regardless of the condition.

💡 Hint

Think about when the condition is checked in each loop type.

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

Practice

1.

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

easy

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

B. A while loop executes the code block at least once before checking the condition.

C. A do-while loop never checks the condition.

D. A while loop always executes the code block twice.

Solution

1. Step 1: Understand do-while loop behavior

   A do-while loop runs the code block first, then checks the condition to decide if it should repeat.

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:

   A do-while loop executes 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, while checks condition first [OK]

Common Mistakes:

• Thinking while loop always runs code once
• Believing do-while skips condition check
• Confusing order of execution and condition check

2.

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

?

easy

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

B. do (condition) { // code };

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

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

Solution

1. Step 1: Recall correct do-while syntax

   The correct syntax starts with do, then the code block in braces, followed by while (condition); with a semicolon.

2. Step 2: Identify the correct option

   do { // code } while (condition); matches the correct syntax exactly. Others have wrong order, missing semicolon, or wrong keywords.

3. Final Answer:

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

4. Quick Check:

   Syntax: do { } while (condition); [OK]

Hint: Look for 'do { } while (condition);' with semicolon [OK]

Common Mistakes:

• Missing semicolon after while condition
• Using 'until' instead of 'while'
• Placing condition before code block

3.

What will be the output of the following Java code?

int count = 1;
do {
    System.out.print(count + " ");
    count++;
} while (count <= 3);

medium

A. 1 2 3 4

B. 2 3 4

C. 1 2 3

D. No output

Solution

1. Step 1: Trace the loop iterations

   Initially, count = 1. The loop prints count, then increments it. It repeats while count <= 3.

2. Step 2: List printed values

   Iterations print: 1 (count=1), 2 (count=2), 3 (count=3). When count becomes 4, condition fails and loop stops.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Prints 1 to 3 inclusive [OK]

Hint: Count starts at 1, loop runs while count ≤ 3 [OK]

Common Mistakes:

• Thinking loop prints 4 because of increment
• Confusing pre-increment and post-increment
• Assuming no output if condition false initially

4.

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

int i = 0;
do {
    System.out.println(i);
    i++
} while (i < 5);

medium

A. Loop should use while instead of do-while

B. Condition should be inside braces

C. Variable i must be declared inside the loop

D. Missing semicolon after i++

Solution

1. Step 1: Check syntax inside the loop

   The statement i++ is missing a semicolon at the end, which causes a syntax error.

2. Step 2: Verify other parts

   The condition is correctly placed after the loop with a semicolon. Variable declaration outside is valid. Loop type is correct.

3. Final Answer:

   Missing semicolon after i++ -> Option D

4. Quick Check:

   Every statement needs a semicolon [OK]

Hint: Check for missing semicolons inside loop body [OK]

Common Mistakes:

• Forgetting semicolon after increment
• Misplacing condition inside braces
• Thinking variable must be declared inside loop

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?

Scanner sc = new Scanner(System.in);
int num;
do {
    System.out.print("Enter a positive number: ");
    num = sc.nextInt();
} while (???);

hard

A. num > 0

B. num <= 0

C. num == 0

D. num < 0

Solution

1. Step 1: Understand the loop goal

   The loop should repeat while the number is not positive, meaning while it is zero or negative.

2. Step 2: Choose the correct condition

   The condition num <= 0 ensures the loop repeats until the user enters a positive number.

3. Final Answer:

   num <= 0 -> Option B

4. Quick Check:

   Repeat while number is not positive [OK]

Hint: Loop while input is zero or negative to get positive number [OK]

Common Mistakes:

• Using condition that stops too early
• Checking for positive instead of non-positive
• Confusing < and <= operators