C program to find the sum of n natural numbers using the formula and a *for* loop.

First n natural numbers are 1, 2, 3, ...,n. The numbers form an arithmetic progression (AP) with one as the first term as well as the common difference.

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

## C program to find sum of n natural numbers

int main()

{

int n, sum;

printf("Enter a number\n");

scanf("%d", &n);

sum = n*(n+1)/2;

printf("Sum of %d natural numbers = %d\n", n, sum);

return 0;

}

Output:

## Sum of n natural numbers in C using a for loop

int main()

{

int n, c, sum = 0;

printf("Enter a number\n");

scanf("%d", &n);

for (c = 1; c <= n; c++)

sum = sum + c;

printf("Sum = %d\n", sum);

return 0;

}

This method is inefficient because it uses a loop that executes n times (time complexity: O(n)). Use the first method, which uses the formula (time complexity: O(1)) to calculate the sum of first n natural numbers.