Kendall tau implementation in C

Kendall_tau.c

double kendalT(double *x, int len_x, double * y, int len_y)
{
    //the two array MUST be have the same length
    int n2=0, n1=0, k, j;
    int is =0;
    double aa, a2, a1;

    for(j=1;j<len_x;j++){
      for(k=(j+1);k<=len_x;k++){
        a1=x[j]-x[k];
        a2=y[j]-y[k];
        aa = a1*a2;
        if (aa) {
          ++n1;
          ++n2;
          aa>0.0 ? ++is: --is;
        }
        else {
          if (a1) ++n1;
          if (a2) ++n2;
        }
      }
    }

    double tau;
    tau = is/(sqrt((double)n1)*sqrt((double)n2) );
    //printf("Tau is %.4f\n", tau);
    return tau;

}