Difference between while and do–while in Java - Visual Side-by-Side Comparison

Concept Flow - Difference between while and do–while

Start

↓

Check condition

|Yes↓

Execute loop body

↓

Repeat Check

No↓

Exit

↓

Start

↓

Execute loop body

↓

Check condition

|Yes↓

Repeat Execute

No↓

Exit

The while loop checks the condition before running the code inside, so it may skip the loop if false. The do-while loop runs the code once before checking the condition, so it always runs at least once.

Execution Sample

Java

int i = 0;
while (i < 2) {
  System.out.println(i);
  i++;
}

int j = 0;
do {
  System.out.println(j);
  j++;
} while (j < 2);

This code prints numbers 0 and 1 using both while and do-while loops, showing how they run.

Execution Table

Step	Loop Type	Variable	Condition	Condition Result	Action	Output
1	while	i=0	i < 2	true	Print i=0, i++	0
2	while	i=1	i < 2	true	Print i=1, i++	1
3	while	i=2	i < 2	false	Exit loop
4	do-while	j=0	N/A before first run	N/A	Print j=0, j++	0
5	do-while	j=1	j < 2	true	Print j=1, j++	1
6	do-while	j=2	j < 2	false	Exit loop

💡 while loop exits when i=2 because 2 < 2 is false; do-while loop exits after j=2 check fails

Variable Tracker

Variable	Start	After 1	After 2	Final
i	0	1	2	2
j	0	1	2	2

Key Moments - 2 Insights

Why does the do-while loop always run at least once?

Because the do-while loop runs the loop body first before checking the condition, as shown in execution_table rows 4 and 5 where output occurs before condition check.

Can the while loop skip running the loop body entirely?

Yes, if the condition is false at the start, the while loop does not run the body at all. This is shown in execution_table row 3 where the condition fails and loop exits.

Visual Quiz - 3 Questions

Look at the execution table, what is the value of i when the while loop condition becomes false?

A 1

B 2

C 0

D 3

💡 Hint

Check execution_table row 3 where condition i < 2 is false.

At which step does the do-while loop print the first output?

A Step 6

B Step 5

C Step 4

D Step 3

💡 Hint

Look at execution_table row 4 where do-while prints j=0 before condition check.

If the initial value of i was 3, what would happen in the while loop?

A It would not print anything

B It would print 3 once

C It would print 0 and 1

D It would run forever

💡 Hint

Since while checks condition before running, if i=3 and condition is i<2, it fails immediately (see key_moments about while loop skipping).

Concept Snapshot

while loop:
- Checks condition first
- May skip loop if false
- Syntax: while(condition) { body }

do-while loop:
- Runs body once before checking
- Always runs at least once
- Syntax: do { body } while(condition);

Full Transcript

This lesson shows the difference between while and do-while loops in Java. The while loop checks the condition before running the loop body, so if the condition is false at the start, the loop body never runs. The do-while loop runs the loop body once before checking the condition, so it always runs at least once. The execution table traces variable values and outputs step-by-step for both loops. Key moments clarify why do-while always runs once and while can skip entirely. The quiz tests understanding of variable values and loop behavior from the execution steps.

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