当向量的两个元素相乘时，向量 c++ 与 operator* 不匹配

Question

This is a part of a code I wrote:

for(int i=1;i<=n;i++) a.push_back(i);
while(a.size()!=1){
    b+=a.begin()*a.end()+a.begin()+a.end();
    a.erase(a.begin());
    a.erase(a.end());
    }

This is the first time I am using vectors, so I dont know where I went wrong. The error I get is:

 error: no match for ‘operator*’ (operand types are ‘std::vector::iterator {aka __gnu_cxx::__normal_iterator >}’ and ‘std::vector::iterator {aka __gnu_cxx::__normal_iterator >}’)
       b+=a.begin()*a.end()+a.begin()+a.end();
          ~~~~~~~~~^~~~~~~~

Answer 1

begin() and end() return iterators . You can't multiply iterators. Use front() and back() to access the first and last value.

b += a.front() * a.back() + a.front() + a.back();

Answer 2

Looks like you over complicating this code. You do not need vector to calculate this value.

int f(int n) {
    int r = 0;
    for(int i=1; i <= (n + 1)/2; i++) {
        auto b = i;
        auto e = n + 1 - i;
        r += b * e + b + e;
    }
    return r;
}