Transfer a matrix into an array - c++

Question

I was solving a problem that asked me to operate a double subscripted array by using a single-subscripted array. Hence, the question is: how can a double-subscripted array be transformed into a single-subscripted array? or if it can't be transformed.. How can you operate a double-subscripted through a single-subscripted array?
For example:
int matrix[ m ][ n ];
//How can you transform matrix into array? Or operate it?
int array[ m * n ];
Basically, the problem asks this: "Create a class DoubleSubscriptedArray. At construction time, the class should be able to create an array of any number of rows and any number of columns.The underlying representation of the double-subscripted array should be a single-subscripted array of integers with rows * columns number of elements"

Answer 1

Your matrix is stored as m times n elements contiguously in memory. What you can do is interpret the first element of your matrix as a int*, and you have your array. It prevents copying the elements.
int main()
{
int matrix[3][3] = { {1, 0, 0}, {0, 1, 0}, {0, 0, 1} };
int* array = &matrix[0][0];
// int array[9] = {1, 0, 0, 0, 1, 0, 0, 0, 1}
return 0;
}
If you need to copy your elements, you can try
#include <memory>
int main()
{
int matrix[3][3] = { {1, 0, 0}, {0, 1, 0}, {0, 0, 1} };
int array[9];
std::memcpy(&array[0], &matrix[0][0], sizeof(array));
// int array[9] = {1, 0, 0, 0, 1, 0, 0, 0, 1}
return 0;
}

Answer 2

The simplest answer:
int array[m * n];
for (int y = 0; y < n; ++y) {
for (int x = 0; x < m; ++x) {
array[y * m + x] = matrix[x][y];
}
}
Where:
y - row number
x - column number
Now you need to serialize all rows into one:
row_no/col_no | 0 | 1 | 2 |
______________|___|___|___|
0 | 1 | 2 | 3 | <-- matrix data
1 | 4 | 5 | 6 |
||
\ /
\/
array_no 0 1 2 3 4 5
| | | | | |
[ 1, 2, 3, 4, 5, 6 ] <-- array data
array_no = row_no * number_of_rows + col_no
You can TEST it :)
P.S You can also do it like this:
int* array = (int*)matrix;
It is version without copying :)

Answer 3

You can recalculate index of one dimension array to store the coresponding value of two dimensional array.
If you need to copy, eficient approuch would be to copy raw by row from two dim array to one dim, cause multidimensional arrays are stored in raw mayor order,
and index can be calculated as
oneDimIndex = row*n+col
where you are fetching element at matrix[row,col]
And after, you can retrieve content from one dimension array with same calculation.