Circular Buffer in C

I need to modify a program to use a circular buffer for the input data window This is the program:

#include <stdio.h>
#include <math.h>
#include "p2.h"
void slide(double y[], double in)
{
 int i;
 for(i=0; i<N-1; i++)  y[i] = y[i+1];
 y[N-1] = in;
}
double filter(double y[] ,double x)
{
 int i;
 double out=0;
 for(i=0;i<N;i++)
 {
  out = y[i]*c[i] + out;
 }
 return out;
}
int main( void)
{
  int s; 
  double d, y[N]={0};
  while( 1 ) 
  { 
    s = scanf("%lf",&d);
    if( s != 1) 
      break;
     {
  slide(y,d);
  d = filter(y,d);
  printf( "%g\n", d);
  }
}
  return 0;
}

This is a bonus question for a homework assignment and i need the extra points, any help would be incredible.