Do-while loop execution model in C Sharp (C#) - Mini Project: Build & Apply

Do-while loop execution model

📖 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 typing zero.

🎯 Goal: You will build a program using a do-while loop that keeps asking for numbers and adds them up. The program will stop asking when the user enters zero, then it will show the total sum of all numbers entered.

📋 What You'll Learn

Create an integer variable sum to hold the total.

Create an integer variable number to hold the current input.

Use a do-while loop to ask the user for a number.

Add the entered number to sum inside the loop.

Stop the loop when the user enters zero.

Print the total sum after the loop ends.

💡 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 a condition, like asking for user input until they want to stop.

💼 Career

Understanding do-while loops helps in many programming jobs where repeated actions are needed, such as menu systems, data entry, or game loops.

1

Create variables for sum and number

Create an integer variable called sum and set it to 0. Also create an integer variable called number and set it to 0.

C Sharp (C#)

int sum = 0;
int number = 0;
// Your code here

Hint

Think of sum as your total score and number as the new score you get each time.

2

Set up the do-while loop

Write a do block that asks the user to enter a number using Console.ReadLine() and converts it to an integer stored in number. Then add number to sum. Use a while condition to continue the loop as long as number is not zero.

C Sharp (C#)

int sum = 0;
int number = 0;
// Your code here

Hint

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

3

Adjust sum to exclude the last zero

After the do-while loop, subtract number from sum to remove the last zero added.

C Sharp (C#)

int sum = 0;
int number = 0;
do
{
    Console.Write("Enter a number (0 to stop): ");
    number = int.Parse(Console.ReadLine() ?? "0");
    sum += number;
} while (number != 0);
// Your code here

Hint

Since zero was added last, subtract it to keep the sum correct.

4

Print the total sum

Write a Console.WriteLine statement to display the text Total sum: followed by the value of sum.

C Sharp (C#)

int sum = 0;
int number = 0;
do
{
    Console.Write("Enter a number (0 to stop): ");
    number = int.Parse(Console.ReadLine() ?? "0");
    sum += number;
} while (number != 0);
sum -= number;
// Your code here

Hint

Use Console.WriteLine with an f-string style to show the sum.

Practice

1. What is the main difference between a do-while loop and a while loop in C#?

easy

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

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

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

D. The while loop executes the code block only if the condition is false.

Solution

1. Step 1: Understand do-while behavior

   The do-while loop runs the code block first, then checks the condition to decide if it repeats.

2. Step 2: Compare with while loop

   The while loop checks the condition first before running the code block, so it may not run at all.

3. Final Answer:

   The 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, then checks condition [OK]

Common Mistakes:

• Thinking while loop runs code before condition check
• Believing do-while never checks condition
• Confusing the order of execution

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

easy

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

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

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

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

Solution

1. Step 1: Recall correct do-while syntax

   The do-while loop syntax is: do { /* code */ } while (condition);

2. Step 2: Identify incorrect options

   Options A, B, and D have incorrect order or missing parentheses/braces.

3. Final Answer:

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

4. Quick Check:

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

Hint: Syntax always: do { } while (condition); with semicolon [OK]

Common Mistakes:

• Omitting semicolon after while condition
• Placing while before do
• Using parentheses incorrectly

3. What will be the output of the following C# code?

int count = 3;
do {
    Console.Write(count + " ");
    count--;
} while (count > 0);

medium

A. 1 2 3

B. 2 1 0

C. 3 2 1 0

D. 3 2 1

Solution

1. Step 1: Trace the loop execution

   Initial count is 3. The loop prints count, then decrements it. It repeats while count > 0.

2. Step 2: List printed values

   First iteration: prints 3, count becomes 2.
   Second: prints 2, count becomes 1.
   Third: prints 1, count becomes 0.
   Loop stops because count is not > 0.

3. Final Answer:

   3 2 1 -> Option D

4. Quick Check:

   Prints 3,2,1 then stops [OK]

Hint: Count down from 3, print before decrement, stop at zero [OK]

Common Mistakes:

• Including 0 in output
• Printing after decrement instead of before
• Confusing loop condition

4. Identify the error in the following do-while loop code snippet:

int i = 0;
do {
    Console.Write(i);
    i++;
} while i < 5;

medium

A. The variable i is not initialized.

B. The loop will never execute because condition is false.

C. Missing parentheses around the condition in the while statement.

D. The semicolon after the while condition is missing.

Solution

1. Step 1: Check syntax of while condition

   The condition in a do-while loop must be enclosed in parentheses: while (condition);

2. Step 2: Identify missing parentheses

   The code has while i < 5; without parentheses, causing a syntax error.

3. Final Answer:

   Missing parentheses around the condition in the while statement. -> Option C

4. Quick Check:

   Condition must be in parentheses [OK]

Hint: Always put condition inside parentheses after while [OK]

Common Mistakes:

• Forgetting parentheses around condition
• Missing semicolon after while statement
• Assuming variable initialization error

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 number greater than zero. Which code snippet correctly implements this?

hard

A. int num;
do {
Console.WriteLine("Enter a positive number:");
num = int.Parse(Console.ReadLine());
} while (num <= 0);

B. int num = 0;
do {
Console.WriteLine("Enter a positive number:");
num = int.Parse(Console.ReadLine());
} while (num > 0);

C. int num;
do {
Console.WriteLine("Enter a positive number:");
num = int.Parse(Console.ReadLine());
} while (num < 0);

D. int num = 1;
do {
Console.WriteLine("Enter a positive number:");
num = int.Parse(Console.ReadLine());
} while (num == 0);

Solution

1. Step 1: Understand the loop condition needed

   The loop should repeat while the number is not positive, i.e., while number is less than or equal to zero.

2. Step 2: Check each option's condition

   int num; do { Console.WriteLine("Enter a positive number:"); num = int.Parse(Console.ReadLine()); } while (num <= 0); uses while (num <= 0); which repeats until user enters a positive number.
   Other options have incorrect conditions that either stop too early or never repeat correctly.

3. Final Answer:

   int num; do { Console.WriteLine("Enter a positive number:"); num = int.Parse(Console.ReadLine()); } while (num <= 0); -> Option A

4. Quick Check:

   Repeat while number ≤ 0 to ensure positivity [OK]

Hint: Repeat while input ≤ 0 to ensure positive number [OK]

Common Mistakes:

• Using wrong comparison operators in condition
• Initializing variable incorrectly
• Stopping loop too early or never