Do–while loop in Java - Mini Project: Build & Apply

Do-while loop

📖 Scenario: Imagine you are creating a simple program that asks a user to enter a number repeatedly until they enter zero. This is like asking a friend to keep telling you numbers until they say "stop" by entering zero.

🎯 Goal: Build a Java program that uses a do-while loop to keep asking the user for numbers and stops when the user enters zero.

📋 What You'll Learn

Use a do-while loop to ask for input at least once

Use a variable called number to store the user input

Stop the loop when number is zero

Print each entered number except zero

💡 Why This Matters

🌍 Real World

Do-while loops are useful when you want to run a task at least once and then repeat it based on user input or a condition, like menus or input validation.

💼 Career

Understanding loops and user input is essential for many programming jobs, including software development, data processing, and automation.

1

Create a variable to store the number

Create an integer variable called number and set it to -1.

Java

int number = -1;  // Your code here

Hint

Use int number = -1; to create the variable.

2

Set up a Scanner to read user input

Create a Scanner object called scanner to read input from System.in.

Java

int number = -1;
Scanner scanner = new Scanner(System.in);  // Your code here

Hint

Use Scanner scanner = new Scanner(System.in); to read input.

3

Write the do-while loop to read numbers

Write a do block that asks the user to enter a number using System.out.println, reads the number into number using scanner.nextInt(), and prints the number if it is not zero. Then write the while condition to continue the loop while number is not zero.

Java

import java.util.Scanner;

int number = -1;
Scanner scanner = new Scanner(System.in);

// Your do-while loop here

Hint

Use do { ... } while (number != 0); and inside the block read and print the number.

4

Print a goodbye message after the loop ends

After the do-while loop, write a System.out.println statement to print "Goodbye!".

Java

import java.util.Scanner;

int number = -1;
Scanner scanner = new Scanner(System.in);

do {
    System.out.println("Enter a number (0 to stop):");
    number = scanner.nextInt();
    if (number != 0) {
        System.out.println("You entered: " + number);
    }
} while (number != 0);

// Your print statement here

Hint

Use System.out.println("Goodbye!"); after the loop.

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