A for loop in C++ is a control structure used to repeat a block of code a fixed number of times.
It is ideal when the number of iterations is known in advance, such as iterating over array elements or performing a calculation a set number of times.

The for loop consists of three parts: initialization, condition, and update.
These are placed inside parentheses and separated by semicolons.

Syntax

for (initialization; condition; update) {
    // loop body
}
  • Initialization: Sets up a loop control variable, such as int i=0
  • Condition: Evaluated before each iteration; loop continues with this is true
  • Update: Modifies the loop control variable after each iteration

For example, to count from 1 to 5:

#include <iostream>
using namespace std;

int main(void){
    for(int i=1; i <= 5; i++){
        cout << i << " ";
    }
    cout << endl;
}

Prints this line:

1 2 3 4 5

Iterating over an Array

You can use a loop to access each element of the array values using the index i:

int values[4] = {10, 20, 30, 40};

for (int i = 0; i < 4; i++) {
    cout << values[i] << endl;
}

Loop Control and Scope

The loop control variable, like int i, is scoped to the loop if declared inside the for statement. It cannot be accessed outside the loop body:

for (int i = 0; i < 3; i++) {
    cout << i << endl;
}
// i is not accessible here

If declared outside the loop, the variable persists after the loop ends.

Infinite Loops

If the condition is always true, the loop will run forever:

for (;;) {
    // infinite loop
}

This is equivalent to while(true) and should be used with caution.

Nested Loops

You can place one for loop inside another to iterate over multi-dimensional data:

for (int row = 0; row < 3; row++) {
    for (int col = 0; col < 4; col++) {
        cout << "(" << row << "," << col << ") ";
    }
    cout << endl;
}

This prints a 3×4 grid of coordinates.

Reading Questions

  1. What are the three components of a for loop?
  2. Write a for loop that prints the numbers from 10 down to 1.
  3. How many times does the loop for (int i = 0; i < 7; i++) execute?
  4. What happens if the condition in a for loop is always false?
  5. Can you declare the loop control variable outside the for loop? What are the implications?
  6. Write a nested for loop that prints a 2×3 grid of asterisks (*).
  7. How would you use a for loop to sum the elements of an array?