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;
    }
}