Write a program to print the sum of first n natural numbers

Posted in COMPUTER SCIENCE

Write a program to print the sum of first n natural numbers
vinaykhatri

Vinay Khatri
Last updated on February 11, 2025

Problem Statment

We need to write a program, that prints the first n natural number and their sum. Basically, we need to write a program that prints the sum of the following series 1+2+3+4+5+...+n . Here n represent the last number of the series.

For example

Input

n=4

Output

10

Explanation

the value of n is 4, so:
=1+2+3+4
=10

Approach 1 (Brute Force)

To compute the sum of the first n natural number problem, we will first use the brute force approach, in which we will take the help of for loop from range 1 to n and add all the numbers in the total sum.

C program to print the sum of first n natural numbers 

#include <stdio.h>
int main()
{	
	int sum =0, i, n;
	printf("Here we will calculate the sum of series 1+2+3+4+5+..n  ...  \n");
	
	//input the value of n
	printf("Enter the value of n: ");
	scanf("%d", &n);
	
	//compute the sum
	
	for(i=1;i<=n; i++)
	{		
		//compute the sum		
		sum += i;
	}
	
	printf("The sum of the series is: %d",sum);

	return 0;
}

Output: 

Here we will calculate the sum of series 1+2+3+4+5+..n ...á
Enter the value of n: 15
The sum of the series is: 120

C++ Program to print the sum of first n natural numbers 

#include<iostream>
using namespace std;
int main()
{
	int sum=0,n; 
	cout<<"Here we will calculate the sum of series 1+2+3+4+5+...n \n";
	
	//input the value of n
	cout<<"Enter the value of n: "; cin>>n;
	
	for(int i=1;i<=n; i++)
	{
		//compute the sum		
		sum += i;
	}
	
	cout<<"The sum of the series is: "<<sum;
	return 0; 
}

Output 

Here we will calculate the sum of series 1+2+3+4+5+...n
Enter the value of n: 4
The sum of the series is: 10

Python program to print the sum of first n natural numbers 

print("Here we will calculate the sum of series 1+2+3+4+5+...n")

# initialize the value of sum
sum_=0

# input the value of n
n = int(input("Enter the value of n: "))

for i in range(1, n+1):
    # compute the sum
    sum_+=i

print(f"The sum of the series is: {sum_}")

Output: 

Here we will calculate the sum of series 1+2+3+4+5+...n
Enter the value of n: 15
The sum of the series is: 120

Complexity analysis

Time Complexity: O(N) Space Complexity: O(1)

Approach 2 (Using Formula)

The time complexity of the above program is O(N), where N is the last number of the series. But in mathematics, we have a dedicated formula that can compute the sum of first N digit natural numbers.

Sum of first N natural numbers = n*(n+1)/2

C Program to print the sum of first n natural numbers 

#include <stdio.h>

int main()
{	
	int sum , i, n;
	printf("Here we will calculate the sum of series 1+2+3+4+5+..n  ...  \n");
	
	//input the value of n
	printf("Enter the value of n: ");
	scanf("%d", &n);
	
	//compute the sum
	sum = n*(n+1)/2;
	
	printf("The sum of the series is: %d",sum);

	return 0;
}

Output 

Here we will calculate the sum of series 1+2+3+4+5+...n
Enter the value of n: 4
The sum of the series is: 10

C++ program to print the sum of first n natural numbers 

#include<iostream>
using namespace std;
int main()
{
	int sum=0,n; 
	cout<<"Here we will calculate the sum of series 1+2+3+4+5+...n \n";
	
	//input the value of n
	cout<<"Enter the value of n: "; cin>>n;
	
	//compute the sum
	sum = n*(n+1)/2;
	
	cout<<"The sum of the series is: "<<sum;
	return 0; 
}

Output 

Here we will calculate the sum of series 1+2+3+4+5+...n
Enter the value of n: 5
The sum of the series is: 15

Python Program to print the sum of first n natural numbers 

print("Here we will calculate the sum of series 1+2+3+4+5+...n")

# input the value of n
n = int(input("Enter the value of n: "))

# compute the sum
sum_ = n*(n+1)//2;

print(f"The sum of the series is: {sum_}")

Output 

Here we will calculate the sum of series 1+2+3+4+5+...n
Enter the value of n: 5
The sum of the series is: 15

Complexity analysis

Time Complexity: O(1)

Space Complexity: O(1)

Wrapping Up!

In this tutorial, we learned how to write a program in C, C++ and python to find out the sum of first n natural numbers. To write the program we follow two different approaches. In the first approach, we use the brute force technique which results in higher time complexity.

But in the second approach, we use the mathematical formula that makes the program more efficient with constant time complexity. If you ever encounter this program in your programming interviews, there use the second approach, because it is more efficient and optimized, and it will make your code faster.

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Vinay

Vinay Khatri
I am a Full Stack Developer with a Bachelor's Degree in Computer Science, who also loves to write technical articles that can help fellow developers.

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