在C ++ STL中搜索映射中的键时，出现以下错误

Question

I have a created a Map with key as string type and the associated value stored in vector. Now I have a string and need to check if the each of the character in the string is present as a key in the map.

#include <iostream>
#include <stdlib.h>
#include <vector>
#include <map>
#include <string>
using namespace std;

int main() {
   map<string, vector<string>> umap;
   umap["1"] = {"a","b","c"};
   umap["2"] = {"d","e","f"};
   string s = "23";
   for(int i=0; i<s.length(); i++) {
      if(umap.find(s[i]) != umap.end()) 
          cout<<"Present"<<endl;
      else
          cout<<"Not Present"<<endl;
      }
}

Error:

main.cpp: In function ‘int main()’:
main.cpp:15:26: error: no matching function for call to ‘std::map<std::__cxx11::basic_string<char>, std::vector<std::__cxx11::basic_string<char> > >::find(__gnu_cxx::__alloc_traits<std::allocator<char> >::value_type&)’
         if(umap.find(s[i]) != umap.end())

Answer 1

The error is maybe a bit cryptic. Lets translate it into something human readable.

main.cpp: In function ‘int main()’:
main.cpp:15:26: error: no matching function for call to ‘std::map<std::__cxx11::basic_string<char>, std::vector<std::__cxx11::basic_string<char> > >::find(__gnu_cxx::__alloc_traits<std::allocator<char> >::value_type&)’
         if(umap.find(s[i]) != umap.end())

First std::__cxx11::basic_string<char> is a complicated way to denote a std::string . Then __gnu_cxx::__alloc_traits<std::allocator<char> >::value_type& is an even more complicated way to denote the return type of s[i] which is actually just char& . Putting this together we get

main.cpp: In function ‘int main()’:
main.cpp:15:26: error: no matching function for call to ‘std::map<std::string, std::vector<std::string> >::find(char&)’
         if(umap.find(s[i]) != umap.end())

I hope now you can see that the error complains that there is no overload of find that would take a char& as parameter.

Instead you should pass a std::string , eg via s.substr(i,1) .