Question

This is the code i was trying to compile for finding primitive root…but it gives me the error as.

int isPrimitive (int q, int a) {
int i,z;
double k,s;

s=1;
i=0;

double *factors=malloc(sizeof(double)*q-2);    

while (s>0 && i<q-2) {
    k=pow(a,i);
    s=k-(q*floor(k/q));
    for (z=0;z<(sizeof(*factors)/sizeof(factors[0]));z++) {
        if (factors[z]==s) {
            return 0;
        }
    }
    factors[i]=s;
    i++;
}

here in the line double *factors=malloc(sizeof(double)*q-2);
// error invalid conversion from void* to double* comes.

Answer 1

The problem is almost certainly using a C++ compiler on C code. In C void* can be converted to any pointer type without a cast, while this is not true in C++, hence the error message. There are basically two solutions: use a C compiler (e.g. gcc rather than g++), or cast the result of malloc.

The latter can be done by:

double *factors = static_cast<double*> (malloc(sizeof(double)*(q-2)));

But if you are actually writing C++, then using the new [] syntax is better:

double *factors = new double [q-2];

(Note that this requires using delete[] factors instead of free(factors))