C Program to Find the Sum of First N Natural Numbers
Sum of First N Natural Numbers in C
In this article, we will see C Program to Find the Sum of First N Natural Numbers. A Natural number is the same as a Counting number. They are used to Count the number of real physical objects. Natural numbers start from 1 and go on infinite.
The positive numbers 1,2,3 ... are known as natural numbers.
Formula for Sum of First n natural numbers is : n(n+1)/2.
If you want to add first 5 Natural number
then we find the Sum of 1+2+3+4+5 =15. 
Methods for C Program to Find the Sum of First N Natural Numbers
- Method 1: Iterative way (Using for loop)
- Method 2: Iterative way (Using while loop)
- Method 3: Direct Formula
- Method 4: Recursive Approach
Method 1: Using For Loop
For an input n
- Create variable sum = 0
- Run a for loop in iteration (I) from 1 -> n
- Each time adding I to sum
- Print the sum value after the loop terminates
Code
Run
#include <stdio.h>
int main()
{
int n;
scanf("%d",&n);
int sum = 0;
for(int i=1;i<=n;i++)
// is same as writing sum = sum + i
sum += i;
printf("Sum is %d",sum);
return 0;
}
// Time complexity : O(n)
// Space complexity : O(1)
Output
5 Sum is 15
Method 2: Using While Loop
Algorithm (in simple words):
- Start
- Take a number N from the user.
- Initialize two variables:
- i = 1 (to start counting from 1)
- sum = 0 (to store the final sum)
4. Use a while loop until i <= N
- Add i to sum
- Increase i by 1
5. When the loop ends, print the value of sum.
6. End
Code:
#include<stdio.h>
int main() {
int N, i = 1, sum = 0;
// Taking input from user
printf("Enter a positive integer: ");
scanf("%d", &N);
// Loop to calculate the sum
while(i <= N) {
sum = sum + i;
i++;
}
// Displaying the result
printf("Sum of first %d natural numbers is: %d\n", N, sum);
return 0;
}
Output
Enter a positive integer: 10 Sum of first 10 natural numbers is: 55
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Method 3: Direct Formulae
For an input n
- Create variable sum = 0
- Use formula sum = n(n+1)/2
- Print the sum value
Code
Run
#include <stdio.h>
int main()
{
int n;
scanf("%d",&n);
int sum = n*(n+1)/2;
printf("The sum is %d",sum);
return 0;
}
// Time complexity : O(1)
// Space complexity : O(1)
Output
6 Sum is 21
Method 4: Recursive Approach
For an input n
- Create variable sum = 0
- Call function getSum(sum, n)
- Base Case, n == 0 return sum
- Others return n + getSum(sum, n-1)
- Print the sum value in main
Code
Run
#include <stdio.h>
int getSum(int sum,int n)
{
if(n==0)
return sum;
return n+getSum(sum,n-1);
}
int main()
{
int n, sum = 0;
scanf("%d",&n);
printf("%d",getSum(sum, n));
return 0;
}
// Time complexity : O(n)
// Space complexity : O(1)
// Auxilary space complexity : O(N)
// Due to function call stack
Output
5 Sum is 15
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