error: 'isinf' or 'isnan' or 'iserr' was not declared in this scope问题解决

        在windows下：

 _isnan(x)!_finite(x)

        在linux下：

isnan(x) isinf(x)

        为了跨平台，应添加如下代码：

int isnan(double x) { return x != x; }int isinf(double x) { return !isnan(x) && isnan(x - x); }int iserr(double x) {return isnan(x)||isinf(x);}