While loop in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a while loop in C++?

A while loop repeats a block of code as long as a given condition is true. It checks the condition before running the code each time.

beginner

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

The code inside the while loop does not run even once if the condition is false at the beginning.

intermediate

How do you stop an infinite while loop?

You stop an infinite while loop by making sure the condition eventually becomes false, usually by changing a variable inside the loop.

beginner

Write a simple while loop that prints numbers from 1 to 5.

#include <iostream> int main() { int i = 1; while (i <= 5) { std::cout << i << std::endl; i++; } return 0; }

beginner

Why is it important to update the variable in the while loop condition?

If you don't update the variable, the condition might never become false, causing the loop to run forever (infinite loop).

What does a while loop do in C++?

A Runs code only if condition is false

B Repeats code while a condition is true

C Runs code once regardless of condition

D Stops the program immediately

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

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

A The loop runs once

B The program crashes

C The loop runs infinitely

D The loop does not run at all

If the condition is false initially, the while loop code block is skipped.

Which of these can cause an infinite while loop?

A Updating the loop variable correctly

B Using a for loop instead

C Condition always true and no variable update

D Condition false at start

If the condition never becomes false and the variable is not updated, the loop runs forever.

How do you usually stop a while loop?

A By making the condition false inside the loop

B By restarting the program

C By using a break outside the loop

D By ignoring the condition

Changing variables inside the loop to make the condition false stops the loop.

What is the output of this code? int i = 3; while (i > 0) { std::cout << i << " "; i--; }

A 3 2 1

B 1 2 3

C 0 1 2

D No output

The loop prints 3, then 2, then 1, decreasing i each time until i is 0.

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

Think about what happens if the condition never changes.

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

Start with int i = 5; then use while (i > 0).

Practice

1.

What does a while loop do in C++?

easy

A. Executes code only once

B. Repeats code a fixed number of times

C. Stops the program immediately

D. Repeats code while a condition is true

Solution

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

   A while loop runs the code inside it repeatedly as long as the condition is true.

2. Step 2: Compare options with definition

   Only Repeats code while a condition is true correctly states this behavior.

3. Final Answer:

   Repeats code while a condition is true -> Option D

4. Quick Check:

   while loop = repeat while true [OK]

Hint: While loops run as long as condition is true [OK]

Common Mistakes:

• Thinking while loops run a fixed number of times
• Confusing while with if statement
• Assuming while loops always run once

2.

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

???
{
    // code
}

easy

A. while condition

B. while {condition}

C. while (condition)

D. while: (condition)

Solution

1. Step 1: Recall correct while loop syntax

   The condition must be inside parentheses after the keyword while.

2. Step 2: Check each option

   while (condition) uses while (condition), which is correct syntax in C++.

3. Final Answer:

   while (condition) -> Option C

4. Quick Check:

   while loop syntax = while (condition) [OK]

Hint: Condition in while must be inside parentheses [OK]

Common Mistakes:

• Omitting parentheses around condition
• Using curly braces instead of parentheses
• Adding colon after while

3.

What is the output of this C++ code?

int i = 1;
while (i <= 3) {
    std::cout << i << " ";
    i++;
}

medium

A. 1 2 3 4

B. 1 2 3

C. 0 1 2

D. 3 2 1

Solution

1. Step 1: Analyze loop condition and increments

   Variable i starts at 1 and increases by 1 each loop until i > 3.

2. Step 2: Trace output values

   i = 1 prints "1 ", then 2 prints "2 ", then 3 prints "3 ". Loop stops after i becomes 4.

3. Final Answer:

   1 2 3 -> Option B

4. Quick Check:

   Prints 1 to 3 with spaces [OK]

Hint: Count from start to condition limit inclusive [OK]

Common Mistakes:

• Printing extra number after loop ends
• Starting count from 0 instead of 1
• Printing numbers in reverse

4.

Find the error in this code snippet:

int count = 5;
while (count > 0)
    std::cout << count << " ";

medium

A. Infinite loop because count is never changed

B. Variable count is not declared

C. Syntax error in while condition

D. Missing semicolon after cout statement

Solution

1. Step 1: Check loop body and variable changes

   The loop prints count but does not change it inside the loop.

2. Step 2: Understand loop behavior

   Since count stays 5, condition is always true, causing an infinite loop.

3. Final Answer:

   Infinite loop because count is never changed -> Option A

4. Quick Check:

   Loop variable must change to avoid infinite loop [OK]

Hint: Update loop variable inside while to avoid infinite loop [OK]

Common Mistakes:

• Forgetting to change loop variable
• Assuming loop ends automatically
• Misreading syntax as error

5.

Write a while loop in C++ that prints all even numbers from 2 to 10 inclusive.

Which code snippet correctly does this?

???

hard

A. int n = 2;
while (n <= 10) {
std::cout << n << " ";
n += 2;
}

B. int n = 1;
while (n < 10) {
if (n % 2 == 0) std::cout << n << " ";
n++;
}

C. int n = 2;
while (n < 10) {
std::cout << n << " ";
n += 2;
}

D. int n = 0;
while (n <= 10) {
std::cout << n << " ";
n += 2;
}

Solution

1. Step 1: Understand the requirement

   We need to print even numbers from 2 to 10 inclusive, so numbers: 2,4,6,8,10.

2. Step 2: Check each option

   int n = 2; while (n <= 10) { std::cout << n << " "; n += 2; } starts at 2, increments by 2, and stops at 10 inclusive, printing correct numbers.

3. Final Answer:

   2 4 6 8 10 -> Option A

4. Quick Check:

   Start at 2, add 2 each time, stop at 10 [OK]

Hint: Start at 2, add 2, stop at 10 inclusive [OK]

Common Mistakes:

• Stopping before 10 (using < instead of <=)
• Starting at 1 and missing even numbers
• Including 0 which is not in range