How to get dimensions of a multidimensional vector in C++

Question

all

I am using multidimensional STL vector to store my data in C++. What I have is a 3D vector

 vector<vector<vector<double>>> vec;

What I want to retrieve from it is :

 &vec[][1][];    // I need a pointer that points to a 2D matrix located at column 1 in vec

Anyone has any idea to do so? I would be extremly appreciate any help!

Regards

Long

Answer 1

It is best to consider vec just as a vector whose elements happen to be vectors-of-vectors-of-double, rather than as a multi-dimensional structure.

You probably know, but just in case you don't I'll mention it, that this datatype does not necessarily represent a rectangular cuboid . vec will only have that "shape" if you ensure that all the vectors are the same size at each level. The datatype is quite happy for the vector vec[j] to be a different size from the one at vec[k] and likewise for vec[j][n] to be a vector of different size from vec[j][m] , so that your structure is "jagged".

So you want to get a pointer to the vector<vector<double>> that is at index 1 in vec . You can do that by:

vector<vector<double>> * pmatrix = &vec[1];

However this pointer will be an unnecessarily awkward means of accessing that vector<vector<double>> . You certainly won't be able to write the like of:

double d = pmatrix[j][k];

and expect to get a double at coordinates (j,k) in the "matrix addressed by a pmatrix ". Because pmatrix is a pointer-to-a-vector-of-vector-of-double; so what pmatrix[j] refers to is the vector-of-vector-of-double (not vector-of-double) at index j from pmatrix , where the index goes in steps of sizeof(vector<vector<double>>) . The statement will reference who-knows-what memory and very likely crash your program.

Instead, you must write the like of:

double d = (*pmatrix)[j][k];

where (*pmatrix) gives you the vector-of-vector-of-double addressed by pmatrix , or equivalently but more confusingly:

double d = pmatrix[0][j][k];

Much simpler - and therefore, the natural C++ way - is to take a reference , rather than pointer, to the vector<vector<double>> at index 1 in vec . You do that simply by:

vector<vector<double>> & matrix = vec[1];

Now matrix is simply another name for the vector<vector<double>> at index 1 in vec , and you can handle it matrix-wise just as you'd expect (always assuming you have made sure it is a matrix, and not a jagged array).

Another thing to consider was raised in a comment by manu343726. Do you want the code that receives this reference to vec[1] to be able to use it to modify the contents of vec[1] - which would include changing its size or the size of any of the vector<double> s within it?

If you allow modification, that's fine. If you don't then you want to get a const reference . You can do that by:

vector<vector<double> > const & matrix = vec[1];

Possibly, you want the receiving code to be able to modify the double s but not the sizes of the vectors that contain them? In that case, std::vector is the wrong container type for your application. If that's your position I can update this answer to offer alternative containers.

Answer 2

Consider using matrix from some linear algebra library. There are some directions here