For loop execution model in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to print numbers from 0 to 4 using a for loop.

C Sharp (C#)

for (int i = 0; i [1] 5; i++)
{
    Console.WriteLine(i);
}

Drag options to blanks, or click blank then click option'

A<=

B>

C>=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= causes the loop to run one extra time.

Using > or >= makes the loop not run at all.

The for loop runs while i is less than 5, so it prints 0 to 4.

2fill in blank

medium

Complete the code to sum numbers from 1 to 5 using a for loop.

C Sharp (C#)

int sum = 0;
for (int num = 1; num [1] 5; num++)
{
    sum += num;
}
Console.WriteLine(sum);

Drag options to blanks, or click blank then click option'

A<=

B<

C>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Using < excludes 5, so sum is less than expected.

Using > or >= causes the loop not to run.

The loop includes 5 by using <=, so sum is 1+2+3+4+5 = 15.

3fill in blank

hard

Fix the error in the for loop condition to count down from 5 to 1.

C Sharp (C#)

for (int i = 5; i [1] 1; i--)
{
    Console.WriteLine(i);
}

Drag options to blanks, or click blank then click option'

A>=

B<

C<=

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using < or <= causes the loop to never run.

Using == runs the loop only once.

The loop runs while i is greater than or equal to 1, counting down.

4fill in blank

hard

Fill both blanks to create a for loop that prints even numbers from 2 to 10.

C Sharp (C#)

for (int i = [1]; i [2] 10; i += 2)
{
    Console.WriteLine(i);
}

Drag options to blanks, or click blank then click option'

A2

B<

C<=

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Starting at 1 prints odd numbers.

Using < excludes 10 from printing.

The loop starts at 2 and runs while i is less than or equal to 10, printing even numbers.

5fill in blank

hard

Fill all three blanks to create a for loop that prints the squares of numbers from 1 to 5.

C Sharp (C#)

for (int [1] = 1; [2] [3] 5; [1]++)
{
    Console.WriteLine([1] * [1]);
}

Drag options to blanks, or click blank then click option'

Ai

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names causes errors.

Using < excludes 5 from the loop.

The loop variable i starts at 1 and runs while it is less than or equal to 5, printing squares.

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