说明：  网上的大部分BP实现曲线拟合都使用MATLAB库函数，这不利于读者理解BP神经网络的算法。本文用C源码实现MATLAB库函数.
(BP to achieve most of the Internet curve fitting using MATLAB library function, this does not help readers understand the BP neural network algorithm. In this paper, the realization of C-source MATLAB library functions.)