C Program To Find Sum of Natural Numbers Using Recursion

Write a recursive function to obtain the running sum of first 25 natural numbers.

1 + 2 + 3 + 4 + 5 + …. + 23 + 24 + 25 = 325

Related Read:
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Recursive Functions In C Programming Language

Video Tutorial: C Program To Find Sum of Natural Numbers Using Recursion

Source Code: C Program To Find Sum of Natural Numbers Using Recursion

#include<stdio.h>

int sum(int num)
{
    if(num)
        return(num + sum(num-1));
    else
        return 0;
}

int main()
{
    int count = 25;

    printf("Sum of 1st 25 natural numbers is %d\n", count, sum(count));

    return 0;
}

Output:
Sum of 1st 25 natural numbers is 325

Logic To Find Sum of Natural Numbers Using Recursion

25 is passed to a function sum, from main method. Inside function sum(), if the passed number is a non-zero then we add sum(num-1) to num. We keep doing it until num value is 0. Once num is 0, code inside else block gets executed and 0 is returned.

Source Code: Find Sum of Natural Numbers Using Recursion – Take input from user

#include<stdio.h>

int sum(int num)
{
    if(num)
        return(num + sum(num-1));
    else
        return 0;
}

int main()
{
    int count;

    printf("Enter a positive no\n");
    scanf("%d", &count);

    printf("Sum of 1st %d natural numbers is %d\n", count, sum(count));

    return 0;
}

Output 1:
Enter a positive no
5
Sum of 1st 5 natural numbers is 15

Output 2:
Enter a positive no
14
Sum of 1st 14 natural numbers is 105

Example:

Lets assume user has input num value as 5.
Recursive Call sum(num – 1).

Value Returning – Control Shifting back.

Finally, after completing the recursive calls and once num is equal to zero, sum() will return 15 to main().

i.e., 1 + 2 + 3 + 4 + 5 = 15.

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