根據Pearson係數求兩個用戶的相似性

template<typename T>
double ComputePearson(vector<T>&user1,vector<T>&user2)
{
 double user1average = 0.0,user2average = 0.0;
 double sumuser12 = 0.0,temp1 = 0.0, temp2 = 0.0;
 for_each(user1.begin(),user1.end(),[&](T num) //sum the average
 {
  user1average += num;
 });
 user1average = user1average/user1.size();
 for_each(user2.begin(),user2.end(),[&](T num) // sum the average
 {
  user2average += num;
 });
 user2average = user2average/user2.size();
 size_t size = user1.size() >= user2.size() ? user2.size():user1.size();
 for (int i = 0 ; i < size; ++i)
 {
  sumuser12 += ((user1[i] - user1average)*(user2[i] - user2average));
 }
 for_each (user1.begin(),user1.begin()+size,[&](T num)
 {
  temp1 += pow(num-user1average,2);
 });
 for_each (user2.begin(),user2.begin()+size,[&](T num)
 {
  temp2 += pow(num-user2average,2);
 });
 return sumuser12 / sqrt(temp1*temp2); 
 
 
}