Common items for C++ that I always forget how to do.

GeneralTips.cpp

// count character occurances in string
std::string s = "the answer; should ;be ;; four";
size_t n = std::count(s.begin(), s.end(), ';');

// Mahalanobis distance
double dist = sqrt((x - u).transposeInPlace() * cov_matrix.inverse() * (x - u)); // untested