#include <iostream>
using namespace std;
int main()
{
    int i;
    i=1;
    double sum;
    sum=0.0;
    for(i=1;i<=20;i++)
    {
          if(i%2)
           sum=sum-1.0/i;
           sum=sum+1.0/i;
    }
              cout<<sum;
              system("pause");
              return 0;
}

你看看i＝1的时候sum是1吗？正负号的问题