While loop in C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in C?

A while loop repeatedly executes a block of code as long as a given condition is true.

beginner

How does the while loop check its condition?

Before each iteration, the while loop checks the condition. If it is true, the loop runs the code inside; if false, it stops.

beginner

What happens if the while loop condition is false at the start?

The loop body does not run even once if the condition is false initially.

beginner

Write a simple while loop that prints numbers 1 to 3.

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

beginner

Why is it important to update the loop variable inside a while loop?

Updating the loop variable prevents an infinite loop by eventually making the condition false.

What does a while loop do in C?

A Repeats code while a condition is true

B Runs code once regardless of condition

C Runs code only if condition is false

D Runs code a fixed number of times

A while loop repeats the code block as long as the condition remains true.

When is the condition in a while loop checked?

A Never

B After each iteration

C Only once at the start

D Before each iteration

The condition is checked before running the loop body each time.

What happens if the loop condition is false at the start?

A Loop runs once

B Loop body does not run

C Loop runs infinitely

D Loop runs twice

If the condition is false initially, the loop body is skipped.

Which of these can cause an infinite while loop?

A Not updating the loop variable

B Updating the loop variable correctly

C Using <code>for</code> loop instead

D Printing inside the loop

If the loop variable is not updated, the condition may never become false, causing an infinite loop.

What is the output of this code? int i = 1; while (i <= 2) { printf("%d ", i); i++; }

A 2 3

B 1 2 3

C 1 2

D No output

The loop prints 1 and 2, then stops when i becomes 3.

Explain how a while loop works in C and why updating the loop variable is important.

Think about what happens if the condition never changes.

Write a simple while loop in C that counts down from 5 to 1 and prints each number.

Start with int i = 5 and decrease i each time.

Practice

1. What does a while loop do in C programming?

easy

A. Repeats a block of code while a condition is true

B. Executes code only once regardless of condition

C. Runs code a fixed number of times without checking condition

D. Stops the program immediately

Solution

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

   A while loop runs repeatedly as long as its condition remains true.

2. Step 2: Compare options with the definition

   Only Repeats a block of code while a condition is true correctly describes this behavior.

3. Final Answer:

   Repeats a block of code while a condition is true -> Option A

4. Quick Check:

   While loop = repeat while condition true [OK]

Hint: While loops run only if condition is true before each repeat [OK]

Common Mistakes:

• Thinking while loop runs at least once even if condition is false
• Confusing while loop with for loop fixed iterations
• Believing while loop stops program immediately

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

easy

A. while (condition) { /* code */ }

B. while condition { /* code */ }

C. while [condition] { /* code */ }

D. while: (condition) { /* code */ }

Solution

1. Step 1: Recall C while loop syntax

   The condition must be inside parentheses and the code block inside braces.

2. Step 2: Check each option

   Only while (condition) { /* code */ } uses correct parentheses and braces syntax.

3. Final Answer:

   while (condition) { /* code */ } -> Option A

4. Quick Check:

   Correct syntax uses parentheses and braces [OK]

Hint: Remember: while(condition) with parentheses and braces [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using square brackets instead of parentheses
• Adding colon after while keyword

3. What is the output of this C code?

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

medium

A. 0 1 2 3

B. 1 2 3 4

C. 3 2 1

D. 1 2 3

Solution

1. Step 1: Trace the loop variable values

   i starts at 1, prints 1, then increments to 2, prints 2, then 3, and stops after printing 3 because i becomes 4 and condition fails.

2. Step 2: Confirm printed output

   The printed numbers are "1 2 3 " with spaces after each number.

3. Final Answer:

   1 2 3 -> Option D

4. Quick Check:

   Prints 1 to 3 with spaces [OK]

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

Common Mistakes:

• Including 4 in output by mistake
• Starting from 0 instead of 1
• Printing numbers in reverse order

4. Identify the error in this code snippet:

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

medium

A. Syntax error due to missing braces

B. Missing semicolon after while condition

C. Infinite loop because i is never updated

D. Variable i is not declared

Solution

1. Step 1: Analyze loop behavior

   Variable i starts at 0 and condition is i < 5, but i is never incremented inside the loop.

2. Step 2: Understand consequence

   Since i never changes, the condition stays true forever, causing an infinite loop.

3. Final Answer:

   Infinite loop because i is never updated -> Option C

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Always update loop variable inside while loop [OK]

Common Mistakes:

• Assuming loop ends without variable update
• Thinking missing braces cause syntax error here
• Ignoring infinite loop possibility

5. How can you modify this code to print all even numbers from 2 to 10 using a while loop?

int num = 1;
while (num <= 10) {
    printf("%d ", num);
    num++;
}

hard

A. Use a for loop instead of while loop

B. Initialize num to 2 and increment by 2 inside the loop

C. Initialize num to 0 and increment by 1 inside the loop

D. Keep num at 1 and print only if num % 2 == 0 without incrementing

Solution

1. Step 1: Set starting point and increment for even numbers

   Start num at 2 (first even number) and increase by 2 each time to get only even numbers.

2. Step 2: Modify loop accordingly

   Change initialization to num = 2 and inside loop do num += 2 instead of num++.

3. Final Answer:

   Initialize num to 2 and increment by 2 inside the loop -> Option B

4. Quick Check:

   Start at 2, add 2 each time for evens [OK]

Hint: Start at first even, add 2 each loop to get evens [OK]

Common Mistakes:

• Not incrementing num causing infinite loop
• Printing odd numbers by starting at 1
• Using for loop when question asks for while loop