Difference between while and do–while in Java - Quick Revision & Key Differences

Recall & Review

beginner

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

The while loop checks the condition before running the loop body, so it may not run at all if the condition is false initially. The do-while loop runs the loop body first, then checks the condition, so it always runs at least once.

beginner

When does the condition get evaluated in a while loop?

The condition is evaluated before the loop body runs. If the condition is false at the start, the loop body does not run at all.

beginner

When does the condition get evaluated in a do-while loop?

The condition is evaluated after the loop body runs. This means the loop body always runs at least once.

beginner

Can a do-while loop run zero times?

No. A do-while loop always runs the loop body at least once before checking the condition.

beginner

Which loop is better if you want to run the code at least once regardless of the condition?

The do-while loop is better because it runs the loop body first before checking the condition.

Which loop checks the condition before executing the loop body?

A while loop

B do-while loop

C for loop

D switch statement

The while loop checks the condition before running the loop body.

Which loop guarantees the loop body runs at least once?

A while loop

B do-while loop

C for loop

D if statement

The do-while loop runs the loop body first, then checks the condition.

If the condition is false initially, which loop will NOT run the loop body?

A both loops run

B do-while loop

C while loop

D none of the above

The while loop checks the condition first and will skip the loop body if false.

What happens first in a do-while loop?

A Loop body execution

B Condition check

C Variable initialization

D Loop termination

The loop body executes first, then the condition is checked.

Which loop is more suitable when you want to repeat an action only if a condition is true from the start?

A do-while loop

B both are equally suitable

C none of the above

D while loop

The while loop only runs if the condition is true initially.

Explain in your own words the key difference between a while loop and a do-while loop.

Think about when the condition is checked in each loop.

Describe a real-life situation where a do-while loop would be better than a while loop.

Imagine you want to show a menu at least once before asking if the user wants to continue.

Practice

1. What is the main difference between while and do-while loops in Java?

easy

A. while runs the loop body once before checking; do-while checks the condition before running.

B. while and do-while both check the condition after running the loop body.

C. while checks the condition before running the loop body; do-while runs the loop body once before checking.

D. while loops run only once; do-while loops run multiple times.

Solution

1. Step 1: Understand while loop behavior

   The while loop checks the condition before running the loop body, so if the condition is false initially, the loop body does not run at all.

2. Step 2: Understand do-while loop behavior

   The do-while loop runs the loop body once first, then checks the condition to decide if it should continue looping.

3. Final Answer:

   while checks the condition before running; do-while runs once before checking. -> Option C

4. Quick Check:

   Condition check timing = while before, do-while after [OK]

Hint: Remember: do-while runs once before condition check [OK]

Common Mistakes:

• Thinking both loops check condition before running
• Assuming do-while never checks condition
• Confusing which loop runs code first

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

easy

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

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

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

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

Solution

1. Step 1: Recall do-while syntax

   The do-while loop syntax starts with do, then the loop body in braces, followed by while with the condition in parentheses and a semicolon.

2. Step 2: Check each option

   do { /* code */ } while (condition); matches the correct syntax exactly. Options B, C, and D have syntax errors or wrong order.

3. Final Answer:

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

4. Quick Check:

   Syntax order: do, body, while(condition); [OK]

Hint: do-while ends with a semicolon after condition [OK]

Common Mistakes:

• Missing semicolon after while(condition)
• Putting condition after do without while
• Using while without do in do-while loop

3. What will be the output of this Java code?

int count = 0;
do {
    System.out.print(count + " ");
    count++;
} while (count < 3);

medium

A. No output

B. 1 2 3

C. 0 1 2 3

D. 0 1 2

Solution

1. Step 1: Trace the loop execution

   The variable count starts at 0. The do-while loop prints count then increments it. It repeats while count < 3.

2. Step 2: List printed values

   First iteration: prints 0, count becomes 1; second: prints 1, count 2; third: prints 2, count 3; loop stops because 3 is not less than 3.

3. Final Answer:

   0 1 2 -> Option D

4. Quick Check:

   Prints 0 to 2 inclusive [OK]

Hint: do-while runs at least once, prints count before increment [OK]

Common Mistakes:

• Assuming count starts at 1
• Expecting output to include 3
• Thinking loop doesn't run if condition false initially

4. Identify the error in this code snippet:

int i = 5;
while (i > 0);
{
    System.out.println(i);
    i--;
}

medium

A. Extra semicolon after while condition causes infinite loop

B. Missing braces around while loop body

C. Variable i is not initialized

D. Condition should be i >= 0

Solution

1. Step 1: Analyze the while loop syntax

   The semicolon immediately after while (i > 0); ends the loop statement, making it an empty loop that runs infinitely if condition is true.

2. Step 2: Understand the block after while

   The block in braces is not part of the loop but runs once after the loop ends, which never happens here due to infinite loop.

3. Final Answer:

   Extra semicolon after while condition causes infinite loop -> Option A

4. Quick Check:

   Semicolon after while() ends loop prematurely [OK]

Hint: No semicolon immediately after while(condition) [OK]

Common Mistakes:

• Thinking braces fix the semicolon error
• Assuming loop runs once
• Confusing condition correctness with syntax error

5. You want to prompt a user to enter a positive number and keep asking until they do. Which loop is best and why?

hard

A. Use a do-while loop because it ensures the prompt runs at least once.

B. Use a for loop because it runs a fixed number of times.

C. Use a while loop because it checks condition before input.

D. Use an if statement to check once and stop.

Solution

1. Step 1: Understand the requirement

   The program must ask the user at least once to enter a positive number, then repeat if the input is invalid.

2. Step 2: Choose the suitable loop

   A do-while loop runs the prompt first, then checks the condition to repeat, ensuring the prompt always appears at least once.

3. Final Answer:

   Use a do-while loop because it ensures the prompt runs at least once. -> Option A

4. Quick Check:

   Prompt runs once before condition check = do-while [OK]

Hint: Use do-while to run code at least once before checking [OK]

Common Mistakes:

• Choosing while loop and missing first prompt
• Using for loop which is not for input validation
• Using if statement which doesn't loop