for Loop

The for loop is the classic choice when you know in advance how many times you want to repeat something — iterating a fixed number of times, walking an index across an array, or counting up or down between two bounds.

Basic Syntax

A for loop has three parts, separated by semicolons, all optional: an initialization (runs once, before the loop starts), a condition (checked before every iteration; the loop stops once it's false), and an increment/update expression (runs after every iteration).

CPP

#include <iostream>
using namespace std;

int main() {
    for (int i = 0; i < 5; i++) {
        cout << i << " ";
    }
    cout << endl;
    // Output: 0 1 2 3 4

    return 0;
}

Counting Down and Custom Steps

CPP

// Count down from 10 to 1
for (int i = 10; i >= 1; i--) {
    cout << i << " ";
}
cout << endl;

// Step by 2
for (int i = 0; i <= 20; i += 2) {
    cout << i << " ";
}
cout << endl;

Multiple Variables with the Comma Operator

The init and increment sections can each hold more than one expression, separated by commas. This is handy when two counters need to move together, such as scanning a range from both ends inward.

CPP

#include <iostream>
using namespace std;

int main() {
    int arr[] = {1, 2, 3, 4, 5, 6, 7, 8};
    int n = 8;

    // i moves forward, j moves backward, in the same loop
    for (int i = 0, j = n - 1; i < j; i++, j--) {
        cout << "swap positions " << i << " and " << j << endl;
    }

    return 0;
}

Infinite for Loops

Omitting all three parts creates a loop with no built-in stopping condition: for (;;). It runs forever unless something inside the body (typically a break or a return) ends it. This pattern is common for event loops or servers that run "until told to stop."

CPP

#include <iostream>
using namespace std;

int main() {
    int count = 0;

    for (;;) {
        cout << "Iteration " << count << endl;
        count++;
        if (count >= 3) {
            break; // the only way this loop ends
        }
    }

    return 0;
}

Warning

An infinite for loop with no break (or other exit path) inside it will hang your program forever, consuming CPU. Always make sure there is a reachable exit condition.

Nested for Loops

A for loop can contain another for loop. The inner loop completes all of its iterations for every single iteration of the outer loop. Nested loops are the standard way to work with two-dimensional data, like a grid or matrix.

CPP

#include <iostream>
using namespace std;

int main() {
    for (int row = 1; row <= 3; row++) {
        for (int col = 1; col <= 3; col++) {
            cout << row * col << "\t";
        }
        cout << endl;
    }

    return 0;
}
// Output:
// 1    2    3
// 2    4    6
// 3    6    9

for vs while

A for loop is ideal when the number of iterations (or the start/stop/step pattern) is known up front. When a loop instead needs to keep running based on a condition that isn't naturally a counter — like "keep reading input until the user types quit" — a while loop usually reads more naturally. The next page covers while and do-while in depth.

Tip

Any for loop can be rewritten as a while loop and vice versa — pick whichever expresses your intent most clearly.

Key Points

A for loop has three parts: initialization, condition, and increment/update.
All three parts are optional; for (;;) creates an infinite loop that needs an internal break to stop.
The comma operator lets you initialize/update multiple variables in one loop header.
Nested for loops iterate the inner loop fully for every outer iteration - useful for grids and matrices.
Prefer for when the number of iterations is known in advance; prefer while for open-ended conditions.