Multidimensional Arrays in C++

C++ does not have a native way of doing multidimensional arrays like some other languages do. There are several ways to workaround this though. Here are some ideas:

Nested Vector method

Using the std::vector object, nest one vector within another like so:

#include <vector>

std::vector< std::vector<int> > MyArray;

// You can index this with normal array syntax:
MyArray[i][j] = x;

Note: the above array has not been initialised with anything, nor does it have a predefined size at compile time as vectors can be allocated dynamically, therefore so can our makeshift 2D Array. It’s effectively a vector of vectors with type int.

Nested array (C++11) method

C++11 has a new feature in the standard library: std::array. Unfortunately it also does not support multidimenional arrays, but you can nest one within another.

#include <array>

std::array< std::array<float, 3>, 3 > MyArray  = { { {5, 8, 2}, {8, 3, 1}, {5, 3, 9} } };

This is a 3x3 array as we’ve specified the dimensions in the declaration. We’ve also intialised the values in the array. Only available with C++11.

Using the Template Numerical Toolkit (TNT)

Eventually, the workarounds above can get a bit tiresome, or difficult to read using higher dimensional arrays. In LSDTopoTools, we use a library called the Template Numerical Toolkit, which adds support for multidimensional arrays, without the awkward looking syntax used above. The ‘user-friendly’ documentation is a bit sparse, but the basic examples are here.

The typical way to declare and intialise a TNT array is like so:

TNT::Array2D <float> mynewarray;  // Empty 2d array
TNT::Array2D <float> myotherarray(4,5);  // 4x5 array
TNT::Array2D <float> mythirdarray(3,3, 0.0); // Initialise zeros.

It’s not mentioned in the brief tutorial, but you can also do it like this:

MyArray = TNT::Array2D <float> (3,3);

Note that assigning ArrayA to ArrayB only produces a shallow copy (a view of the other array). To get a deep copy, you have to do B = A.copy(). Also, in the LSDTT code, we tend to use a namespace declaration to avoid typing TNT::Array... all the time:

#include "TNT/tnt.h"
using namespace TNT;

Array2D <int> somearray(3,3);

Other Methods

You could define your own template (good luck!) or use C-style arrays (ugh…).

Other Libraries

In the LSDTT code you might also see the Matrix Template Library (MTL). This is another matrix/array library with some more powerful functions for doing matrix algebra. There’s also the Eigen library, which can do a lot of things that the TNT can’t, but is a much bigger library.

In some languages, there is a useful method that will sort the contents of an array based on the order of another:

For example, C# has this:

Array.sort(array, array)

This doesn’t quite translate easily into C++. But don’t worry, you can use this handy template:

template <class T1, class T2, class Pred = std::less <class T> >
struct sort_pair_second
{
  bool operator()(const std::pair<T1,T2>&left, constd std::pair<T1,T2>&right)
  {
    Pred p;
    return p(left.second, right.second);
  }
};

// Usage:

std::sort(v.begin(),v.end(), sort_pair_second <int,int>() );

I have been having trouble with bil files if I import ASTER data on my windows machine. It seemed to be limited to windows but I now think the problem could be systematic.

I think I have fixed it but you’ll need to check by rerunning some test analyses and making sure the behaviour is what you expect.

ENVI files have a ‘DATATYPE’ entry in the hdr file that give the number of bytes in each data point. ASTER data is 2 bytes. It turns out this is a ‘short int’ type. It can only go up to ~32000.

So 1) I have changed the read raster functions to read short ints now 2) I have changed the LSDIndexRaster to go back to printing normal 4 bit ints so that we don’t run out of numbers…I think this has happened to big DEMs where we need the NodeIndex.

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