for Loop in C++

A for loop repeatedly executes a block of code while a specified condition remains true.

for (initialization; condition; update) {
    // statements
}

The loop continues until the condition becomes false.

for (initialization; condition; update) {

    // code to repeat

}

The three parts are separated by semicolons.

• Initialization: Sets the starting value.
• Condition: Determines whether the loop continues.
• Update: Changes the loop variable after each iteration.

for (int i = 1; i <= 5; i++) {
    std::cout << i << std::endl;
}

Output:

1
2
3
4
5

The initialization part runs once before the loop starts.

for (int i = 1; i <= 5; i++) {
    std::cout << i;
}

Here, int i = 1 initializes the loop variable with 1.

The condition is checked before each iteration.

for (int i = 1; i <= 5; i++) {
    std::cout << i;
}

The loop continues while i <= 5 is true.

The update part changes the loop variable after each iteration.

for (int i = 1; i <= 5; i++) {
    std::cout << i;
}

Here, i++ increases i by 1 after every iteration.

For this loop:

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

The execution is:

1. i is initialized to 1.
2. The condition i <= 3 is checked.
3. The loop body executes.
4. i++ increases the value.
5. The condition is checked again.
6. The process continues until the condition becomes false.

#include <iostream>

int main() {

    for (int i = 1; i <= 10; i++) {
        std::cout << i << std::endl;
    }

    return 0;
}

The decrement operator can be used to count backward.

for (int i = 10; i >= 1; i--) {
    std::cout << i << std::endl;
}

The output starts from 10 and ends at 1.

for (int i = 2; i <= 20; i += 2) {
    std::cout << i << std::endl;
}

The update expression i += 2 increases the value by 2, so only even numbers are printed.

for (int i = 1; i <= 19; i += 2) {
    std::cout << i << std::endl;
}

The loop starts from 1 and increases by 2.

The i++ operator increases the loop variable by 1.

for (int i = 1; i <= 5; i++) {
    std::cout << i;
}

This is one of the most common forms of a for loop.

The i-- operator decreases the loop variable by 1.

for (int i = 5; i >= 1; i--) {
    std::cout << i << std::endl;
}

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

This loop prints values with a difference of 2.

int sum = 0;

for (int i = 1; i <= 10; i++) {
    sum += i;
}

std::cout << "Sum = " << sum;

The loop adds the numbers from 1 to 10 to the sum variable.

int number = 5;

for (int i = 1; i <= 10; i++) {
    std::cout << number << " x "
              << i << " = "
              << number * i
              << std::endl;
}

This program prints the multiplication table of 5.

#include <iostream>

int main() {

    int n;

    std::cout << "Enter a number: ";
    std::cin >> n;

    for (int i = 1; i <= n; i++) {
        std::cout << i << std::endl;
    }

    return 0;
}

The number of iterations depends on the value entered by the user.

int number = 5;
int factorial = 1;

for (int i = 1; i <= number; i++) {
    factorial *= i;
}

std::cout << "Factorial = " << factorial;

The factorial of 5 is 120.

#include <iostream>
#include <string>

int main() {

    std::string name = "SOOPRO";

    for (int i = 0; i < name.length(); i++) {
        std::cout << name[i] << std::endl;
    }

    return 0;
}

The loop accesses each character using its index.

int numbers[] = {10, 20, 30, 40, 50};

for (int i = 0; i < 5; i++) {
    std::cout << numbers[i] << std::endl;
}

The loop uses the index to access each element of the array.

A for loop can contain an if statement.

for (int i = 1; i <= 20; i++) {

    if (i % 2 == 0) {
        std::cout << i << std::endl;
    }

}

This prints only even numbers between 1 and 20.

The break statement immediately terminates the loop.

for (int i = 1; i <= 10; i++) {

    if (i == 5) {
        break;
    }

    std::cout << i << std::endl;
}

The loop stops when i becomes 5.

The continue statement skips the current iteration and moves to the next iteration.

for (int i = 1; i <= 5; i++) {

    if (i == 3) {
        continue;
    }

    std::cout << i << std::endl;
}

The number 3 is skipped.

A for loop can be placed inside another for loop. This is called a nested loop.

for (int i = 1; i <= 3; i++) {

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

}

for (int i = 1; i <= 5; i++) {

    for (int j = 1; j <= i; j++) {
        std::cout << "* ";
    }

    std::cout << std::endl;
}

Output:

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

• Forgetting the semicolons between initialization, condition, and update.
• Using the wrong loop condition.
• Forgetting to update the loop variable.
• Creating an infinite loop accidentally.
• Using an incorrect starting value.
• Using <= when < is required, or vice versa.
• Accessing an array outside its valid index range.

Correct structure:

for (int i = 0; i < 10; i++) {
    std::cout << i;
}

#include <iostream>

int main() {

    int n;

    std::cout << "Enter a number: ";
    std::cin >> n;

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

        if (i % 2 == 0) {
            std::cout << i
                      << " is even"
                      << std::endl;
        }
        else {
            std::cout << i
                      << " is odd"
                      << std::endl;
        }

    }

    return 0;
}

When a variable is declared inside the initialization part of a for loop, its scope is normally limited to the loop.

for (int i = 1; i <= 5; i++) {
    std::cout << i;
}

// i is not available here

The variable i can be used inside the loop but not after the loop.

• Use meaningful loop variables when appropriate.
• Keep the loop condition simple.
• Make sure the loop variable changes correctly.
• Use braces to keep the loop body clear.
• Avoid unnecessary changes to the loop variable inside the body.
• Check array boundaries carefully.
• Use break and continue only when they make the logic clearer.
• Choose the correct starting and ending values.

for (int i = 1; i <= 10; i++) {
    std::cout << i << std::endl;
}

for (int i = 1; i <= 10; i++) {
    std::cout << i;
}