C Program to Find Sum of 1+11+111 Using Recursive Function

In this program, we first read number of terms n from user. Then we pass value of n to recSum() which calculates sum of 1 + 11 + ... series up to n terms recursively.

This C program calculates sum of 1 + 11 + 111 + 1111 + .... up to n terms using recursive function.

C Source Code

#include<stdio.h>

int recSum( int n );

int main()
{
    int n,res;
    printf("Enter number of terms:\n");
    scanf("%d",&n);
    res=recSum(n);
    printf("Required sum = %d", res);
    
    return 0;
}

int recSum( int n )
{
    if (n<=0)
    {
        return 0;
    }
    else
    {
        return ( n + 10 * recSum ( n-1 ) );
    }
}

Output

Enter number of terms:
4 ↲
Required sum = 1234 


Note: ↲ indicates ENTER is pressed.