C program to find sum of natural numbers in given range using recursion

Write a recursive function in C programming to find sum of all natural numbers between 1 to n. How to find sum of all natural numbers using recursion in C program. Logic to find sum of natural numbers in given range using recursion.

Input lower limit: 1
Input upper limit: 10

Sum of natural numbers from 1 to 10 = 55

Required knowledge

Basic C programming, If else, Functions, Recursion

Also learn – Program to find sum of natural numbers in given range using loop.

Declare recursive function to find sum of natural numbers

1. First give a meaningful name to the function, say sumOfNaturalNumbers().
2. Next the function must accept two inputs i.e. the lower and upper limit to find sum. Hence, pass two integer parameters to the function say sumOfNaturalNumbers(int start, int end).
3. Finally, the function must return sum of natural numbers between start and end. Therefore return type of function should be int.

The final function declaration to find sum of all natural numbers in given range is – int sumOfNaturalNumbers(int start, int end);

Logic to find sum of natural numbers using recursion

Above is the mathematical recursive function to find sum of natural numbers. Where n is lower limit and x is upper limit. n=x is base condition to exit control from function returning n.

If n < x then return sum of current number i.e. n and n+1. To find sum of n+1 we will make a recursive call to sumOfNaturalNumbers() function i.e. sumOfNaturalNumbers(start + 1, end);.

Program to find sum of natural numbers using recursion

/**
 * C program to find sum of natural numbers from 1 to n using recursion
 */

#include <stdio.h>


/* Function declaration */
int sumOfNaturalNumbers(int start, int end);


int main()
{
    int start, end, sum;
    
    /* Input lower and upper limit from user */
    printf("Enter lower limit: ");
    scanf("%d", &start);
    printf("Enter upper limit: ");
    scanf("%d", &end);
    
    sum = sumOfNaturalNumbers(start, end);
    
    printf("Sum of natural numbers from %d to %d = %d", start, end, sum);
    
    return 0;
}


/**
 * Recursively find the sum of natural number
 */
int sumOfNaturalNumbers(int start, int end)
{
    if(start == end)
        return start;
    else
        return start + sumOfNaturalNumbers(start + 1, end); 
}

Enter lower limit: 1
Enter upper limit: 100
Sum of natural numbers from 1 to 100 = 5050

Happy coding 😉