Do-while loop execution model in C Sharp (C#) - Time & Space Complexity

Time Complexity: Do-while loop execution model

O(n)

Understanding Time Complexity

We want to understand how the time a do-while loop takes changes as the input size grows.

How many times does the loop run when the input gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int i = 0;
do
{
    Console.WriteLine(i);
    i++;
} while (i < n);
    

This code prints numbers from 0 up to n-1 using a do-while loop.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The do-while loop runs the print statement repeatedly.
• How many times: It runs exactly n times, once for each number from 0 to n-1.

How Execution Grows With Input

As n grows, the loop runs more times, directly matching n.

Input Size (n)	Approx. Operations
10	10
100	100
1000	1000

Pattern observation: The number of operations grows in a straight line with n.

Final Time Complexity

Time Complexity: O(n)

This means the time taken grows directly in proportion to the input size.

Common Mistake

[X] Wrong: "A do-while loop always runs at least twice, so it has constant time."

[OK] Correct: The do-while loop runs at least once, but the total runs depend on n, so time grows with input size.

Interview Connect

Understanding how loops like do-while behave helps you explain how your code scales, a key skill in programming.

Self-Check

"What if the loop condition changed to run while i <= n? How would the time complexity change?"

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