# Sum of First n Natural Numbers

Sum of First n Natural Numbers” is a very basic programming exercise problem for beginners. Here, we are given a number ‘n’ and our task is to compute the sum of first ‘n’ natural numbers.

Natural Numbers: 1,2,3,………

Example:

```INPUT:
N = 5
OUTPUT:
15
Explanation:
1+2+3+4+5 = 15```

There are two methods to compute the sum of first ‘n’ natural numbers:

1. Using loop
2. Using formula

METHOD 1: Using loop

This method computes the sum of first ‘n’ natural numbers in O(n).

C++ Program to compute sum of first ‘n’ natural numbers is as follows:

```#include<bits/stdc++.h>
using namespace std;
int main()
{
//Scanning the number 'n'
int n;
cout<<"Enter the number 'n' : ";
cin>>n;

//Computing the sum of first 'n' natural numbers
int sum = 0;
for(int i = 1; i <= n;i++)
{
sum = sum + i;
}

//Printing the sum of first 'n' natural numbers
cout<<"Sum of first "<<n<<" natural numbers is "<<sum;
}

```
```OUTPUT:
Enter the number 'n' : 5
Sum of first 5 natural numbers is 15```

METHOD 2: Using formula

Sum of first ‘n’ natural numbers: n*(n+1)/2.

This method computes the sum of first ‘n’ natural numbers in O(1).

C++ Program to compute sum of first ‘n’ natural numbers is as follows:

```#include<bits/stdc++.h>
using namespace std;
int main()
{
//Scanning the number 'n'
int n;
cout<<"Enter the number 'n' : ";
cin>>n;

//Computing the sum of first 'n' natural numbers
int sum = n*(n+1)/2;

//Printing the sum of first 'n' natural numbers
cout<<"Sum of first "<<n<<" natural numbers is "<<sum;
}
```
```OUTPUT:
Enter the number 'n' : 5
Sum of first 5 natural numbers is 15```

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