While loop in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a simple while loop

What is the output of this C++ code?

C++

#include <iostream>
int main() {
    int i = 1;
    while (i <= 3) {
        std::cout << i << " ";
        i++;
    }
    return 0;
}

A 0 1 2 3

B 2 3 4

C 1 2 3 4

D 1 2 3

💡 Hint

The loop runs while i is less than or equal to 3, starting from 1.

The loop starts with i=1 and prints i, then increments i by 1 each time until i becomes 4, which breaks the loop. So it prints 1 2 3

🧠 Conceptual

intermediate

Understanding while loop condition

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

A The loop body never runs.

B The loop runs twice.

C The loop runs infinitely.

D The loop body runs once and then stops.

💡 Hint

Think about when the condition is checked in a while loop.

A while loop checks the condition before running the loop body. If the condition is false at the start, the loop body does not run at all.

❓ Predict Output

advanced

Output with nested while loops

What is the output of this C++ code?

C++

#include <iostream>
int main() {
    int i = 1;
    while (i <= 2) {
        int j = 1;
        while (j <= 2) {
            std::cout << i * j << " ";
            j++;
        }
        i++;
    }
    return 0;
}

A 1 1 2 2

B 1 2 2 4

C 1 2 3 4

D 2 4 6 8

💡 Hint

The outer loop runs twice, and for each iteration, the inner loop runs twice.

For i=1, j=1 and 2: prints 1*1=1 and 1*2=2. For i=2, j=1 and 2: prints 2*1=2 and 2*2=4. So output is '1 2 2 4 '.

🔧 Debug

advanced

Identify the error in this while loop

What error does this code produce when compiled and run?

C++

#include <iostream>
int main() {
    int i = 0;
    while (i < 3) {
        std::cout << i << " ";
    }
    return 0;
}

A Infinite loop (program never ends)

B Syntax error: missing semicolon

C Runtime error: division by zero

D No output

💡 Hint

Look at how the variable controlling the loop changes inside the loop.

Variable i is never incremented inside the loop, so the condition i < 3 is always true, causing an infinite loop.

🚀 Application

expert

Calculate sum using while loop

What is the value of 'sum' after running this code?

C++

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

A 5

B 10

C 15

D 0

💡 Hint

Sum of numbers from 1 to 5 is what you need to find.

The loop adds 1 + 2 + 3 + 4 + 5 = 15 to sum.

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