C++ STL map I don't want it to sort!

Question

This is my code

map<string,int> persons;

persons["B"] = 123;
persons["A"] = 321;


for(map<string,int>::iterator i = persons.begin();
    i!=persons.end();
    ++i)
{
    cout<< (*i).first << ":"<<(*i).second<<endl;
}

Expected output:

  B:123
  A:321

But output it gives is:

  A:321
  B:123

I want it to maintain the order in which keys and values were inserted in the map<string,int> .

Is it possible? Or should I use some other STL data structure? Which one?

Answer 1

There is no standard container that does directly what you want. The obvious container to use if you want to maintain insertion order is a vector. If you also need look up by string, use a vector AND a map. The map would in general be of string to vector index, but as your data is already integers you might just want to duplicate it, depending on your use case.

Answer 2

Like Matthieu has said in another answer, the Boost.MultiIndex library seems the right choice for what you want. However, this library can be a little tough to use at the beginning especially if you don't have a lot of experience with C++. Here is how you would use the library to solve the exact problem in the code of your question:

struct person {
    std::string name;
    int id;
    person(std::string const & name, int id) 
    : name(name), id(id) { 
    }
};

int main() {

    using namespace::boost::multi_index;
    using namespace std;

    // define a multi_index_container with a list-like index and an ordered index
    typedef multi_index_container<
      person,        // The type of the elements stored
      indexed_by<    // The indices that our container will support
        sequenced<>,                           // list-like index
        ordered_unique<member<person, string, 
                              &person::name> > // map-like index (sorted by name)
      >
    > person_container;

    // Create our container and add some people
    person_container persons;
    persons.push_back(person("B", 123));
    persons.push_back(person("C", 224));
    persons.push_back(person("A", 321));

    // Typedefs for the sequence index and the ordered index
    enum { Seq, Ord };
    typedef person_container::nth_index<Seq>::type persons_seq_index;
    typedef person_container::nth_index<Ord>::type persons_ord_index;

    // Let's test the sequence index
    persons_seq_index & seq_index = persons.get<Seq>();
    for(persons_seq_index::iterator it = seq_index.begin(), 
                                    e = seq_index.end(); it != e; ++it)
        cout << it->name << ":"<< it->id << endl;
    cout << "\n";

    // And now the ordered index
    persons_ord_index & ord_index = persons.get<Ord>();
    for(persons_ord_index::iterator it = ord_index.begin(), 
                                    e = ord_index.end(); it != e; ++it)
        cout << it->name << ":"<< it->id << endl;
    cout << "\n";

    // Thanks to the ordered index we have fast lookup by name:
    std::cout << "The id of B is: " << ord_index.find("B")->id << "\n";
}

Which produces the following output:

B:123
C:224
A:321

A:321
B:123
C:224

The id of B is: 123

Answer 3

Map is definitely not right for you:

"Internally, the elements in the map are sorted from lower to higher key value following a specific strict weak ordering criterion set on construction."

Quote taken from here .

Unfortunately there is no unordered associative container in the STL, so either you use a nonassociative one like vector , or write your own :-(

Answer 4

I had the same problem every once in a while and here is my solution: https://github.com/nlohmann/fifo_map . It's a header-only C++11 solution and can be used as drop-in replacement for a std::map .

For your example, it can be used as follows:

#include "fifo_map.hpp"
#include <string>
#include <iostream>

using nlohmann::fifo_map;

int main()
{
    fifo_map<std::string,int> persons;

    persons["B"] = 123;
    persons["A"] = 321;

    for(fifo_map<std::string,int>::iterator i = persons.begin();
        i!=persons.end();
        ++i)
    {
        std::cout<< (*i).first << ":"<<(*i).second << std::endl;
    }
}

The output is then

B:123
A:321

Answer 5

maps and sets are meant to impose a strict weak ordering upon the data. Strick weak ordering maintains that no entries are equavalent (different to being equal).

You need to provide a functor that the map/set may use to perform a<b . With this functor the map/set sorts its items (in the STL from GCC it uses a red-black tree). It determines weather two items are equavalent if !a<b && !b<a -- the equavelence test.

The functor looks like follows:

template <class T>
struct less : binary_function<T,T,bool> {
  bool operator() (const T& a, const T& b) const {
    return a < b;
  }
};

If you can provide a function that tells the STL how to order things then the map and set can do what you want. For example

template<typename T>
struct ItemHolder
{
    int insertCount;
    T item;
};

You can then easily write a functor to order by insertCount. If your implementation uses red-black trees your underlying data will remain balanced -- however you will get a lot of re-balancing since your data will be generated based on incremental ordering (vs. Random) -- and in this case a list with push_back would be better. However you cannot access data by key as fast as you would with a map/set.

If you want to sort by string -- provide the functor to search by string, using the insertCount you could potentiall work backwards. If you want to search by both you can have two maps.

map<insertcount, string> x; // auxhilary key
map<string, item> y; //primary key

I use this strategy often -- however I have never placed it in code that is run often. I'm considering boost::bimap .

Answer 6

如果您需要保持插入顺序和按键搜索的能力，除了 Neil 推荐的组合向量 + 映射之外，您还可以考虑使用 boost 多索引库，它提供了以多种方式可寻址的容器。

Answer 7

Well, there is no STL container which actually does what you wish, but there are possibilities.

1. STL

By default, use a vector . Here it would mean:

struct Entry { std::string name; int it; };

typedef std::vector<Entry> container_type;

If you wish to search by string, you always have the find algorithm at your disposal.

class ByName: std::unary_function<Entry,bool>
{
public:
  ByName(const std::string& name): m_name(name) {}
  bool operator()(const Entry& entry) const { return entry.name == m_name; }
private:
  std::string m_name;
};

// Use like this:
container_type myContainer;
container_type::iterator it =
  std::find(myContainer.begin(), myContainer.end(), ByName("A"));

2. Boost.MultiIndex

This seems way overkill, but you can always check it out here .

It allows you to create ONE storage container, accessible via various indexes of various styles, all maintained for you (almost) magically.

Rather than using one container ( std::map ) to reference a storage container ( std::vector ) with all the synchro issues it causes... you're better off using Boost.

Answer 8

Use a vector. It gives you complete control over ordering.

Answer 9

For preserving all the time complexity constrains you need map + list:

struct Entry
{
   string key;
   int val;
};

typedef list<Entry> MyList;
typedef MyList::iterator Iter;
typedef map<string, Iter> MyMap;

MyList l;
MyMap m;

int find(string key)
{
   Iter it = m[key]; // O(log n)
   Entry e = *it;
   return e.val;
}

void put(string key, int val)
{
   Entry e;
   e.key = key;
   e.val = val;
   Iter it = l.insert(l.end(), e); // O(1)
   m[key] = it;                    // O(log n)
}

void erase(string key)
{
   Iter it = m[key];  // O(log n)
   l.erase(it);       // O(1)
   m.erase(key);      // O(log n)
}

void printAll()
{
   for (Iter it = l.begin(); it != l.end(); it++)
   {
       cout<< it->key << ":"<< it->val << endl;
   }
}

Enjoy

Answer 10

您可以使用成对向量，它与未排序的地图容器几乎相同

std::vector<std::pair<T, U> > unsorted_map;

Answer 11

Instead of map you can use the pair function with a vector! ex:

vector<::pair<unsigned,string>> myvec;
myvec.push_back(::pair<unsigned,string>(1,"a"));
myvec.push_back(::pair<unsigned,string>(5,"b"));
myvec.push_back(::pair<unsigned,string>(3,"aa"));`

Output:

myvec[0]=(1,"a"); myvec[1]=(5,"b"); myvec[2]=(3,"aa");

or another ex:

vector<::pair<string,unsigned>> myvec2;
myvec2.push_back(::pair<string,unsigned>("aa",1));
myvec2.push_back(::pair<string,unsigned>("a",3));
myvec2.push_back(::pair<string,unsigned>("ab",2));

Output: myvec2[0]=("aa",1); myvec2[1]=("a",3); myvec2[2]=("ab",2); myvec2[0]=("aa",1); myvec2[1]=("a",3); myvec2[2]=("ab",2);

Hope this can help someone else in the future who was looking for non sorted maps like me!

Answer 12

I also think Map is not the way to go. The keys in a Map form a Set; a single key can occur only once. During an insert in the map the map must search for the key, to ensure it does not exist or to update the value of that key. For this it is important (performance wise) that the keys, and thus the entries, have some kind of ordering. As such a Map with insert ordering would be highly inefficient on inserts and retrieving entries.

Another problem would be if you use the same key twice; should the first or the last entry be preserved, and should it update the insert order or not?

Therefore I suggest you go with Neils suggestion, a vector for insert-time ordering and a Map for key-based searching.

Answer 13

Yes, the map container is not for you.
As you asked, you need the following code instead:

   struct myClass {
      std::string stringValue;
      int         intValue;
      myClass( const std::string& sVal, const int& iVal ):
               stringValue( sVal ),
               intValue( iVal) {}
   };

   std::vector<myClass> persons;

   persons.push_back( myClass( "B", 123 ));
   persons.push_back( myClass( "A", 321 ));


   for(std::vector<myClass>::iterator i = persons.begin();
      i!=persons.end();
      ++i)
   {
      std::cout << (*i).stringValue << ":" << (*i).intValue << std::endl;
   }

Here the output is unsorted as expected.

Answer 14

I'd vote for typedef std::vector< std::pair< std::string, int > > UnsortedMap;

Assignment looks a bit different, but your loop remains exactly as it is now.

Answer 15

Map is ordered collection (second parametr in template is a order functor), as set. If you want to pop elements in that sequenses as pushd you should use deque or list or vector.

Answer 16

In order to do what they do and be efficient at it, maps use hash tables and sorting. Therefore, you would use a map if you're willing to give up memory of insertion order to gain the convenience and performance of looking up by key.

If you need the insertion order stored, one way would be to create a new type that pairs the value you're storing with the order you're storing it (you would need to write code to keep track of the order). You would then use a map of string to this new type for storage. When you perform a look up using a key, you can also retrieve the insertion order and then sort your values based on insertion order.

One more thing: If you're using a map, be aware of the fact that testing if persons["C"] exists (after you've only inserted A and B) will actually insert a key value pair into your map.

Answer 17

Use a Map along with a vector of iterators as you insert in Map. (Map iterators are guaranteed not to be invalidated)

In the code below I am using Set set myset; vector::iterator> vec;

void printNonDuplicates(){ vector::iterator>::iterator vecIter; for(vecIter = vec.begin();vecIter!=vec.end();vecIter++){ cout<<(*vecIter)->c_str()<

void insertSet(string str){ pair::iterator,bool> ret; ret = myset.insert(str); if(ret.second) vec.push_back(ret.first); }

Answer 18

There is std::unordered_map that you can check out. From first view, it looks like it might solve your problem.

Answer 19

If you don't want to use Boost::multi_index, I have put a proof of concept class template up for review here:

https://codereview.stackexchange.com/questions/233157/wrapper-class-template-for-stdmap-stdlist-to-provide-a-sequencedmap-which

using std::map and std::list which uses pointers to maintain the order.

It will take O(n) linear time for the delete because it needs to search the whole list for the right pointer. To avoid that would need another cross reference in the map.

Answer 20

``struct Compare : public binary_function { bool operator() (int a, int b) {return true;} };

use this to get all the elements of a map in the reverse order in which you entered(ie: the first entered element will be the last and the last entered element will be the first). Not as good as the same order but it might serve your purpose with a little inconvenience.