NIVEN/HARSHAD NUMBER:

Any number which is divisible by sum of its own digits.

digit = 21
sum=2+1 = 3           
division =21%3=7.

conditions true for niven/harshad number. 

Program. 

#include<stdio.h>
#include<math.h>
int main()
{
    int j,p,n,temp,sum=0;
    printf("enter number: ");
    scanf("%d",&n);
    temp=n;
    while(n>0)
    {
        j=n%10;
        sum=sum+j;
        n=n/10;
    }
    n=temp;
    p=temp/sum;
    printf("%d/%d=%d",n,sum,p);
    if(temp%sum==0)
        printf("\n(niven/harshad) number ");
    else
        printf("\nnot a (niven/harshad) number");
    return 0;
}

OUTPUT :

enter number: 6804
6804/18=378
(niven/harshad) number
enter number: 35
35/8=4
not a (niven/harshad) number