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Sum of n natural numbers in C

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

#include <stdio.h>

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 program output.

Sum of n natural numbers in C using a for loop

#include <stdio.h>

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 to calculate the sum of first n natural numbers.