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?

Solution 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;
    }
}

Solution 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>.

Solution 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

Solution 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
}

Solution 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

View more solutions

Comments

[Nim]

refer to: cplusplus.com/reference/stl/map/begin

[ROAR]

In the latest version of C++ standard you can replace the lengthy iterator decelerations with the auto keyword.

[Matthieu M.]

I do wonder: in generally, does a compiler automatically lifts the myMap.end() outside of the loop ? Lifting invariants is a common optimization but I do wonder if it gets detected, I suppose it could with the implementation being inlined, but I don't know. Thoughts ?

[fredoverflow]

@RA: Or even better, a range based for loop. g++ 4.6.0 already supports this.

[Steve Jessop]

@RA: strictly speaking, in the latest proposal for the next version of the C++ standard, you can do that thing you said.

[Steve Jessop]

@Matthieu: main thought is that myMap.end() is almost certainly a trivial operation. Barely worth lifting, especially considering that iterating a map isn't a very cache-friendly operation.

[Steve Jessop]

@Matthieu: after some header-diving: in the GNU implementation, an iterator holds a pointer to a node, and the "end node" of a map is embedded in the object. So I think that assuming several layers of calls can be inlined, constructing and returning an end iterator is basically a pointer copy, and if you're lucky i != myMap.end() could just be a compare of the address stored in i, with myMap+offset. I haven't disassembled any code to check, though.

[Steve Jessop]

@John: I think you can use auto & val in your range-based for statement. We'll reach a proper type-inferred language some day...

[John Dibling]

@Steve: Edited. I'll verify once i get G++ installed. Until then I'm trusting you!

[Steve Jessop]

@John: I'm pretty sure just by looking at the definition of range-based for loop that auto is OK. My doubt about auto & is that auto does a somewhat fiddly thing equivalent to template argument deduction, so I may have missed something. I don't have a good enough compiler to test either.

[Martin York]

You keep copying the value out of the iterator. You should use a references as this then allows manipulation of the data within the map. string& value = val.second; I think any (average) user manipulating the value would expect the internal representation of the map to change.

[EddieV223]

This is wrong, not all STL container iterators yield std::pair. There is no need for a pair in vector or list for example.

[fredoverflow]

@EddieV223 fair enough, I clarified my answer.

[thecoshman]

As this is now a c++-faq answer, perhaps it can be cleaned up so that it is neater.