Nested Dictionary/Array in C++

Question

Everybody, I am a Python/C# guy and I am trying to learn C++. In Python I used to do things like:
myRoutes = {0:[1,2,3], 1:[[1,2],[3,4]], 2:[[1,2,3],[[1,2,3],[1,2,3]],[4]]}

Basically when you have arrays of variable length and you don't want to wast a 2D matrix for them, nesting arrays into a dictionary to keep track of them is a good option.

In C++ I tried std::map<int, std:map<int, std:map<int, int> > > and it works but I feel there got to bo a better way to do this.

I prefer to stick to the standard libraries but popular libraries like boost are also acceptable for me.

I appreciate your help,
Ali

Good luck shoehorning that kind of thing into a STL data structure. The best you can do is probably std::map<int, std::list<int>> or something similar with a fixed number of levels, not arbitrarily deep data structures, as in Python. That's not to say it can't be done, just that it's not trivial. — Chinmay Kanchi
– Chinmay Kanchi, Commented Mar 3, 2011 at 0:57
@Chinmay, so what data structure do you use in C++ when you need a dictionary of arrays? — Ali
– Ali, Commented Mar 3, 2011 at 1:02
Dictionary of arrays is no problem. std::map<int, std::list<int>> or std::map<int, std::vector<int>> will do that job just fine. The problem is that your arrays can hold more arrays and so on ad infinitum. Boost::Any or Boost::Variant might meet your needs, as @Phooji says. — Chinmay Kanchi
– Chinmay Kanchi, Commented Mar 3, 2011 at 1:38
Remember, Python is written in C, so it is very possible to write code that allows this, but also something I'd think long and hard before doing. Most of the time, wanting/needing this kind of code comes from not having fully shifted into thinking in the new language. I would wager that you're still trying to write Python code in C++, an effort, which IMHO, is doomed to failure. — Chinmay Kanchi
– Chinmay Kanchi, Commented Mar 3, 2011 at 1:42

phooji · Accepted Answer · 2011-03-03 01:02:32Z

It looks like part of the question is: 'How do I store heterogeneous data in a container?' There are several different approaches:

1) Use a ready-made class of array types that abstracts away the exact details (i.e., dimensionality). Example: Boost Basic Linear Algebra

2) Make an explicit list of element types, using Boost.variant

   #import "boost/variant.hpp"

   typedef boost::variant< ArrayTypeA, ArrayTypeB > mapelement;
   typedef std::map<int, mapelement> mappingtype;

Building a visitor for variant type is a bit involved (it involves writing a subclass of boost::static_visitor<desired_return_type> with a single operator() overload for each type in your variant). On the plus side, visitors are statically type checked to ens ure that they implement the exact right set of handlers.

3) Use a wrapper type, like Boost.Any to wrap the different types of interest.

Overall, I think the first option (using a specialized array class) is probably the most reliable. I have also used variants heavily in recent code, and although the compile errors are long, once one gets used to those, it is great to have compile time checking, which I prefer strongly over Python's "run and find out you were wrong later" paradigm.

wheaties · Accepted Answer · 2011-03-03 01:10:52Z

2

Many of us share the pain you're experiencing right now but there are solutions to them. One of those solutions is the Boost library (it's like the 2nd standard library of C++.) There's quite a few collection libraries. In your case I'd use Boost::Multi-Dimentional Arrays.

Looks like this:

boost::multi_array<double,3> myArray(boost::extents[2][2][2]);

Which creates a 2x2x2 array. The first type in the template parameters "double" specifies what type the array will hold and the second "3" the dimension count of the array. You then use the "extents" to impart the actual size of each of the dimensions. Quite easy to use and syntatically clear in its intent.

Now, if you're dealing with something in Python like foo = {0:[1,2,3], 1:[3,4,5]} what you're really looking for is a multimap. This is part of the standard library and is essentially a Red-Black tree, indexed by key but with a List for the value.

answered Mar 3, 2011 at 1:10

wheaties

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2 Comments

phooji Over a year ago

I think the foo = {0:[1,2,3], 1:[3,4,5]} part of your answer needs a bit more explanation. What you're saying is (I think) that it could be modeled in C++ using a multimap<int, int>, which has the nice property of dealing with the corner cases for you, or the more direct translation, map<int, list<int> >, which doesn't.

CashCow Over a year ago

multimap allows duplicate keys. Python arrays do not. So I don't see how it solves the problem here. The main issue is that the "value" type of the array varies so he needs some kind of variant, albeit they are always arrays of some dimension.

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