can anybody offer a simple hash_map example in C++?

Question

Hi,

I heard a lot about hash function but I don't know how to use it in C++. I learnt that we can use hash_map. Does g++ support that by simply including #include <hash_map>? Can anybody offer me a simple example using hash_map?

Thanks a lot!

Answer 1

Wikipedia never lets down:

http://en.wikipedia.org/wiki/Hash_map_(C%2B%2B)

Answer 2

hash_map is a non-standard extension. unordered_map is part of std::tr1, and will be moved into the std namespace for C++0x. http://en.wikipedia.org/wiki/Unordered_map_%28C%2B%2B%29

Answer 3

The current C++ standard does not have hash maps, but the coming C++0x standard does, and these are already supported by g++ in the shape of "unordered maps":

#include <unordered_map>
#include <iostream>
#include <string>
using namespace std;

int main() {
    unordered_map <string, int> m;
    m["foo"] = 42;
    cout << m["foo"] << endl;
}

In order to get this compile, you need to tell g++ that you are using C++0x:

g++ -std=c++0x main.cpp

These maps work pretty much as std::map does, except that instead of providing a custom operator<() for your own types, you need to provide a custom hash function - suitable functions are provided for types like integers and strings.

Answer 4

#include <tr1/unordered_map> will get you next-standard C++ unique hash container. Usage:

std::tr1::unordered_map<std::string,int> my_map;
my_map["answer"] = 42;
printf( "The answer to life and everything is: %d\n", my_map["answer"] );

Answer 5

The name accepted into TR1 (and the draft for the next standard) is std::unordered_map, so if you have that available, it's probably the one you want to use.

Other than that, using it is a lot like using std::map, with the proviso that when/if you traverse the items in an std::map, they come out in the order specified by operator<, but for an unordered_map, the order is generally meaningless.