Pointer to portions of array-CodePudding

I have an object of std::vector<std::array<double, 16>>

vector entry    Data
[0]           - 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[1]           - 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[2]           - 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
[...]         - 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

This is intended to represent a 4x4 matrix in ravel format.

To not duplicate information I would like to create a pointer to extract a 3x3 from the above structure: I have mathematical operations for the 3x3 structure (std::array<double, 9>)

someStructure: pointing to data elements [0, 1, 2, 4, 5, 6, 8, 9 10]

The end goal is do: std::array<double, 9> tst = someStructure[0] someStructure[1];

Is this doable?

Best Regards

CodePudding user response：

The 3x3 part is not contiguous, hence a pointer alone wont help here.

You can write a view_as_3x3 that allows you to access elements of the submatrix of the 4x4 as if it was contiguous:

struct view_as_3x3 {
    double& operator[](size_t index) {
        static size_t mapping[] = {0, 1, 2, 4, 5, 6, 8, 9, 10};
        return parent[mapping[index]];
    }
    std::array<double, 16>& parent;
};

Such that for example

for (size_t = 0; i< 9;   i) std::cout << " " << view_as_3x3{orignal_matrix}[i];

is printing the 9 elements of the 3x3 sub-matrix of the original 4x4 original_matrix.

Then you could more easily apply your 3x3 algorithms to the 3x3 submatrix of a 4x4 matrix. You just need to replace the std::array<double, 9> with some generic T. For example change

double sum_of_elements(const std::array<double, 9>& arr) {
       double res = 0;
       for (int i=0;i <9;   i) res  = arr[i];
       return res;
}

To:

template <typename T>
double sum_of_elements(const T& arr) {
       double res = 0;
       for (int i=0;i <9;   i) res  = arr[i];
       return res;
}

The calls are then

 std::array<double, 16> matrix4x4;
 sum_of_elements(view_as_3x3{matrix4x4});
 // or
 std::array<double, 9> matrix3x3;
 sum_of_elements(matrix3x3);

It would be nicer to use iterators instead of indices, however, writing the view with custom iterators requires considerable amount of boilerplate. On the other hand, I would not suggest to use naked std::arrays in the first place, but rather some my_4x4matrix that holds the array as member and provides iterators and more convenience methods.