std :: accumulate没有按预期行事

Question

我使用std :: accumulate和测试代码得到了意想不到的结果.我试图添加一个大的双向量,但由于某种原因,值溢出:

#include <iostream>
#include <vector>
#include <functional>
#include <numeric>

using namespace std;

double sum(double x, double y)
{
    // slows things down but shows the problem:
    //cout << x << " + " << y << endl;
    return (x+y);
}

double mean(const vector<double> & vec)
{
    double result = 0.0;

    // works:
    //vector<double>::const_iterator it;
    //for (it = vec.begin(); it != vec.end(); ++it){
    //      result += (*it);
    //}

    // broken:
    result = accumulate(vec.begin(), vec.end(), 0, sum);

    result /= vec.size();

    return result;
}


int main(int argc, char ** argv)
{

    const unsigned int num_pts = 100000;

    vector<double> vec(num_pts, 0.0);

    for (unsigned int i = 0; i < num_pts; ++i){
        vec[i] = (double)i;
    }

    cout << "mean = " << mean(vec) << endl;

    return 0;
}

总和里面的cout的部分输出:

2.14739e + 09 + 65535
2.14745e + 09 + 65536
-2.14748e + 09 + 65537
-2.14742e + 09 + 65538
-2.14735e + 09 + 65539

正确的输出(迭代):

平均值= 49999.5

输出不正确(使用累积):

平均值= 7049.5

我可能犯了一个累了的错误？我之前已成功使用累积...

谢谢

Answer 1

你需要传递double给accumulate:

result = accumulate(vec.begin(), vec.end(), 0.0, sum);
                                            ^^^

否则,使用int,然后将结果转换为double来执行累积.