Programming in C++ – Looping Structures

Comprehensive guide to C++ looping structures — while, do-while, and for loops — with clear syntax, annotated examples, menu-driven program patterns, and practical tips for beginners.

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While Loop

do..while Loop

What is a do-while loop?

A do-while loop executes the code at least once, then checks the condition.

Syntax

do {
    // code to execute
} while (condition);

👉 Key point: The condition is checked after the loop body.

Example 1: Print Numbers from 1 to 5

#include <iostream>
using namespace std;

int main() {
    int i = 1;

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

    return 0;
}

Output

1 2 3 4 5

Explanation:

• i starts at 1
• The loop prints the number
• i increases by 1
• The loop runs until i becomes greater than 5

Example 2: Print “Hello” 3 Times

#include <iostream>
using namespace std;

int main() {
    int count = 1;

    do {
        cout << "Hello\n";
        count++;
    } while (count <= 3);

    return 0;
}

Explanation:

• The loop runs exactly 3 times
• Useful when you know how many times to repeat something

Example 3: Sum of First 5 Natural Numbers

#include <iostream>
using namespace std;

int main() {
    int i = 1, sum = 0;

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

    cout << "Sum = " << sum;
    return 0;
}

Output

Sum = 15

Example 4: Menu-Driven Program (Real-Life Use)

#include <iostream>
using namespace std;

int main() {
    int choice;

    do {
        cout << "\n1. Say Hello";
        cout << "\n2. Say Bye";
        cout << "\n3. Exit";
        cout << "\nEnter your choice: ";
        cin >> choice;

        if (choice == 1)
            cout << "Hello!\n";
        else if (choice == 2)
            cout << "Bye!\n";

    } while (choice != 3);

    return 0;
}

Why do-while here?

• The menu must appear at least once
• Perfect for menus and user input programs

Example 5: Loop Runs At Least Once (Important Concept)

#include <iostream>
using namespace std;

int main() {
    int i = 10;

    do {
        cout << "This will print once\n";
    } while (i < 5);

    return 0;
}

Explanation:

• Condition is false
• Still, the message prints once
• This is the main difference from while loop

Comparison: while vs do-while

When to Use do-while

✅ When the loop must execute at least once ✅ For menus, password checks, user input

for loop

Syntax of a for loop:

for (initialization; condition; update) {
    // code to execute in each iteration
}

• Initialization: Runs once at the start of the loop. Usually used to declare a loop variable.
• Condition: Checked before each iteration. If true, the loop continues; if false, the loop stops.
• Update: Runs after each iteration. Usually increments/decrements the loop variable.

Example 1: Print numbers 1 to 5

#include <iostream>
using namespace std;

int main() {
    for (int i = 1; i <= 5; i++) { // i starts at 1, loop until i <= 5, i increments by 1
        cout << i << " ";
    }
    return 0;
}

Output:

1 2 3 4 5

✅ Explanation:

• int i = 1 → start counting from 1
• i <= 5 → stop after 5
• i++ → increment i by 1 each time

Example 2: Print even numbers from 2 to 10

#include <iostream>
using namespace std;

int main() {
    for (int i = 2; i <= 10; i += 2) { // i increases by 2 each time
        cout << i << " ";
    }
    return 0;
}

Output:

2 4 6 8 10

✅ Explanation:

• i += 2 means add 2 to i each time, so we get even numbers.

Example 3: Sum of first 10 natural numbers

#include <iostream>
using namespace std;

int main() {
    int sum = 0;
    for (int i = 1; i <= 10; i++) {
        sum += i; // sum = sum + i
    }
    cout << "Sum = " << sum;
    return 0;
}

Output:

Sum = 55

✅ Explanation:

• sum += i adds each number to the total sum.
• After the loop finishes, sum contains the total.

Key Points for Beginners

1. A for loop is great for known number of iterations.
2. You can control the start, end, and increment.
3. The loop can run forward (i++) or backward (i–).

Example 4: Basic Multiplication Table

Question: Write a C++ program that asks the user to enter a number and then displays its multiplication table from 1 to 10 using a for loop.

Example:

• If the user enters 7, the program should display:

7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
...
7 x 10 = 70

Requirements:

1. Use cin to take input from the user.
2. Use a for loop to generate the multiplication table.
3. Display the results in a readable format.

#include <iostream>
using namespace std;

int main() {
    int number;

    // Ask the user for the number
    cout << "Enter a number to display its multiplication table: ";
    cin >> number;

    // Loop from 1 to 10
    for (int i = 1; i <= 10; i++) {
        cout << number << " x " << i << " = " << number * i << endl;
    }

    return 0;
}

✅ How it works:

1. cin >> number; → User inputs the number for which they want the table.
2. for (int i = 1; i <= 10; i++) → Loop runs from 1 to 10.
3. Inside the loop, number * i calculates each multiplication.
4. cout prints the result in a readable format.

Example Output (if user enters 5):

5 x 1 = 5
5 x 2 = 10
5 x 3 = 15
...
5 x 10 = 50

Example 5: Print Even Numbers from 1 to N

Question: Write a C++ program that asks the user to enter a positive number N and then prints all even numbers from 1 to N using a for loop.

Example:

• Input: 10
• Output:

Even numbers from 1 to 10 are: 2 4 6 8 10

Requirements:

1. Use cin to take input from the user.
2. Use a for loop to iterate from 1 to N.
3. Use an if statement to check if a number is even.
4. Display the even numbers in a single line, separated by spaces.

#include <iostream>
using namespace std;

int main() {
    int n;

    // Ask the user for a number
    cout << "Enter a positive number: ";
    cin >> n;

    cout << "Even numbers from 1 to " << n << " are: ";

    // For loop to print even numbers
    for (int i = 1; i <= n; i++) {
        if (i % 2 == 0) {  // Check if number is even
            cout << i << " ";
        }
    }

    cout << endl;
    return 0;
}

✅ How it works:

1. User inputs a number n.
2. Loop for (int i = 1; i <= n; i++) runs from 1 to n.
3. if (i % 2 == 0) checks if the number is even.
4. Prints only even numbers.

Example Output:

Enter a positive number: 10
Even numbers from 1 to 10 are: 2 4 6 8 10

Example 6: Print a Simple Star Pattern

Question: Write a C++ program that asks the user to enter a positive number N and then prints a right-angled triangle star pattern with N rows using nested for loops.

Example:

• Input: 5
• Output:

*
**
***
****
*****

Requirements:

1. Use cin to take input from the user.

2. Use a nested for loop:

   • Outer loop to control the rows.
   • Inner loop to print stars in each row.
3. Each row should have an increasing number of stars, starting from 1 star in the first row.

#include <iostream>
using namespace std;

int main() {
    int n;

    // Ask the user for the number of rows
    cout << "Enter the number of rows: ";
    cin >> n;

    // Outer loop for each row
    for (int i = 1; i <= n; i++) {
        // Inner loop to print stars in each row
        for (int j = 1; j <= i; j++) {
            cout << "*";
        }
        cout << endl; // Move to next row
    }

    return 0;
}

✅ How it works:

1. User inputs the number of rows n.
2. Outer loop for (int i = 1; i <= n; i++) → Controls the rows.
3. Inner loop for (int j = 1; j <= i; j++) → Prints stars in each row.
4. Each row has an increasing number of stars.

Example Output:

Enter the number of rows: 5
*
**
***
****
*****