View Compilation Error

0_0_37769952_13174.cpp: In function 'int main()':
0_0_37769952_13174.cpp:37:42: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'char' for argument '1' to 'void calc(char, int&)'
         if (aa.size() == 1) calc(aa, a[1]);
                                          ^
0_0_37769952_13174.cpp:39:40: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'char' for argument '1' to 'void calc(char, int&)'
         if (b.size() == 1) calc(b, a[2]);
                                        ^
0_0_37769952_13174.cpp:41:40: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'char' for argument '1' to 'void calc(char, int&)'
         if (c.size() == 1) calc(c, a[3]);
                                        ^
0_0_37769952_13174.cpp:43:40: error: cannot convert 'std::string {aka std::basic_string<char>}' to 'char' for argument '1' to 'void calc(char, int&)'
         if (d.size() == 1) calc(d, a[4]);
                                        ^