For loop in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the basic structure of a for loop in C?

A for loop in C has three parts inside parentheses: initialization, condition, and update. It looks like:

for (initialization; condition; update) {
  // code to repeat
}

beginner

What happens if the condition in a for loop is false at the start?

The code inside the for loop does not run at all. The loop ends immediately.

beginner

How can you use a for loop to print numbers from 1 to 5?

You can write:

for (int i = 1; i <= 5; i++) {
  printf("%d\n", i);
}

This prints numbers 1, 2, 3, 4, 5 each on a new line.

intermediate

Can the update part of a for loop be empty?

Yes, the update part can be empty, but you must update the loop variable inside the loop body to avoid an infinite loop.

intermediate

What is an infinite loop and how can it happen with a for loop?

An infinite loop happens when the loop's condition never becomes false. For example, if you forget to update the loop variable, the loop runs forever.

Which part of the for loop runs first?

A Condition check

B Initialization

C Update

D Loop body

The initialization runs first, setting up the loop variable before checking the condition.

What happens if the condition in a for loop is false initially?

A The loop body runs once

B The loop runs infinitely

C The update runs once

D The loop body never runs

If the condition is false at the start, the loop body does not run at all.

Which of these is a correct for loop header to count from 0 to 9?

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

B for (int i = 1; i <= 10; i++)

C for (int i = 10; i > 0; i--)

D for (int i = 0; i <= 10; i--)

Option A correctly counts from 0 up to 9 by increasing i each time.

What must you do if you leave the update part empty in a for loop?

A Add a break statement

B Nothing, it works fine

C Update the loop variable inside the loop body

D Use a while loop instead

If the update is empty, you must update the loop variable inside the loop body to avoid infinite loops.

What is the output of this code?
for (int i = 1; i <= 3; i++) { printf("%d", i); }

A 123

B 1 2 3

C 321

D Error

The loop prints 1, then 2, then 3 without spaces, so output is '123'.

Explain the three parts of a for loop and their roles.

Think about what happens first, what keeps the loop running, and how the loop variable changes.

Describe how a for loop can cause an infinite loop and how to prevent it.

Focus on the condition and updating the loop variable.

Practice

1. What does a for loop in C primarily do?

easy

A. Repeats a block of code a fixed number of times

B. Defines a new variable

C. Stops the program execution

D. Declares a function

Solution

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

   A for loop is designed to repeat a block of code multiple times based on a condition.

2. Step 2: Identify the correct description

   Among the options, only repeating code a fixed number of times matches the for loop's purpose.

3. Final Answer:

   Repeats a block of code a fixed number of times -> Option A

4. Quick Check:

   For loop = repeat code [OK]

Hint: For loops repeat code a set number of times [OK]

Common Mistakes:

• Confusing for loop with variable declaration
• Thinking for loop stops program
• Mixing for loop with function declaration

2. Which of the following is the correct syntax for a for loop in C?

easy

A. for i = 0 to 5 { }

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

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

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

Solution

1. Step 1: Review standard for loop syntax in C

   The correct syntax is: for (initialization; condition; update) { body }

2. Step 2: Check each option

   for (int i = 0; i < 5; i++) { } matches the correct syntax with initialization, condition, and update in proper order.

3. Final Answer:

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

4. Quick Check:

   Syntax = for(init; cond; update) [OK]

Hint: For loop syntax: for(init; condition; update) [OK]

Common Mistakes:

• Using 'for i = 0 to 5' which is not C syntax
• Swapping condition and initialization
• Missing semicolons inside for parentheses

3. What is the output of this code?

for(int i = 1; i <= 3; i++) {
    printf("%d ", i);
}

medium

A. 3 2 1

B. 0 1 2 3

C. 1 2 3

D. 1 2 3 4

Solution

1. Step 1: Analyze the loop range

   The loop starts at i=1 and runs while i is less than or equal to 3, incrementing i by 1 each time.

2. Step 2: Determine printed values

   It prints i each time: 1, then 2, then 3, then stops.

3. Final Answer:

   1 2 3 -> Option C

4. Quick Check:

   Loop prints 1 to 3 [OK]

Hint: Check loop start, end, and increments carefully [OK]

Common Mistakes:

• Starting loop at 0 instead of 1
• Including an extra number beyond 3
• Printing in reverse order

4. Identify the error in this for loop:

for(int i = 0; i < 5; i)
    printf("%d ", i);

medium

A. Missing semicolon after printf statement

B. Missing increment operator in for loop update

C. Incorrect loop condition syntax

D. Variable i not declared

Solution

1. Step 1: Examine the for loop update part

   The update section is 'i' without increment operator, so i never changes.

2. Step 2: Understand the effect of missing increment

   Without 'i++' or similar, the loop runs infinitely or behaves incorrectly.

3. Final Answer:

   Missing increment operator in for loop update -> Option B

4. Quick Check:

   Update must change loop variable [OK]

Hint: For loop update must change variable (e.g., i++) [OK]

Common Mistakes:

• Forgetting to increment loop variable
• Confusing semicolon placement
• Assuming variable auto-increments

5. How can you use a for loop to print only even numbers from 2 to 10 inclusive in C?

hard

A. for(int i = 2; i <= 10; i += 2) { printf("%d ", i); }

B. for(int i = 0; i <= 10; i++) { if(i % 2 == 0) printf("%d ", i); }

C. for(int i = 2; i < 10; i++) { if(i % 2 == 0) printf("%d ", i); }

D. for(int i = 0; i <= 10; i += 3) { printf("%d ", i); }

Solution

1. Step 1: Understand the goal

   We want to print even numbers from 2 to 10 inclusive.

2. Step 2: Check each option

   for(int i = 2; i <= 10; i += 2) { printf("%d ", i); } starts at 2 and increments by 2, printing 2,4,6,8,10 exactly.

3. Final Answer:

   for(int i = 2; i <= 10; i += 2) { printf("%d ", i); } -> Option A

4. Quick Check:

   Increment by 2 prints evens directly [OK]

Hint: Increment by 2 to print evens directly [OK]

Common Mistakes:

• Using wrong increment step
• Off-by-one errors in loop condition
• Printing odd numbers by mistake