/in/foo.cc: In function 'int main()':
/in/foo.cc:15:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     for (int i = 0; i < s.length(); ++i) {
                     ~~^~~~~~~~~~~~
/in/foo.cc:24:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < a.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:27:38: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
                 else if (in == false && a[j] == '+' || a[j] == '-') {
                          ~~~~~~~~~~~~^~~~~~~~~~~~~~
/in/foo.cc:38:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < a.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:41:38: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
                 else if (in == false && a[j] == '+' || a[j] == '-') {
                          ~~~~~~~~~~~~^~~~~~~~~~~~~~
/in/foo.cc:52:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < a.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:55:38: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
                 else if (in == false && a[j] == '+' || a[j] == '-' || a[j] == '*' || a[j] == '/') {
                          ~~~~~~~~~~~~^~~~~~~~~~~~~~
/in/foo.cc:62:31: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
             for (int j = 0; j < b.length(); ++j) {
                             ~~^~~~~~~~~~~~
/in/foo.cc:65:38: warning: suggest parentheses around '&&' within '||' [-Wparentheses]
                 else if (in == false && b[j] == '+' || b[j] == '-') {
                          ~~~~~~~~~~~~^~~~~~~~~~~~~~
/in/foo.cc:14:9: warning: unused variable 'n' [-Wunused-variable]
     int n = s.length();
         ^