在网上学习的filter的写法
    #include<stdio.h>
   #include<math.h>

void filter(int xlen,int nfilt, int* a, int* b,double* zi)
{
    int i,j,x,y;
    for(i=0;i<xlen;i++)
    {
        for(j=0;i>=j&&j<nfilt;j++)
           {y[i]+=(b[j]*x[i-j]-a[j]*y[i-j]);}    报错expression must have pointer-to-object type
        if(zi&&i<nfilt-1)
                {y[i] += zi[i];}
    }
    a[0]=1.0;
}