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

Concept Flow - For loop execution model

Initialize i = 0

↓

Check: i < 3?

No→EXIT

Yes↓

Execute loop body

↓

Update: i = i + 1

↩Back to Check

The for loop starts by setting i to 0, then checks if i is less than 3. If yes, it runs the loop body, then increases i by 1 and repeats. If no, it stops.

Execution Sample

C Sharp (C#)

for (int i = 0; i < 3; i++)
{
    Console.WriteLine(i);
}

This code prints numbers 0, 1, and 2 using a for loop.

Execution Table

Step	i	Condition (i < 3)	Action	Output
1	0	True	Print 0	0
2	1	True	Print 1	1
3	2	True	Print 2	2
4	3	False	Exit loop

💡 i reaches 3, condition 3 < 3 is False, loop stops

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
i	0	1	2	3	3

Key Moments - 2 Insights

Why does the loop stop when i equals 3?

Because the condition i < 3 becomes false at step 4 in the execution_table, so the loop exits.

When is the value of i increased?

i is increased after executing the loop body, as shown between steps 1 to 2, 2 to 3, and 3 to 4 in the variable_tracker.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of i at step 3?

A 2

B 3

C 1

D 0

💡 Hint

Check the 'i' column in the execution_table at step 3.

At which step does the loop condition become false?

A Step 2

B Step 3

C Step 4

D Step 1

💡 Hint

Look at the 'Condition (i < 3)' column in the execution_table.

If the loop condition changed to i < 2, how many times would the loop body run?

A 1 time

B 2 times

C 3 times

D 0 times

💡 Hint

Refer to the variable_tracker and how many times i satisfies the condition.

Concept Snapshot

for (int i = 0; i < limit; i++)
{
  // code runs while condition true
}

1. Initialize i
2. Check condition
3. Run body if true
4. Increment i
5. Repeat or exit

Full Transcript

This visual execution shows how a for loop works in C#. It starts by setting i to 0. Then it checks if i is less than 3. If yes, it runs the loop body and prints i. After that, it increases i by 1. This repeats until i is 3, when the condition becomes false and the loop stops. The execution_table shows each step with i's value, condition check, action, and output. The variable_tracker shows how i changes after each iteration. Key moments explain why the loop stops and when i increases. The quiz tests understanding of i's value at steps, when the loop ends, and how changing the condition affects the loop count.

Practice

1. What are the three main parts of a for loop in C#?

easy

A. Declaration, execution, and termination

B. Initialization, condition check, and increment/decrement

C. Start, middle, and end

D. Input, process, and output

Solution

1. Step 1: Understand the structure of a for loop

   A for loop has three parts inside parentheses: initialization, condition, and increment/decrement.

2. Step 2: Match parts to options

   Initialization sets the start value, condition checks if the loop continues, and increment/decrement changes the variable each time.

3. Final Answer:

   Initialization, condition check, and increment/decrement -> Option B

4. Quick Check:

   for loop parts = initialization, condition, increment [OK]

Hint: Remember: start, check, change in for loop [OK]

Common Mistakes:

• Confusing for loop parts with general program steps
• Thinking for loop has only two parts
• Mixing up condition and increment roles

2. Which of the following is the correct syntax for a basic for loop in C# that counts from 0 to 4?

easy

A. for (int i = 0; i < 5; i++) { }

B. for int i = 0; i < 5; i++ { }

C. for (int i = 0; i < 4; i++) { }

D. for (i = 0; i < 5; i++) { }

Solution

1. Step 1: Check syntax correctness

   In C#, the for loop syntax requires parentheses around initialization, condition, and increment, and braces for the loop body.

2. Step 2: Validate each option

   for (int i = 0; i < 4; i++) { } only counts 0 to 3 (missing 4); for int i = 0; i < 5; i++ { } misses parentheses; for (int i = 0; i < 5; i++) { } is correct syntax counting 0 to 4; for (i = 0; i < 5; i++) { } misses type declaration for i.

3. Final Answer:

   for (int i = 0; i < 5; i++) { } -> Option A

4. Quick Check:

   Correct for loop syntax uses parentheses and semicolons [OK]

Hint: Use parentheses and semicolons in for loop header [OK]

Common Mistakes:

• Omitting parentheses around for loop parts
• Missing type declaration for loop variable
• Using wrong comparison operators

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

for (int i = 1; i <= 3; i++)
{
    Console.Write(i + " ");
}

medium

A. 1 2 3

B. 0 1 2 3

C. 1 2 3 4

D. 3 2 1

Solution

1. Step 1: Analyze loop range

   The loop starts at i=1 and runs while i is less than or equal to 3, so it runs for i=1, 2, 3.

2. Step 2: Determine output per iteration

   Each iteration prints the current i followed by a space, so output is "1 ", "2 ", "3 ".

3. Final Answer:

   1 2 3 -> Option A

4. Quick Check:

   Loop prints 1 to 3 with spaces [OK]

Hint: Check loop start and end values carefully [OK]

Common Mistakes:

• Assuming loop starts at 0 instead of 1
• Including an extra number beyond the condition
• Printing in reverse order

4. Identify the error in this C# for loop:

for (int i = 0; i < 5; i++)
    Console.WriteLine(i);
    Console.WriteLine("Done");

medium

A. Infinite loop due to wrong condition

B. Syntax error: semicolon missing after for statement

C. Loop variable i is not declared

D. Missing braces causes only first line to loop

Solution

1. Step 1: Check loop body structure

   Without braces, only the first statement after for is inside the loop; the second statement runs once after the loop.

2. Step 2: Understand effect on output

   "Done" prints once after loop finishes, which might be unexpected if intended inside loop.

3. Final Answer:

   Missing braces causes only first line to loop -> Option D

4. Quick Check:

   Use braces to include multiple statements in loop [OK]

Hint: Use braces {} for multiple statements in loops [OK]

Common Mistakes:

• Assuming all lines are inside loop without braces
• Thinking semicolon is needed after for header
• Confusing variable declaration errors

5. You want to print all even numbers from 2 to 10 using a for loop in C#. Which loop header correctly achieves this?

hard

A. for (int i = 0; i < 10; i += 2)

B. for (int i = 1; i <= 10; i++) if (i % 2 == 0)

C. for (int i = 2; i <= 10; i += 2)

D. for (int i = 2; i < 10; i++) if (i % 2 == 0)

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10 inclusive, so loop variable should start at 2 and go up to 10.

2. Step 2: Check each option

   for (int i = 2; i <= 10; i += 2) increments by 2 starting at 2, so it prints 2,4,6,8,10. for (int i = 1; i <= 10; i++) if (i % 2 == 0) has a condition inside loop but header increments by 1; it works but is less efficient. for (int i = 0; i < 10; i += 2) starts at 0 and stops before 10, missing 10. for (int i = 2; i < 10; i++) if (i % 2 == 0) increments by 1 and stops before 10, missing 10.

3. Final Answer:

   for (int i = 2; i <= 10; i += 2) -> Option C

4. Quick Check:

   Increment by 2 from 2 to 10 for even numbers [OK]

Hint: Increment by 2 starting at 2 for even numbers [OK]

Common Mistakes:

• Using i++ and filtering inside loop instead of incrementing by 2
• Starting loop at 0 or 1 instead of 2
• Stopping loop before reaching 10