foo.cc: In function 'int evalexpression(const std::string&)':
foo.cc:35:16: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   35 |     while (idx < expression.size()) {
      |            ~~~~^~~~~~~~~~~~~~~~~~~
foo.cc:42:24: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
   42 |             while (idx < expression.size() && (isdigit(expression[idx]) || expression[idx] == '.')) {
      |                    ~~~~^~~~~~~~~~~~~~~~~~~
foo.cc: In function 'int calculate(int, int, char)':
foo.cc:28:1: warning: control reaches end of non-void function [-Wreturn-type]
   28 | }
      | ^

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