计算标准差,均值和方差的函数

//计算标准差
float cal_stdev(vector<float> nums) {
    float sum = accumulate(nums.begin(), nums.end(), 0.0);
    float temp = 0.0;
    float mean = sum / nums.size();
    for (auto num : nums) {
        temp += (num - mean) * (num - mean);
    }
    return sqrt(temp / (nums.size() - 1));
}

//计算均值和方差
void cal_mean_stdev(vector<float> A, float& mean, float& stdev)
{
    float sum = std::accumulate(std::begin(A), std::end(A), 0.0);
    mean = sum / A.size(); //均值  

    float accum = 0.0;
    std::for_each(std::begin(A), std::end(A), [&](const float d) {
        accum += (d - mean) * (d - mean);
        });

    stdev = sqrt(accum / (A.size() - 1)); //方差
}