While loop execution model in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple while loop

What is the output of this C# code snippet?

C Sharp (C#)

int i = 0;
while (i < 3)
{
    Console.Write(i);
    i++;
}

A 012

B 123

C 0 1 2

D 0123

💡 Hint

Remember the loop runs while i is less than 3 and increments i each time.

The loop starts with i=0 and prints i before incrementing. It prints 0, then 1, then 2, then stops because i becomes 3.

❓ Predict Output

intermediate

While loop with break statement

What will be printed by this C# code?

C Sharp (C#)

int count = 0;
while (count < 5)
{
    if (count == 3) break;
    Console.Write(count);
    count++;
}

A 01234

B 0123

C 012

D 123

💡 Hint

The break stops the loop when count equals 3.

The loop prints 0, 1, 2 and then breaks when count is 3, so 3 and 4 are not printed.

🔧 Debug

advanced

Identify the infinite loop

Which option causes an infinite loop when run?

C Sharp (C#)

int x = 0;
while (x < 5)
{
    Console.Write(x);
    // missing increment
}

A

int x = 0;
while (x < 5)
{
    Console.Write(x);
}

B

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

C

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

D

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

💡 Hint

Check if the loop variable changes inside the loop.

Option A never increments x, so x stays 0 and the condition x < 5 is always true, causing an infinite loop.

❓ Predict Output

advanced

While loop with nested condition

What is the output of this code?

C Sharp (C#)

int i = 0;
while (i < 5)
{
    if (i % 2 == 0)
        Console.Write(i);
    i++;
}

A 246

B 135

C 01234

D 024

💡 Hint

Only print i when it is even.

The loop prints i only when i is even (0, 2, 4). Odd numbers are skipped.

❓ Predict Output

expert

While loop with complex condition and post-loop value

What is the value of variable 'result' after this code runs?

C Sharp (C#)

int i = 1;
int result = 0;
while (i < 10)
{
    if (i % 3 == 0)
        result += i;
    i += 2;
}

A 21

B 12

C 15

D 18

💡 Hint

Add i to result only if i is divisible by 3, increment i by 2 each time.

i takes values 1,3,5,7,9. Only 3 and 9 are divisible by 3. Sum is 3 + 9 = 12.

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