double *a = malloc(sizeof(double)*nx*ny*nz);
int id;
for(int k = 0; k < nz; k++) {
for(int j = 0; j < ny; j++) {
for(int i = 0; < i < nx; i++) {
id = k*nx*ny + j*nx + i;
a[id] = /* some mathematical operation */
}
}
}