C++ for Loop

Introduction

The for loop in C++ is a control flow statement that allows code to be executed repeatedly based on a given condition. It is particularly useful when you know in advance how many times you need to iterate through a block of code. The for loop is one of the most commonly used loops in programming due to its compact structure and clarity.

Syntax

The basic syntax of a for loop is as follows:

for (initialization; condition; increment) {
    // code to be executed
}

Key Points

• Initialization: Sets up a loop control variable and is executed once at the beginning.
• Condition: Evaluated before each iteration. If true, the loop body is executed. If false, the loop terminates.
• Increment: Updates the loop control variable after each iteration.

Example: Basic for Loop

#include <iostream>
using namespace std;

int main() {
    for (int i = 0; i < 5; i++) {
        cout << "i = " << i << endl;
    }

    return 0;
}

Output

i = 0
i = 1
i = 2
i = 3
i = 4

Explanation

• Initialization: int i = 0 initializes the loop control variable i to 0.
• Condition: i < 5 checks if i is less than 5. If true, the loop body executes.
• Increment: i++ increments i by 1 after each iteration.

Example: for Loop with Different Steps

You can control the step size in a for loop by modifying the increment part.

#include <iostream>
using namespace std;

int main() {
    for (int i = 0; i < 10; i += 2) {
        cout << "i = " << i << endl;
    }

    return 0;
}

Output

i = 0
i = 2
i = 4
i = 6
i = 8

Explanation

• Increment: i += 2 increments i by 2 after each iteration.

Example: for Loop in Reverse Order

You can also use a for loop to iterate in reverse order.

#include <iostream>
using namespace std;

int main() {
    for (int i = 5; i > 0; i--) {
        cout << "i = " << i << endl;
    }

    return 0;
}

Output

i = 5
i = 4
i = 3
i = 2
i = 1

Explanation

• Initialization: int i = 5 initializes the loop control variable i to 5.
• Condition: i > 0 checks if i is greater than 0. If true, the loop body executes.
• Increment: i-- decrements i by 1 after each iteration.

Example: Nested for Loop

You can nest for loops to create a loop within a loop, which is useful for multidimensional data structures like matrices.

#include <iostream>
using namespace std;

int main() {
    for (int i = 1; i <= 3; i++) {
        for (int j = 1; j <= 3; j++) {
            cout << "i = " << i << ", j = " << j << endl;
        }
    }

    return 0;
}

Output

i = 1, j = 1
i = 1, j = 2
i = 1, j = 3
i = 2, j = 1
i = 2, j = 2
i = 2, j = 3
i = 3, j = 1
i = 3, j = 2
i = 3, j = 3

Explanation

• The outer loop (for (int i = 1; i <= 3; i++)) iterates 3 times.
• The inner loop (for (int j = 1; j <= 3; j++)) iterates 3 times for each iteration of the outer loop.

Example: Sum of First N Natural Numbers

A for loop can be used to calculate the sum of the first N natural numbers.

#include <iostream>
using namespace std;

int main() {
    int n, sum = 0;
    cout << "Enter a positive integer: ";
    cin >> n;

    for (int i = 1; i <= n; ++i) {
        sum += i;
    }

    cout << "Sum of the first " << n << " natural numbers is: " << sum << endl;

    return 0;
}

Output

Enter a positive integer: 10
Sum of the first 10 natural numbers is: 55

Example: Multiplication Table

A for loop can be used to print the multiplication table of a given number.

#include <iostream>
using namespace std;

int main() {
    int num;
    cout << "Enter a number: ";
    cin >> num;

    for (int i = 1; i <= 10; ++i) {
        cout << num << " * " << i << " = " << num * i << endl;
    }

    return 0;
}

Output

Enter a number: 5
5 * 1 = 5
5 * 2 = 10
5 * 3 = 15
5 * 4 = 20
5 * 5 = 25
5 * 6 = 30
5 * 7 = 35
5 * 8 = 40
5 * 9 = 45
5 * 10 = 50

Conclusion

The for loop is a powerful and flexible control flow statement in C++ that allows you to execute a block of code multiple times based on a given condition. This chapter covered the basic syntax of the for loop, examples of using different step sizes, iterating in reverse order, and using nested for loops. Additionally, we demonstrated simple programs to calculate the sum of the first N natural numbers and print a multiplication table. Understanding how to use the for loop effectively will help you write more efficient and readable code. In the next chapter, we will explore the while loop in C++.