API函数原型测试——1维真数组的使用，给出了如下三个自定义API函数：

mean：计算由num个浮点数组成的数组的平均值

std_dev：计算标准均方差

find_mode：返回样本的众数值，如果众数超过一个，则返回最后找到的那个

-API function prototypes- 1-dimensional real array of uses, given the following three custom API function: mean: calculated by the num array consisting of a floating point average std_dev: Calculation of the standard mean square find_mode: Back to sample public value, and if more than one mode, then returned to find that the final