Translating python dictionary to C++

Question

I have python code that contains the following code.

d = {}

d[(0,0)] = 0
d[(1,2)] = 1
d[(2,1)] = 2
d[(2,3)] = 3
d[(3,2)] = 4

for (i,j) in d:
    print d[(i,j)], d[(j,i)]

Unfortunately looping over all the keys in python isn't really fast enough for my purpose, and I would like to translate this code to C++. What is the best C++ data structure to use for a python dictionary that has tuples as its keys? What would be the C++ equivalent of the above code?

I looked at sparse matrices in the boost library, but couldn't find an easy way to loop only over the non-zero elements.

Have you considered another data layout. If (i,j) is always paired with (j,i) then you might not really need both. You could just build the association at the time you are building the dictionary. You could just store a dict of (i,j) in entry store ( d(i,j), d(j,i) ). This assumes the numbers in the tuple could be arbitrarily ordered, which is probably true. — Sanjaya R, Dec 03 '09 at 21:52
Also, if you are just walking the tuples, store them in a list and walk the list. — Sanjaya R, Dec 03 '09 at 21:53

Thomas · Accepted Answer · 2009-12-03T22:30:30.740

41

A dictionary would be a std::map in c++, and a tuple with two elements would be a std::pair.

The python code provided would translate to:

#include <iostream>
#include <map>

typedef std::map<std::pair<int, int>, int> Dict;
typedef Dict::const_iterator It;

int main()
{
   Dict d;

   d[std::make_pair(0, 0)] = 0;
   d[std::make_pair(1, 2)] = 1;
   d[std::make_pair(2, 1)] = 2;
   d[std::make_pair(2, 3)] = 3;
   d[std::make_pair(3, 2)] = 4;

   for (It it(d.begin()); it != d.end(); ++it)
   {
      int i(it->first.first);
      int j(it->first.second);
      std::cout <<it->second <<' '
                <<d[std::make_pair(j, i)] <<'\n';
   }
}

edited Dec 03 '09 at 22:30

answered Dec 03 '09 at 21:17

Thomas

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That's very nice indeed. Good job. Very instructional for someone like me working on upgrading their skillset from old school C++ to modern C++. – Bill Forster Dec 03 '09 at 21:34
1

`d[*it]` should be `it->second` – Dec 03 '09 at 21:43
2

It seems like the error was introduced by the edit by `joshperry` – chollida Dec 03 '09 at 22:09
Note that with C++14, `std::make_pair(a, b)` can be replaced by simply writing `{a, b}`, which makes it look a lot cleaner. – Romeo Valentin Aug 23 '19 at 12:57

catwalk · Answer 2 · 2009-12-03T22:00:09.757

8

The type is

std::map< std::pair<int,int>, int>

The code to add entries to map is like here:

typedef  std::map< std::pair<int,int>, int> container;

container m;

m[ make_pair(1,2) ] = 3; //...

for(container::iterator i = m.begin();  i != m.end(); ++i){
   std::cout << i.second << ' '; 
   // not really sure how to translate [i,j] [j,i] idiom here easily
}

edited Dec 03 '09 at 22:00

answered Dec 03 '09 at 21:12

catwalk

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You cannot use `typename` when the type is not a dependent name. – Dec 03 '09 at 21:42
@Roger: you're right, fixed that "typename container::iterator" – catwalk Dec 03 '09 at 22:00

score 5 · Answer 3 · answered Dec 03 '09 at 21:22

Have a look at Boost.python. It's for interaction between python and C++ (basically building python libs using C++, but also for embedding python in C++ programs). Most pythons data structures and their C++ equivalents are described (didn't checked for the one you want).

score 3 · Answer 4 · answered Dec 03 '09 at 21:26

std::map or more likely std::tr1::unordered_map / boost::unordered_map (aka hash_map) is what you want.

Also, as kriss said, Boost.Python is a good idea to look at here. It provides a C++ version of python's dict class already, so if you're doing cross-language stuff, it might be useful.

score 2 · Answer 5 · answered May 20 '16 at 20:24

2

Map is often implemented as a balanced binary tree not a hash table. This not the case for a Python dict. So you need a C++ O(1) equivalent data structure to use your pairs with.

answered May 20 '16 at 20:24

mszlazak

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Pete · Answer 6 · 2009-12-03T22:15:07.853

Do you want to call an optimized C++ routine via Python? If so, read on:

Often times I use PyYaml when dealing with dictionaries in Python. Perhaps you could link in something like LibYAML or yamlcpp to:

Translate a Python dictionary into a YAML string
Use Python to call a C++ function wrapped using something like SWIG, taking the YAML string as a parameter.
Use a C++ library to parse the YAML & obtain a std::map object
Operate on std::map object

Warning: I have never tried this, but using everyone's favorite search engine on "yaml std::map" yields lots of interesting links

score 0 · Answer 7 · answered Dec 03 '09 at 21:37

As a direct answer to your question (for the python part look at my other answer). You can forget the tuple part if you want. You can use any mapping type key/value (hash, etc.) in C++, you just have to find a unique key function. In some cases that can be easy. For instance if you two integers are integers between 1 and 65536 you just could use a 32 bits integer with each 16 bits part one of the keys. A simple shift and an 'or' or + to combine the two values would do the trick and it's very efficient.

Translating python dictionary to C++

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