Extract rows/columns from ** matrices in C/C++ - c++

Question

I have a square matrix double **A
I know how to iterate through this matrix:
for (int i = 0; i < MATRIX_SIZE; i++) {
for (int j = 0; j < MATRIX_SIZE; j ++) {
int val = A[i][j];
printf("val: %d\n", val);
}
}
However, I'm wondering how I can assign an entire row or column to a variable given I have this ** matrix (The ** pointer to pointer nonsense is still a little confusing. I believe its saying a list of list of doubles).
To add a little more background, I'm trying to extract rows and columns so I can perform a cuda matrix multiplication. I see a lot of documentation online that uses one-dimensional vectors to represent matrices (i.e. double* A) However, I am getting confused with the **

Answer 1

A two-dimensional array of doubles (double **) can be looked at as a one-dimensional array of one-dimensional arrays of doubles.
double **arr; // properly initialized
for(int rowNumber = 0; rowNumber < MATRIX_SIZE; ++rowNumber)
{
double *row = arr[rowNumber];
// do something with this row
for(int colNumber = 0; colNumber < MATRIX_SIZE; ++colNumber)
{
double value = row[colNumber];
// do something with value
}
}
In the above example, row is a pointer to a contiguous row of values from the initial array. This works because a two dimensional array is usually allocated like this:
double **arr = new double*[ROW_COUNT];
for(int rowNumber = 0; rowNumber < ROW_COUNT; ++rowNumber)
{
arr[rowNumber] = new double[COL_COUNT];
}
Getting a pointer to a column in the matrix (like we did with row above) is not possible because the values in a column are not contiguous, only the values in each row are contiguous.

Answer 2

In C++, you can use std::array
std::array< std::array<int, MATRIX_SIZE>, MATRIX_SIZE> A;
std::array<int, MATRIX_SIZE> ith_row = A[i];
std::array<int, MATRIX_SIZE> &ith_row_ref = A[i];

Answer 3

A[i][j] is a type of int, but A[i] is a int pointer, so if you want get a row to a variable, you can do this:
for (int i = 0; i < MATRIX_SIZE; i ++) {
int* val = A[i];
for (int j = 0; j < MATRIX_SIZE; j ++) {
printf("%d\n", val[j]);
}
}
but you can't get a column to a variable.

Answer 4

You can assign rows to variables easily but you can't assign columns because of the way that memory is laid out.
You can think of double pointers like this.
The first pointer points to the item to give you the row.
I'm going to make a 3w 4c matrix to show you an example
Theoretical (How you should think about it in your head)
Your first double pointer
p
|
V 0 1 2 <-indexes
0 [p1]->[1,2,3]
1 [p2]->[0,2,3]
2 [p3]->[1,0,3]
3 [p4]->[1,2,0]
which corresponds to the matrix
1,2,3
0,2,1
1,0,3
1,2,0
So you can thinking about getting the 0 at index (1,0) as
int **p = //some place that holds the matrix;
int *row2 = p[1];
int value = p[0];
The reason why it's not as straightforward as declaring a two dimensional
array is because when get the double pointer, you're not sure of the layout of the memory. The numbers could be stored like this
p1 p3 p2 p4
| | | |
[123103021120...] <- //this is basically RAM or "memory"
and you would have no idea as the programmer.
I hope this cleared some things up!