Do–while loop in Java - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start a do-while loop that prints "Hello" once.

Java

do {
    System.out.println("Hello");
} [1] (false);

Drag options to blanks, or click blank then click option'

Awhile

Bfor

Cif

Dswitch

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'if' instead of 'while' after the do block.

Forgetting the parentheses around the condition.

The do-while loop in Java uses the keyword while after the do block to specify the loop condition.

2fill in blank

medium

Complete the code to declare an integer variable i initialized to 0 before the do-while loop.

Java

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

Drag options to blanks, or click blank then click option'

AString

Bfloat

Cint

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Using float or boolean instead of int.

Using String for a number variable.

In Java, int is used to declare integer variables like i.

3fill in blank

hard

Fix the error in the do-while loop condition to correctly check if i is less than 5.

Java

int i = 0;
do {
    System.out.println(i);
    i++;
} while (i [1] 5);

Drag options to blanks, or click blank then click option'

A>=

B>

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using > or >= which would cause the loop to never run or run incorrectly.

Using == which only runs when i equals 5.

The loop should continue while i is less than 5, so the condition uses the < operator.

4fill in blank

hard

Fill both blanks to create a do-while loop that prints numbers from 1 to 3.

Java

int num = 1;
do {
    System.out.println(num);
    num[1];
} [2] (num <= 3);

Drag options to blanks, or click blank then click option'

A++

B--

Cwhile

Dif

Attempts:

3 left

💡 Hint

Common Mistakes

Using decrement operator -- which would count down instead of up.

Using if instead of while after the do block.

The variable num is increased by 1 each loop using ++. The loop continues while num is less than or equal to 3 using while.

5fill in blank

hard

Fill all three blanks to create a do-while loop that prints "Count: x" for x from 1 to 4.

Java

int count = [1];
do {
    System.out.println("Count: " + [2]);
    [3];
} while (count <= 4);

Drag options to blanks, or click blank then click option'

A1

Bcount

Ccount++

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Starting count at 0 which would print from 0 instead of 1.

Forgetting to increment count inside the loop.

The variable count starts at 1. Inside the loop, it prints the current count and then increases it by 1 using count++.

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