RockAndaHardPlace RockAndaHardPlace - 1 month ago 9
C++ Question

c++ - iterating through a map of 3 elements

I'm very new to the use of STL containers in C++.

I have a map of 3 elements (2 strings as a pair - acting as the key, and an int acting as the value.)

map<pair<string, string>, int> wordpairs;


But when I try to iterate through it like this:

for (map<pair<string, string>, int> iterator i = wordpairs.begin(); i != wordpairs.end(); i++) {
cout << i->first << " " << i->second << "\n";
}


the compiler is throwing errors:

error: expected ‘;’ before ‘i’
for (map<pair<string, string>, int> iterator i = wordpairs.begin(); i != wordpairs.
^
error: name lookup of ‘i’ changed for ISO ‘for’ scoping [-fpermissive]
a7a.cpp:46:50: note: (if you use ‘-fpermissive’ G++ will accept your code)

error: cannot convert ‘std::map<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >, int>::iterator {aka std::_Rb_tree_iterator<std::pair<const std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >, int> >}’ to ‘int’ in assignment
for (map<pair<string, string>, int> iterator i = wordpairs.begin(); i != wordpairs.
^
error: no match for ‘operator!=’ (operand types are ‘int’ and ‘std::map<std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >, int>::iterator {aka std::_Rb_tree_iterator<std::pair<const std::pair<std::__cxx11::basic_string<char>, std::__cxx11::basic_string<char> >, int> >}’)
for (map<pair<string, string>, int> iterator i = wordpairs.begin(); i != wordpairs.
^

error: expected ‘)’ before ‘;’ token
pair<string, string>, int> iterator i = wordpairs.begin(); i != wordpairs.end(); i++) {
^
error: expected ‘;’ before ‘)’ token
pair<string, string>, int> iterator i = wordpairs.begin(); i != wordpairs.end(); i++) {


Not sure what I'm doing wrong here - this should be a simple fix though.

Answer
  1. You got the type wrong (you used spaces instead of ::).

  2. Map iterator gives you a key value pair -- and your key is a pair! So you have a pair with a pair as a member. Here's an example that does roughly what you want to do.

    #include <iostream>
    #include <map>
    #include <string>
    #include <utility>
    using namespace std;
    
    int main() {
      pair<string, string> my_key("To", "Be");
      map<pair<string, string>, int> wordpairs { { {"Hello", "World"}, 33} };
      for (const auto& kv : wordpairs) {
        cout << kv.first.first << ", " 
             << kv.first.second << static_cast<char>(kv.second);
      }
      return 0;
    }