Dear Mailing List, for a project I need to convert a std::string to an ex, diff it and then convert it back to a std::string. For the conversion from std::string to ex I found this constructor : /** Construct example from string and a list of symbols. The input grammar is * similar to the GiNaC output format. All symbols and indices to be used * in the expression must be specified in a lst in the second argument. * Undefined symbols and other parser errors will throw an exception. */ ex(const std::string &s, const ex &l); at http://www.ginac.de/reference/classGiNaC_1_1ex.html#ace68037326c1a2b8c93a567... So I tried doing the following : symbol x("x"); ex expression; ex e("x^2",expression); std::cout << diff(e,x) << std::endl; But this leads me to this exception : std::invalid_argument: find_or_insert_symbol: symbol "x" not found Note that I tried using expression and e in diff(e,x). Unfortunately I could not find any documentation on how to convert the expression diff returns to a std::string. Thanks a lot for your help in advance ! Robert Hey