So much errors in the code! I guess you want:
#include <algorithm>
#include <vector>
double vecMed(std::vector<double> vec) {
if(vec.empty()) return 0;
else {
std::sort(vec.begin(), vec.end());
if(vec.size() % 2 == 0)
return (vec[vec.size()/2 - 1] + vec[vec.size()/2]) / 2;
else
return vec[vec.size()/2];
}
}