How to port C Lowpass filter code into Julia

I am having difficulty porting Julius Smith's lowpass filter C code into Julia. In particular, I am having difficulty understanding how this particular C for loop can be executed within Julia.

/* C function implementing the simplest lowpass:
 *      y(n) = x(n) + x(n-1)
double simplp (double *x, double *y, 
               int M, double xm1)
  int n;
  y[0] = x[0] + xm1;
  for (n=1; n < M ; n++) {
    y[n] =  x[n]  + x[n-1];
  return x[M-1];

Here is the webpage where the code resides for more context:

Thanks, Nakul