如何阅读地图 <string,vector<pair<int, string> &gt;&gt;

Question

I am having a map like this

typedef vector< pair<int, string> > Categories;  
map<string,Categories> cats;

but when I am trying to read elements of the map like

for(map<string,Categories>::const_iterator it = cats.begin(); it != cats.end(); ++it)
 {
    std::cout << it->first << " "  << it->second.first<<"\n";
 }

I do get errors

error: 'const class std::vector<std::pair<int, std::basic_string<char>
' has no member named 'first'  std::cout << it->first << " "  << it-> second.first<<"\n";

Answer 1

error : 'const class std::vector ' has no member named 'first' std::cout << it->first << " " << it->second.first<<"\\n";

Its clear as Crystal, that you might have many elements in your value s of your map, which is a std::vector< std::pair<int, std::string>>

Then how would you print elements of a vector? The options are:

1. random access (ie, vec[index] )
2. iterator (ie, std::vector< std::pair<int, std::string>>::const_iterator itr; )
3. or by a range based for loop (ie, for(const auto& it: vec) )

In your case, if you wanna have something simple and easy code is using a range based loop:

   for(const auto& it: cats)
   {
      std::cout << it.first << " = "; // keys
      for(const auto& ve: it.second)  // values vec
         std::cout << ve.first << "-" << ve.second << "\t";
      std::cout << std::endl;
   }

If you still wanna have long iterator loops, here is it.

see live action here: https://www.ideone.com/3bS1kR

#include <string>
#include <iostream>
#include <vector>
#include <map>
#include <iterator>

typedef std::vector< std::pair<int, std::string>> Categories;

int main()
{
   std::map<std::string, Categories> cats;

   cats["key1"] = {std::make_pair(1, "pair1"), std::make_pair(1, "pair2"), std::make_pair(1, "par3")};
   cats["key2"] = {std::make_pair(2, "pair1"), std::make_pair(2, "pair2")};
   cats["key3"] = {std::make_pair(3, "pair1")};

   std::cout << "Range based loop \n";
   for(const auto& it: cats)
   {
      std::cout << it.first << " = ";  // keys
      for(const auto& ve: it.second)   // values vec
         std::cout << ve.first << "-" << ve.second << "\t";
      std::cout << std::endl;
   }

   std::cout << "\nIterator loop \n";
   std::map<std::string, Categories>::const_iterator it;
   std::vector< std::pair<int, std::string>>::const_iterator curr_val_it;
   for(it = cats.cbegin(); it != cats.cend(); ++it)
   {
      std::cout << it->first << " = "; // keys

      for(curr_val_it = it->second.cbegin(); curr_val_it != it->second.cend(); ++curr_val_it )
          std::cout << curr_val_it->first << "-" << curr_val_it->second << "\t";  // values vec

      std::cout << std::endl;
   }


   return 0;
}

Answer 2

you need to access a element of the vector first, then the pair within.

... it->second[0].first<< ...

better impl of loop:

for(const auto& cat : cats)
 {
     string mapidx = cat.first;
     vector<pair<int, std::string>> catvect = cat.second;
 }

then you can have a seperate loop to read the contents of the vector:

for(const auto& cat : cats)
 {
     string mapidx = cat.first;
     vector<pair<int, std::string>> catvect = cat.second;
     for (const auto& entry : catvect)
     {
         int number = entry.first;
         string whatever = entry.second;
     }
 }

the temp variables are just for readability, no need for all the copies ;)

Answer 3

Error is exacly what compiler told you:

const class std::vector ' has no member named 'first'

so, you have do decide how to print your map by overloading ostream opeartor, below example how it can be achived:

#include <iostream>
#include <map>
#include <string>
#include <utility>
#include <vector>

typedef std::vector<std::pair<int, std::string>> Categories;  
std::map<std::string,Categories> cats;

std::ostream& operator << (std::ostream& os, const std::vector<std::pair<int, std::string>>& v) 
{
    os << "[";
    for (auto& el : v) {
        os << " " << el.first << " : " << el.second;
    }
    os << "]";
    return os;
}

int main() {
    cats.emplace("cat1", std::vector<std::pair<int, std::string>>(1, std::make_pair(1, "category1")));
    for(auto& cat : cats) {
        std::cout << cat.first << " "  << cat.second << "\n";
    }
}

Answer 4

Since we are storing Vector Categories inside a Map we will have to iterate Vector too:

 for(map<string,Categories>::const_iterator it = cats.begin(); it != cats.end(); ++it)
    {
        //iterator for vector (it->second)
        for(Categories::const_iterator it2 = it->second.begin(); it2 != it->second.end(); it2++ )
        {
              std::cout << it->first << " "  << it2->first<<" "<<it2->second <<"\n";
        }
    }