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C++ for Loop

C++ for Loop

In this tutorial, we will learn about the C++ for loop and its working with the help of some examples.

In computer programming, loops are used to repeat a block of code.

For example, let's say we want to show a message 100 times. Then instead of writing the print statement 100 times, we can use a loop.

That was just a simple example; we can achieve much more efficiency and sophistication in our programs by making effective use of loops.

There are 3 types of loops in C++.

  • for loop
  • while loop
  • do...while loop

This tutorial focuses on C++ for loop. We will learn about the other type of loops in the upcoming tutorials.


C++ for loop

The syntax of for-loop is:

for (initialization; condition; update) {
    // body of-loop 
}

Here,

  • initialization - initializes variables and is executed only once
  • condition - if true, the body of for loop is executed
    if false, the for loop is terminated
  • update - updates the value of initialized variables and again checks the condition

To learn more about conditions, check out our tutorial on C++ Relational and Logical Operators.


Flowchart of for Loop in C++

C++ for loop flowchart
Flowchart of for loop in C++

Example 1: Printing Numbers From 1 to 5

#include <iostream>

using namespace std;

int main() {
        for (int i = 1; i <= 5; ++i) {
        cout << i << " ";
    }
    return 0;
}

Output

1 2 3 4 5

Here is how this program works

Iteration Variable i <= 5 Action
1st i = 1 true 1 is printed. i is increased to 2.
2nd i = 2 true 2 is printed. i is increased to 3.
3rd i = 3 true 3 is printed. i is increased to 4.
4th i = 4 true 4 is printed. i is increased to 5.
5th i = 5 true 5 is printed. i is increased to 6.
6th i = 6 false The loop is terminated

Example 2: Display a text 5 times

// C++ Program to display a text 5 times

#include <iostream>

using namespace std;

int main() {
    for (int i = 1; i <= 5; ++i) {
        cout <<  "Hello World! " << endl;
    }
    return 0;
}

Output

Hello World!
Hello World!
Hello World!
Hello World!
Hello World!

Here is how this program works

Iteration Variable i <= 5 Action
1st i = 1 true Hello World! is printed and i is increased to 2.
2nd i = 2 true Hello World! is printed and i is increased to 3.
3rd i = 3 true Hello World! is printed and i is increased to 4.
4th i = 4 true Hello World! is printed and i is increased to 5.
5th i = 5 true Hello World! is printed and i is increased to 6.
6th i = 6 false The loop is terminated

Example 3: Find the sum of first n Natural Numbers

// C++ program to find the sum of first n natural numbers
// positive integers such as 1,2,3,...n are known as natural numbers

#include <iostream>

using namespace std;

int main() {
    int num, sum;
    sum = 0;

    cout << "Enter a positive integer: ";
    cin >> num;

    for (int count = 1; count <= num; ++count) {
        sum += count;
    }

    cout << "Sum = " << sum << endl;

    return 0;
}

Output

Enter a positive integer: 10
Sum = 55

In the above example, we have two variables num and sum. The sum variable is assigned with 0 and the num variable is assigned with the value provided by the user.

Note that we have used a for loop.

for(int count = 1; count <= num; ++count)

Here,

  • int count = 1: initializes the count variable
  • count <= num: runs the loop as long as count is less than or equal to num
  • ++count: increase the count variable by 1 in each iteration

When count becomes 11, the condition is false and sum will be equal to 0 + 1 + 2 + ... + 10.


Ranged Based for Loop

In C++11, a new range-based for loop was introduced to work with collections such as arrays and vectors. Its syntax is:

for (variable : collection) {
    // body of loop
}

Here, for every value in the collection, the for loop is executed and the value is assigned to the variable.


Example 4: Range Based for Loop

#include <iostream>

using namespace std;

int main() {
  
    int num_array[] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10};
  
    for (int n : num_array) {
        cout << n << " ";
    }
  
    return 0;
}

Output

1 2 3 4 5 6 7 8 9 10

In the above program, we have declared and initialized an int array named num_array. It has 10 items.

Here, we have used a range-based for loop to access all the items in the array.


C++ Infinite for loop

If the condition in a for loop is always true, it runs forever (until memory is full). For example,

// infinite for loop
for(int i = 1; i > 0; i++) {
    // block of code
}

In the above program, the condition is always true which will then run the code for infinite times.


Check out these examples to learn more:


In the next tutorial, we will learn about while and do...while loop.