c++实现python的filtfilt函数

filtfilt函数是一个双向滤波器，它可以使输入信号通过正向和反向滤波器两次，从而消除滤波器引入的相位延迟。下面是一个C语言实现filtfilt函数的示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define N 1000
#define M 6

void filtfilt(double *b, double *a, double *x, double *y, int n, int m)
{
    int i, j;
    double tmp[N];

    for (i = 0; i < n; i++) {
        y[i] = 0;
        tmp[i] = 0;
    }

    for (i = 0; i <= m-2; i++) {
        for (j = 0; j <= i; j++) {
            y[i] += b[j] * x[i-j];
            tmp[i] += a[j] * y[i-j];
        }
    }

    for (i = m-1; i < n; i++) {
        for (j = 0; j < m; j++) {
            y[i] += b[j] * x[i-j];
            tmp[i] += a[j] * y[i-j];
        }
    }

    for (i = n-m+1; i < n; i++) {
        for (j = i-n+m; j < m; j++) {
            y[i] += b[j] * x[i-j];
            tmp[i] += a[j] * y[i-j];
        }
    }

    for (i = n-1; i >= m; i--) {
        for (j = 1; j < m; j++) {
            y[i] += b[j] * x[i-j];
            tmp[i] += a[j] * y[i-j];
        }
    }

    for (i = 0; i < n; i++) {
        y[i] = tmp[i];
    }
}

int main()
{
    double b[M] = {0.2, 0.2, 0.2, 0.2, 0.2, 0.2};
    double a[M] = {1.0, -0.25, -0.25, -0.25, -0.25, -0.25};
    double x[N], y[N];
    int i, n;

    for (i = 0; i < N; i++) {
        x[i] = sin(i*0.1);
    }

    n = N;

    filtfilt(b, a, x, y, n, M);

    for (i = 0; i < n; i++) {
        printf("%lf ", y[i]);
    }

    printf("\n");

    return 0;
}

该示例代码使用一个6阶低通滤波器对正弦波进行滤波，并输出滤波后的结果。在实际使用中，需要根据具体的滤波器设计和信号特征来确定滤波器的系数和滤波器阶数。