C++ equivalent of Python dictionaries

Question

I\'m currently making a tic-tac-toe program with AI and i\'m having a bit of a trouble translating this line of code (python) :

RANKS = dict([(4,3),                      


        

Answer 1

The closest match in C++ would be an std::unordered_map<int, int>. This is a hash table mapping int keys to int values.

#include <unordered_map>


std::unordered_map<int, int> RANKS = {
        { 4, 3 },
        { 0, 2 }, { 2, 2 }, { 6, 2 }, { 8, 2 },
        { 1, 1 }, { 3, 1 }, { 5, 1 }, { 7, 1 }
};

You can access elements using operator[], for example

std::cout << RANKS[0] << std::endl; // prints "2"

Note that the C++ standard library also has the std::map class template, which allows you to create a similar but ordered look-up table std::map<int, int>, with logarithmic look-up and insertion complexity. But python dicts are hash tables, so unordered_map is a closer match in terms of behaviour.

Answer 2

You could use a map or unordered_map for this (and they'd work fine) but given that your keys are a dense set of integers (I.e. all the integers from 0 to N) there are better choices.

I'd probably use an std::array instead. It would look something like this:

std::array <char, 9> vals = { 2, 1, 2, 1, 3, 1, 2, 1, 2 };

This gives pretty much the same syntax and observable behavior, but will typically save quite a bit of memory and probably CPU time as well.

Answer 3

The C++ equivalent of Python's dict is std::map. To initialize a map using a similar syntax, do this:

std::map<int,int> myMap = {{4,3},                       # center  = 3
                           {0,2},{2,2},{6,2},{8,2},     # corners = 2
                           {1,1},{3,1},{5,1},{7,1}};    # sides   = 1

Note that this needs C++11.

If you cannot use C++11, turn to map_list_of in Boost.Assign. The example from their page is:

using namespace boost::assign; // bring 'map_list_of()' into scope
std::map<int,int> next = map_list_of(1,2)(2,3)(3,4)(4,5)(5,6);

Answer 4

In C++ this would be a std::unordered_map

#include <unordered_map>

std::unordered_map<int, int> dict
{
    {
        { 4, 3 },
        { 0, 2 }, { 2, 2 }, { 6, 2 }, { 8, 2 },
        { 1, 1 }, { 3, 1 }, { 5, 1 }, { 7, 1 }
    }
};

Answer 5

Although a "language equivalent" might me something like an std::unordered_map your use case may be more efficiently served with a straight array:

int RANKS[] = {2, 1, 2, 1, 3, 1, 2, 1, 2};