Is there a way to switch the dimensions order in a multi-dimensional vector?

Question

I have a 3D vector and I want to be able to chose which dimension to plot as a function of another dimension.

So far, I am doing this manually: I create a second 3D vector and re-organize the data accordingly. This solution is not very practical since I need to switch the indexes (inside the nested loop) every time I want to switch the dimensions...

Is there a better/cleaner solution ?

Thanks.

Answer 1

C++ does not provide multidimensional containers for the same reason that containers like std::vector do not provide a standard operator+ etc.: there is no standard that fits everyone's needs (in the case of a + operator, this could be concatenation, element-wise addition, increasing the dimensionality, who knows). If instead of a vector you take a class

<template typename T>
class volume {

private:
std::vector<T>       data;    // e.g. 3x2 array { 0, 1, 2, 3, 4, 5 }
std::vector<size_t>  sizes;   // e.g. 3x2 array { 3, 2 }
std::vector<size_t>  strides; // e.g. 3x2 array { 1, 3 }

};

then you have all the flexibility you want - no need to stop at 3D!

As an example the data vector of a 3x2 array could be the first 6 natural numbers, the sizes vector would be { 3, 2 } and the strides array { 1, 3 }: in a row (of which there are 2) the elements are next to each other, to increase the row you need to move 3 positions forward.

In the general n-dimensional case you can make an at() operator that takes a vector (or an initializer_list) as a position argument, and the offset corresponding to that position is its inner product with strides.

If you don't feel like programming this from scratch then libraries like Blitz++ already provide this functionality.