Arithmetic-Derivative/arithmetic_derivative.c

#include <stdio.h>

long long derive (long long n);
void split (long long value, long long *a, long long *b);

int main () {
    long long n, res;
    int sgn;
    char again;
    do {
        sgn = 1;
        printf ("Enter an integer: ");
        scanf ("%lli", &n);
        if (n < 0)
            sgn = -1;
        if ( sgn * n > 9999999999999999)
            printf ("Warning! Number too big. The result could be wrong\n");
        res = derive (sgn * n); // derive (abs (n))
        res = sgn * res; // (-n)' = -(n')
        printf ("The arithmetic derivative of %lli is: %lli", n, res);
        getchar ();
        printf ("\nAgain? [y/n] ");
        scanf ("%c", &again);
    } while (again != 'n');
}

// if n=ab  (a and b > 1) -> n'=a'b + ab'
long long derive (long long n){
    long long a = 1, b = 1;
    switch (n) {
        case 0:	return 0; break; // 0' = 0
        case 1: return 0; break; // 1' = 0;
        default:
            split (n, &a, &b); // n = a * b
            if (b > 1) // if n is not a prime
                n = derive (a) * b + a * derive (b);
            else
                n = 1; // if n is a prime
            return n; 
            break;
    }
}

// n = a * b
void split (long long value, long long *a, long long *b) {
	long long i = 2;
	int stop = 0;
	do {
        if (value % i == 0) {
			*a = i;
			*b = value / *a;
			stop++;
		}
		i++;
	} while (stop == 0);
}