c++编程求1-1/2+1/3-......1/n,用while和do while做，哪里错了？

#include <iostream>
using namespace std;
int main()
{
int a,n;
float sum,sum1,sum2;
sum1=0,sum2=0,sum=0,a=0;
cin>>n;
while(a<=n)
{
if(a%2!=0)
sum1=sum1+(1.0/n);
else
sum2=sum2-(1.0/n);}
sum=sum1+sum2;
cout<<sum;

system("pause");
return 0;
}

1. 赋初值时，a = 1
2. sum1 = sum1 + (1.0 / a);
sum2 = sum2 - (1.0 / a);
3. 漏了一句a++;

#include <iostream> using namespace std; int main() { int n,a; float sum; sum=0,a=0; cin>>n; while(a++<n) { if(a%2!=0) sum=sum+(1.0/a); else sum=sum-(1.0/a); } cout<<sum; system("pause"); return 0; }

#include <iostream> using namespace std; int main() { int a,n; float sum,sum1,sum2; sum1=0,sum2=0,sum=0,a=1; //改过！改过！改过！改过！ cin>>n; while(a<=n) { if(a%2!=0) sum1=sum1+(1.0/a); //改过！改过！改过！改过！ else sum2=sum2-(1.0/a);a++;} //改过！改过！改过！改过！ sum=sum1+sum2; cout<<sum; system("pause"); return 0; }