Multi-dimensional dynamic arrays in classes in C++ - c++

Question

I am a relative beginner to C++. I am working on a model related to forecasting property financials, and I am having a few issues getting my data structures setup.
A bit of background - the specific task I am trying to do it setup class variables for key data structures - one such structure called PropFinance. This structure will house all of my key information on a given property (with iterations for each property in a collection of them), including forecasts of future performance. Two of the main arguments being passed to the program are (applicable to all properties to be evaluated)
(1) number of iterations (Iterations) - how many times we are going to generate a forecast (random iterations)
(2) length of forecast (NumofPeriods) - how many periods we are going to forecast
The PropFinance class has 79 variables in it containing property details. A simple example - Expenses. For expenses, and many of my variables like it, I will need to create a 3D array of doubles - one dimension for each iteration, one dimension for each forecasted period. So ideally, I would have a variable for Expenses of:
class PropFinance {
double Expenses[Iterations][NumofPeriods];
}
but, I don't know Iterations and NumofPeriods at compile time. I do know the value of these two variables at the outset of runtime (and they will be constant for all iterations/properties of the current program execution)
My question is how can I have the size of these arrays dynamically updated when the program runs? Based on my research on this site and others, it seems like the two main ways to accomplish this are
(1) Use
(2) Use a pointer in the class definition and then use new and delete to manage
But even with those two options, I am not sure if it will work with a third dimension (all of the examples I saw needed just a single dimension to be dynamically sized). Could someone post either a verbal explanation or (better) a simple code example showing how this would work in (1) or (2) above? Any guidance on which option is preferable would be appreciated (but don't want to start a "what's better" debate). It seems like vector is more appropriate when the size of the array is going to be constantly changing, which is not the case here...
The overall speed of this model is critical, and as we expand the number of iterations and properties things get large quickly - so I want to do things as efficiently as possible.
Sorry I didn't post code - I can try to put something together if people are unable to discern what I am asking from above.

Answer 1

The idiomatic solution is to avoid direct heap allocations of C-arrays, and to prefer an STL container like std::vector, which automatically handles resizing, iteration, and element access in an efficient, portable manner. I would highly recommend Scott Meyers' Effective STL, which talks about appropriateness of each container for different applications - insertion/removal/retrieval complexity gaurantees, etc.

Answer 2

If you need more than 2 dimensions(3, 4, 5 and so on).The most easiest solution I know is using the multi_array provided by boost.
If you only need two dimension array, use vector
std::vector<std::vector<double> > Expenses;
Since you are a beginner, you better start with the higher level components provided by c++, even you are familiar with c++, you should stay with those high level components too.The basic elements of c++ are used when you need to develop some infrastructure(vector, list, smart pointers, thread and so on).
#include <iostream>
#include <vector>
int main()
{
std::vector<std::vector<double> > expenses[10]; //contains 10 std::vector<double>
expenses[0].push_back(100);
std::cout<<expenses[0][0]<<std::endl;
expenses.push_back(std::vector<double>()); //now expenses has 11 std::vector<double>
return 0;
}
how to use vector
multi array

Answer 3

I think you are approaching object oriented programming wrong.
Instead of having a master class PropFinance with everything in many dimensions arrays. Have you considered having classes like Iteration which has multiple Period such as
class Iteration
{
std::vector<Period­> _periods;
}
class Period
{
public:
double Expense;
}
Then as you add more dimensions you can create super classes PropFinance
class PropFinance
{
std::vector<Iteration> _iterations;
}
This makes everything more manageable instead of having deeply nested arrays [][][][]. As a rule of thumb, whenever you have multiple dimension arrays, consider creating subclasses containing the other dimension.