Do-while loop execution model in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a do-while loop in C#?

A do-while loop is a control flow statement that executes a block of code at least once and then repeats the loop as long as a specified condition is true.

beginner

How does the execution order of a do-while loop differ from a while loop?

In a do-while loop, the code block runs first, then the condition is checked. In a while loop, the condition is checked first before running the code block.

beginner

What happens if the condition in a do-while loop is false on the first check?

The code inside the do-while loop still runs once before the condition is checked and the loop stops if the condition is false.

beginner

Write the basic syntax of a do-while loop in C#.

do { // code to execute } while (condition);

beginner

Why might you choose a do-while loop over a while loop?

You choose a do-while loop when you want the code inside the loop to run at least once, regardless of the condition.

What is guaranteed when using a do-while loop?

A The loop body executes at least once

B The loop body may never execute

C The condition is checked before the first execution

D The loop runs exactly once

A do-while loop always runs the code block once before checking the condition.

Where is the condition checked in a do-while loop?

A At the start of the program

B Before the loop body

C After the loop body

D Only once at the end of the program

The condition is checked after the loop body executes in a do-while loop.

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

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

B while (condition) { /* code */ }

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

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

Option A shows the correct do-while loop syntax.

If the condition is false initially, how many times does the do-while loop run?

A Infinite times

B Zero times

C Twice

D Once

The do-while loop runs once before checking the condition.

Which scenario is best suited for a do-while loop?

A When you want to run code only if a condition is true

B When you want to run code at least once regardless of condition

C When you want to run code zero or more times

D When you want to run code exactly twice

Do-while loops ensure the code runs at least once, regardless of the condition.

Explain how a do-while loop works and how it differs from a while loop.

Think about when the condition is checked in each loop.

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

Imagine you want to show a menu and ask for input at least once.

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