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C Program to find the sum of natural numbers upto N terms

Last updated on July 27, 2020


What are Natural numbers? #

Integers used for counting are known as natural numbers. Natural numbers start with 1 and goes on to infinity. For example:

1, 2, 3, 4, 5, 6, 7, 8, 9, 10 . . .

The following is a C program to calculate the sum of natural numbers upto n terms:

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/**********************************************************
 * Program to find the sum of natural numbers upto n terms
***********************************************************/

#include<stdio.h> // include stdio.h

int main() 
{
    int terms, i;
    unsigned long int sum = 0;

    printf("Enter the number of terms: ");
    scanf("%d", &terms);

    for(int i = 1 ; i <= terms; i++)
    {
        sum += i;                
    }

    printf("%ld", sum);

    return 0;
}

Expected Output:

1st run:

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2
Enter the number of terms: 4 
10

2nd run:

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2
Enter the number of terms: 50000
1250025000

How it works #

The following table demonstrates what happens at each iteration of the for loop, assuming terms = 4.

Iteration sum `i
After 1st iteration sum = sum + i = 0+1 = 1 i = 2
After 2nd iteration sum = 1+2 = 3 i = 3
After 3rd iteration sum = 3+3 = 6 i = 4
After 4th iteration sum = 6+4 = 10 i = 5

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