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

Recall & Review

beginner

What is the basic structure of a while loop in C#?

A while loop repeatedly executes a block of code as long as its condition is true. The structure is:

while (condition) {
  // code to repeat
}

beginner

When does the condition in a while loop get evaluated?

The condition is checked before each iteration. If the condition is false at the start, the loop body does not run at all.

intermediate

What happens if the while loop condition never becomes false?

The loop runs forever, causing an infinite loop. This can freeze or crash the program if not stopped.

intermediate

How can you safely exit a while loop before the condition becomes false?

You can use the break statement inside the loop to exit immediately when a certain condition is met.

intermediate

Explain the difference between while and do-while loops.

while checks the condition before running the loop body, so it may run zero times.
do-while runs the loop body first, then checks the condition, so it runs at least once.

When is the condition in a while loop checked?

A After each iteration

B Before each iteration

C Only once before the loop starts

D Only once after the loop ends

The while loop checks its condition before every iteration to decide whether to run the loop body.

What happens if the while loop condition is false at the start?

A The program crashes

B The loop body runs once

C The loop runs infinitely

D The loop body does not run at all

If the condition is false initially, the while loop skips the body completely.

Which statement can be used to exit a while loop early?

A continue

B exit

C break

D return

The break statement immediately stops the loop and continues with the code after it.

What is a risk of a while loop if the condition never becomes false?

A An infinite loop occurs

B The loop runs once

C The loop runs zero times

D The loop throws an error

If the condition never becomes false, the while loop runs forever, causing an infinite loop.

How does a do-while loop differ from a while loop?

A It runs the loop body at least once before checking the condition

B It checks the condition before running the loop body

C It cannot run infinitely

D It does not use a condition

do-while loops run the loop body first, then check the condition, so they always run at least once.

Describe how a while loop executes step-by-step in C#.

Think about what happens before and after each time the loop runs.

Explain why it is important to update variables inside a while loop.

Consider what happens if the condition never changes.

Practice

1. What does a while loop do in C#?

easy

A. Stops the program immediately

B. Executes code only once regardless of condition

C. Repeats code a fixed number of times

D. Repeats a block of code as long as a condition is true

Solution

1. Step 1: Understand the purpose of a while loop

   A while loop runs repeatedly while its condition remains true.

2. Step 2: Compare with other options

   Options B, C, and D do not describe the behavior of a while loop correctly.

3. Final Answer:

   Repeats a block of code as long as a condition is true -> Option D

4. Quick Check:

   while loop = repeats while condition true [OK]

Hint: While loops run only if condition is true before each repeat [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with do-while loops
• Assuming code runs at least once without checking condition

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

easy

A. while (condition) { /* code to repeat */ }

B. while condition { /* code to repeat */ }

C. while: (condition) { /* code to repeat */ }

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

Solution

1. Step 1: Recall correct while loop syntax

   The correct syntax requires parentheses around the condition and braces for the code block.

2. Step 2: Identify syntax errors in other options

   A has a colon after while, B misses parentheses around the condition, D has a semicolon after the condition making the loop body empty.

3. Final Answer:

   while (condition) { /* code to repeat */ } -> Option A

4. Quick Check:

   Correct syntax = while (condition) { /* code to repeat */ } [OK]

Hint: Always use parentheses for condition and braces for loop body [OK]

Common Mistakes:

• Omitting parentheses around condition
• Adding semicolon after while condition
• Using colon instead of parentheses

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

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

medium

A. 0 1 2 3

B. 3 2 1

C. 3 2 1 0

D. 1 2 3

Solution

1. Step 1: Trace the loop iterations

   count starts at 3, prints 3, then decrements to 2, prints 2, then 1, prints 1, then decrements to 0 and stops.

2. Step 2: Confirm output sequence

   The printed numbers are "3 2 1 " exactly in that order.

3. Final Answer:

   3 2 1 -> Option B

4. Quick Check:

   Loop prints count down from 3 to 1 [OK]

Hint: Count down loops print from start to 1 before stopping [OK]

Common Mistakes:

• Including 0 in output
• Printing numbers in ascending order
• Forgetting to decrement count

4. Identify the error in this code snippet:

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

medium

A. Missing braces causes only Console.Write(i) to loop, i++ runs once

B. Syntax error due to missing semicolon after while

C. Infinite loop because i is never incremented

D. No error, code runs correctly

Solution

1. Step 1: Analyze loop body without braces

   Only the first statement after while loops; here, only Console.Write(i) is inside the loop.

2. Step 2: Understand effect on i++

   i++ runs once after the loop finishes, so i never increments inside the loop, causing an infinite loop if braces were not considered.

3. Final Answer:

   Missing braces causes only Console.Write(i) to loop, i++ runs once -> Option A

4. Quick Check:

   Braces needed to include i++ in loop [OK]

Hint: Use braces to include multiple statements in while loop [OK]

Common Mistakes:

• Assuming both statements are inside loop without braces
• Thinking missing semicolon after while is error
• Confusing infinite loop cause

5. You want to read user input repeatedly until they type "exit". Which while loop correctly implements this in C#?

hard

A. while (true) {
string input = Console.ReadLine();
if (input != "exit") break;
}

B. string input;
do {
input = Console.ReadLine();
} while (input != "exit");

C. string input = Console.ReadLine();
while (input != "exit") {
input = Console.ReadLine();
}

D. string input;
while (input != "exit") {
input = Console.ReadLine();
}

Solution

1. Step 1: Understand the need to read input before loop

   We must read input once before the while condition to avoid skipping the first input.

2. Step 2: Check each option's logic

   string input = Console.ReadLine(); while (input != "exit") { input = Console.ReadLine(); } uses priming read pattern correctly. string input; do { input = Console.ReadLine(); } while (input != "exit"); is do-while, not while loop. while (true) { string input = Console.ReadLine(); if (input != "exit") break; } breaks incorrectly on non-exit input, stopping too early. string input; while (input != "exit") { input = Console.ReadLine(); } uses uninitialized input, compile error.

3. Final Answer:

   string input = Console.ReadLine(); while (input != "exit") { input = Console.ReadLine(); } -> Option C

4. Quick Check:

   Read input before while, then loop while not "exit" [OK]

Hint: Read input once before while to check condition correctly [OK]

Common Mistakes:

• Not reading input before while loop
• Using do-while instead of while
• Using infinite loop with break instead of while condition