How can I traverse/iterate an STL map?

Question

I want to traverse an STL map. I don't want to use its key. I don't care about the ordering, I just look for a way to access all elements it contains. How can I do this?

Answer 1

Yes, you can traverse a Standard Library map. This is the basic method used to traverse a map, and serves as guidance to traverse any Standard Library collection:

C++03/C++11:

#include <cstdlib>
#include <map>
#include <string>
using namespace std;

int main()
{
    typedef map<int,string> MyMap;
    MyMap my_map;
    // ... magic

    for( MyMap::const_iterator it = my_map.begin(); it != my_map.end(); ++it )
    {
      int key = it->first;
      string value = it->second;
    }
}

If you need to modify the elements:

• Use iterator rather than const_iterator.

• Instead of copying the values out of the iterator, get a reference and modify the values through that.

  for( MyMap::iterator it = my_map.begin(); it != my_map.end(); ++it ) { int key = it->first; string& value = it->second; if( value == "foo" ) value = "bar"; }

This is how you typically traverse Standard Library containers by hand. The big difference is that for a map the type of *it is a pair rather than the element itself

C++11

If you have the benefit of a C++11 compiler (for example, latest GCC with --std=c++11 or MSVC), then you have other options as well.

First you can make use of the auto keyword to get rid of all that nasty verbosity:

#include <cstdlib>
#include <map>
#include <string>
using namespace std;

int main()
{
    map<int,string> my_map;
    // ... magic

    for( auto it = my_map.begin(); it != my_map.end(); ++it )
    {
      int key = it->first;
      string& value = it->second;
    }
}

Second, you can also employ lambdas. In conjunction with decltype, this might result in cleaner code (though with tradeoffs):

#include <cstdlib>
#include <map>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
    map<int,string> my_map;
    // ... magic

    for_each(my_map.begin(), my_map.end(), [](decltype(*my_map.begin()) val)
    {
        string& value = val.second;
        int key = val.first;
    });
}

C++11 also instroduces the concept of a range-bases for loop, which you may recognize as similar to other languages. However, some compilers do not fully support this yet -- notably, MSVC.

#include <cstdlib>
#include <map>
#include <string>
#include <algorithm>
using namespace std;

int main()
{
    map<int,string> my_map;
    // ... magic

    for(auto val : my_map )
    {
        string& value = val.second;
        int key = val.first;
    }
}

Answer 2

As with any STL container, the begin() and end() methods return iterators that you can use to iterate over the map. Dereferencing a map iterator yields a std::pair<const Key, Value>.

Answer 3

C++17

Since C++17 you can use range-based for loops together with structured bindings for iterating over a map. The resulting code, e.g. for printing all elements of a map, is short and well readable:

std::map<int, std::string> m{ {3, "a"}, {5, "b"}, {9, "c"} };

for (const auto &[k, v] : m)
    std::cout << "m[" << k << "] = " << v << std::endl;

Output:

m[3] = a
m[5] = b
m[9] = c

Code on Coliru

Answer 4

You can traverse STL map in the same way as any other STL container: using iterators, e.g.

for (std::map<key, value>::const_iterator
     i = myMap.begin(), end = myMap.end(); i != end; ++i)
{
    // *i is a key-value pair
}

Answer 5

Using for with auto for C++11 and above usage

map<int,int> map_variable; //you can use any data type for keys, as well as value

for(auto &x:map_variable)
{ 
    cout<<x.first ;// gives the key
    cout<<x.second; //gives the value
}

The newer format of for using auto was introduced in C++11

To give it functionality like some higher level languages like python

Where there was already an implementation of such type of iteration

P.S. : map variable keeps values sorted, so when iterating you will get keys in sorted order

Answer 6

You can iterate map by using auto iterator.

Code Snippet:

#include<bits/stdc++.h>
using namespace std;

int main()
{
      ios::sync_with_stdio(false);
      map<string, int> mp;

      mp["a"]=500;
      mp["b"]=200;
      mp["d"]=300;
      mp["c"]=400;

      for(auto it=mp.begin(); it != mp.end(); it++)
      {
         cout<<it->first <<" : "<<it->second<<endl;
      }
      return 0;
}