Do-while loop execution model in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Do-while loop execution model

Start

↓

Execute body

↓

Check condition

↓

Exit

The do-while loop runs the code inside the loop first, then checks the condition. If true, it repeats; if false, it stops.

Execution Sample

C Sharp (C#)

int i = 1;
do {
    Console.WriteLine(i);
    i++;
} while (i <= 3);

This code prints numbers 1 to 3 using a do-while loop that runs the body before checking the condition.

Execution Table

Step	i value	Action	Condition (i <= 3)	Loop continues?	Output
1	1	Execute body: print i, increment i	1 <= 3 is True	Yes	1
2	2	Execute body: print i, increment i	2 <= 3 is True	Yes	2
3	3	Execute body: print i, increment i	3 <= 3 is True	Yes	3
4	4	Condition checked, body skipped	4 <= 3 is False	No

💡 At step 4, i is 4, condition 4 <= 3 is False, so loop stops.

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	1	2	3	4	4

Key Moments - 3 Insights

Why does the loop body run even when the condition is false at the start?

Because in a do-while loop, the body runs first before the condition is checked, as shown in execution_table step 1.

When does the condition get checked in the do-while loop?

The condition is checked after the loop body runs, as seen in the 'Condition' column after each action in the execution_table.

What happens when the condition becomes false?

The loop stops immediately after the condition check shows false, as in execution_table step 4 where the loop exits.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i when the loop stops?

A 3

B 4

C 1

D 0

💡 Hint

Check the 'i value' column at step 4 in the execution_table where the loop exits.

At which step does the condition first become false?

A Step 1

B Step 3

C Step 4

D Step 2

💡 Hint

Look at the 'Condition' column in the execution_table to find when condition is false.

If the initial value of i was 5, how many times would the loop body run?

A 1 time

B 5 times

C 0 times

D Infinite times

💡 Hint

Remember do-while runs the body once before checking condition; see key_moments about first run.

Concept Snapshot

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

Runs loop body first, then checks condition.
Repeats if condition true, stops if false.
Always runs at least once.

Full Transcript

This visual execution shows how a do-while loop works in C#. The loop starts by running the code inside the body once before checking the condition. After running the body, it checks if the condition is true. If yes, it repeats the body again. If no, it stops. The example code prints numbers 1 to 3. The variable i starts at 1 and increases by 1 each time. The loop runs while i is less than or equal to 3. The execution table shows each step: the value of i, the action taken, the condition check, whether the loop continues, and what is printed. The variable tracker shows how i changes after each loop. Key moments explain why the body runs first and when the loop stops. The quiz questions help check understanding of the loop's behavior and variable changes.

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