Share Your Feedback

C++ Loops: Fix the Mistakes

Practice identifying and correcting common C++ beginner mistakes in syntax, variables, operators, and output with simple debugging exercises.

Instructions

For each question:

Read the problem statement.
Examine the given code carefully.
Identify the mistake or mistakes.
Explain why the code is incorrect.
Correct the code.
Compile and run the corrected program.
Explain how your correction solves the problem.

Important: Try to find the mistake yourself before checking the solution.

Section 1: `while` Loop

Question 1: Missing Semicolon

Task: Find and fix the mistake in the following program.

Incorrect Code

int i = 1

while (i <= 5)
{
    cout << i << endl;
    i++;
}

Question 2: Incorrect Condition

Task: Find and fix the mistake so the program displays numbers from 1 to 5.

Incorrect Code

int i = 1;

while (i >= 5)
{
    cout << i << endl;
    i++;
}

Question 3: Missing Update Statement

Task: Find and fix the mistake so the program displays numbers from 1 to 5 and terminates.

Incorrect Code

int i = 1;

while (i <= 5)
{
    cout << i << endl;
}

Question 4: Incorrect Initialization

Task: Find and fix the mistake so the program displays numbers from 1 to 10.

Incorrect Code

int i = 0;

while (i <= 10)
{
    cout << i << endl;
    i++;
}

Question 5: Incorrect Increment

Task: Find and fix the mistake so the program displays even numbers from 2 to 10.

Incorrect Code

int i = 2;

while (i <= 10)
{
    cout << i << endl;
    i = i + 1;
}

Question 6: Incorrect Decrement

Task: Find and fix the mistake so the program displays numbers from 10 down to 1.

Incorrect Code

int i = 10;

while (i >= 1)
{
    cout << i << endl;
    i++;
}

Question 7: Incorrect Comparison Operator

Task: Find and fix the mistake so the program displays numbers from 1 to 5.

Incorrect Code

int i = 1;

while (i < 5)
{
    cout << i << endl;
    i++;
}

Question 8: Incorrect Loop Variable

Task: Find and fix the mistake so the program displays numbers from 1 to 5.

Incorrect Code

int i = 1;

while (i <= 5)
{
    cout << i << endl;
    j++;
}

Question 9: Incorrect Sum Calculation

Task: Find and fix the mistake so the program calculates the sum of numbers from 1 to 5. The expected sum is 15.

Incorrect Code

int i = 1;
int sum = 0;

while (i <= 5)
{
    sum = i;
    i++;
}

cout << "Sum = " << sum;

Question 10: Incorrect Condition for Odd Numbers

Task: Find and fix the mistake so the program displays odd numbers from 1 to 9.

Incorrect Code

int i = 1;

while (i <= 9)
{
    cout << i << endl;
    i = i + 2;
}

Question 11: Incorrect Input Condition

Task: Find and fix the mistake so the program continues asking for a number until the user enters 0.

Incorrect Code

int num;

cout << "Enter a number: ";
cin >> num;

while (num == 0)
{
    cout << "Enter a number: ";
    cin >> num;
}

Question 12: Incorrect Counter Update

Task: Find and fix the mistake so the program counts from 1 to 10 and displays the correct count.

Incorrect Code

int i = 1;
int count = 0;

while (i <= 10)
{
    count++;
    i++;
}

cout << "Count = " << i;

Question 13: Incorrect Factorial Calculation

Task: Find and fix the mistake so the program calculates the factorial of 5. The expected result is 120.

Incorrect Code

int n = 5;
int fact = 0;

while (n >= 1)
{
    fact = fact * n;
    n--;
}

cout << "Factorial = " << fact;

Question 14: Incorrect Average Calculation

Task: Find and fix the mistake so the program calculates the average of the numbers 10, 20, and 30. The expected average is 20.

Incorrect Code

int i = 1;
int sum = 0;

while (i <= 3)
{
    sum = sum + i * 10;
    i++;
}

double average = sum / 2;
cout << "Average = " << average;

Question 15: Incorrect Digit Counting

Task: Find and fix the mistake so the program counts the digits in 12345. The expected result is 5.

Incorrect Code

int num = 12345;
int count = 0;

while (num > 0)
{
    num = num / 10;
    count = 1;
}

cout << "Digits = " << count;

Question 16: Incorrect Reverse Number Logic

Task: Find and fix the mistake so the program reverses 1234 and displays 4321.

Incorrect Code

int num = 1234;
int reverse = 0;

while (num > 0)
{
    int digit = num % 10;
    reverse = reverse + digit;
    num = num / 10;
}

cout << "Reverse = " << reverse;

Question 17: Incorrect Multiple Condition

Task: Find and fix the mistake so the program displays numbers from 1 to 20 that are divisible by both 2 and 3.

Incorrect Code

int i = 1;

while (i <= 20)
{
    if (i % 2 == 0 && i % 3 == 0);
    {
        cout << i << endl;
    }

    i++;
}

Question 18: Incorrect Prime Number Check

Task: Find and fix the mistake so the program correctly checks whether 7 is a prime number.

Incorrect Code

int num = 7;
int i = 2;
bool isPrime = true;

while (i < num)
{
    if (num % i == 0)
    {
        isPrime = false;
    }

    i++;
}

if (isPrime = true)
{
    cout << "Prime";
}
else
{
    cout << "Not Prime";
}

Question 19: Incorrect Power Calculation

Task: Find and fix the mistake so the program calculates \(2^5\). The expected result is 32.

Incorrect Code

int base = 2;
int exponent = 5;
int result = 0;
int i = 1;

while (i <= exponent)
{
    result = result * base;
    i++;
}

cout << "Result = " << result;

Question 20: Incorrect Termination Condition

Task: Find and fix the mistake so the program displays numbers from 1 to 100 in steps of 10 and terminates correctly.

Incorrect Code

int i = 1;

while (i != 100)
{
    cout << i << endl;
    i = i + 10;
}

Section 2: `do-while` Loop

## Question 1: Missing Semicolon

Task: Find and fix the syntax error in the program so that it displays numbers from 1 to 5.

### Incorrect Code

int i = 1;

do
{
    cout << i << endl;
    i++;
}
while (i <= 5)

Expected Output

## Question 2: Incorrect Loop Condition

Task: A student wants to display numbers from 1 to 10, but the program does not display any numbers correctly. Fix the condition.

### Incorrect Code

int i = 1;

do
{
    cout << i << " ";
    i++;
}
while (i >= 10);

Expected Output

1 2 3 4 5 6 7 8 9 10

## Question 3: Missing Counter Update

Task: Fix the program so that it displays numbers from 1 to 5 and terminates without running forever.

### Incorrect Code

int i = 1;

do
{
    cout << i << " ";
}
while (i <= 5);

Expected Output

1 2 3 4 5

## Question 4: Incorrect Initialization

Task: A countdown program should display numbers from 10 down to 1. Find and fix the mistake.

### Incorrect Code

int i = 1;

do
{
    cout << i << " ";
    i--;
}
while (i >= 1);

Expected Output

10 9 8 7 6 5 4 3 2 1

## Question 5: Incorrect Increment

Task: A program should display even numbers from 2 to 20. Correct the mistake in the update statement.

### Incorrect Code

int i = 2;

do
{
    cout << i << " ";
    i++;
}
while (i <= 20);

Expected Output

2 4 6 8 10 12 14 16 18 20

## Question 6: Incorrect Sum Calculation

Task: A student wants to calculate the sum of numbers from 1 to 10. Fix the calculation so the final sum is correct.

### Incorrect Code

int i = 1;
int sum = 0;

do
{
    sum = i;
    i++;
}
while (i <= 10);

cout << "Sum = " << sum;

Expected Output

Sum = 55

## Question 7: Multiplication Table Error

Task: Fix the program so that it displays the multiplication table of 5 from 1 to 10.

### Incorrect Code

int num = 5;
int i = 1;

do
{
    cout << num << " x " << i << " = " << num * i << endl;
    i++;
}
while (i < 10);

Expected Output

x 1 = 5
x 2 = 10
x 3 = 15
x 4 = 20
x 5 = 25
x 6 = 30
x 7 = 35
x 8 = 40
x 9 = 45
x 10 = 50

## Question 8: Incorrect Digit Counter

Task: A program should count the digits in the number 12345. Find and fix the mistake.

### Incorrect Code

int num = 12345;
int count = 0;

do
{
    num = num / 10;
    count = 1;
}
while (num > 0);

cout << "Digits = " << count;

Expected Output

Digits = 5

## Question 9: Incorrect Factorial Calculation

Task: A program should calculate the factorial of 5. Find and fix the initialization or calculation mistake.

### Incorrect Code

int n = 5;
int fact = 0;

do
{
    fact = fact * n;
    n--;
}
while (n >= 1);

cout << "Factorial = " << fact;

Expected Output

Factorial = 120

## Question 10: Login Attempt System

Task: A login system should keep asking the user to enter a PIN until the correct PIN (1234) is entered. Fix the condition so the program works correctly.

### Incorrect Code

int pin;

do
{
    cout << "Enter PIN: ";
    cin >> pin;
}
while (pin == 1234);

cout << "Access Granted";

Expected Output

Enter PIN: 1111
Enter PIN: 2222
Enter PIN: 1234
Access Granted

## Question 11: ATM Menu

Task: An ATM menu should repeat until the user selects option 3 to exit. Find and fix the loop condition.

### Incorrect Code

int choice;

do
{
    cout << "\n1. Check Balance";
    cout << "\n2. Withdraw Cash";
    cout << "\n3. Exit";
    cout << "\nEnter choice: ";
    cin >> choice;
}
while (choice == 3);

cout << "ATM Closed";

Expected Output for Input 3

1. Check Balance
2. Withdraw Cash
3. Exit
Enter choice: 3
ATM Closed

## Question 12: Guess the Secret Number

Task: A number-guessing game should continue until the user guesses the secret number, 7. Correct the condition.

### Incorrect Code

int guess;

do
{
    cout << "Guess the number: ";
    cin >> guess;
}
while (guess == 7);

cout << "Correct guess!";

Expected Output for Input 3, 5, 7

Guess the number: 3
Guess the number: 5
Guess the number: 7
Correct guess!

## Question 13: Count Even Numbers

Task: A program should count the even numbers from 1 to 10. Fix the logic so that the counter stores the correct number of even values.

### Incorrect Code

int i = 1;
int count = 0;

do
{
    if (i % 2 == 0)
    {
        count = 1;
    }

    i++;
}
while (i <= 10);

cout << "Even numbers = " << count;

C++ Loops: Fix the Mistakes

Instructions

Section 1: while Loop

Question 1: Missing Semicolon

Incorrect Code

Question 2: Incorrect Condition

Incorrect Code

Question 3: Missing Update Statement

Incorrect Code

Question 4: Incorrect Initialization

Incorrect Code

Question 5: Incorrect Increment

Incorrect Code

Question 6: Incorrect Decrement

Incorrect Code

Question 7: Incorrect Comparison Operator

Incorrect Code

Question 8: Incorrect Loop Variable

Incorrect Code

Question 9: Incorrect Sum Calculation

Incorrect Code

Question 10: Incorrect Condition for Odd Numbers

Incorrect Code

Question 11: Incorrect Input Condition

Incorrect Code

Question 12: Incorrect Counter Update

Incorrect Code

Question 13: Incorrect Factorial Calculation

Incorrect Code

Question 14: Incorrect Average Calculation

Incorrect Code

Question 15: Incorrect Digit Counting

Incorrect Code

Question 16: Incorrect Reverse Number Logic

Incorrect Code

Question 17: Incorrect Multiple Condition

Incorrect Code

Question 18: Incorrect Prime Number Check

Incorrect Code

Question 19: Incorrect Power Calculation

Incorrect Code

Question 20: Incorrect Termination Condition

Incorrect Code

Section 2: do-while Loop

Section 1: `while` Loop

Section 2: `do-while` Loop