Re: Question about 2D VB arrays

Dmitriy,

You state the following:

"So if you want to have the same memory layout in both languages, then you
just
need to switch "meaning" of subscripts to the opposite - if you think that
the first
dimension in C is a row, then you should think that the first subscript in
VB is a column."

When you stated this, it leads one to believe that one of the langues will
now be
more inefficient when accessing the array. If the array is stored in
contiguous
memory in column major order (i.e. VB) and accessed by C, the C code will
be much more inefficient, due to the fact that it expects it's data to be in
row
major order. There is going to be a big hit due to memory access.

John


"Dmitriy Antonov" wrote:


"John" <John@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:9B1BA774-2D4E-4C7D-8026-B4F909F2115A@xxxxxxxxxxxxxxxx
This following question has started several "heated" discussions within my
company between the C/C++ and VB programmers and it deals with 2d visual
basic arrays.

Say for instance that I want to define an array that consists of 480 rows
by
640 columns. In C++ you would allocate and populate the array in the using
the
following syntax:

LONG i, j, k = 0;
LONG* larray = new LONG[480* 640];
for (i=0; i<480; i++) { // loop for rows
for (j=0; j<640; j++) { // loop for columns
larray[i * 640 + j] = k; // row * columns + column
}
}
delete [] larray;

In memory the array is stored in the following manner

Hex
0x0000 0, 1, 2, 3, 4, 5......639 (Row 1)
0x0A00 641, 642........... (Row 2)

Now when I look at how these arrays are allocated and populated within our
company in VB it's done in the following manner

Dim i as long, j as long, k as long
Dim larray (0 to 639, 0 to 479) as long

k = 0
For i = 0 To 639 // columns
For j = 0 to 479 // rows
larray(i, j) = k
k = k + 1
Next J
Next I

The VB code above seems wrong, allocated wrong and populated wrong! They
allocate the array by columns then rows. All the documentation I have
found
on
arrays for visual basic state that they are allocated by rows/columns.

This really screws things up when you call a COM object using a safearray
as a
parameter, when you get the number of rows and columns, they are backwards
and the data will be scrambled.

So here are my questions:

- How are arrays supposed to be allocated and populated within VB?
- How are arrays layed out within memory?
- Is VB column or row major order?
- If column major order why does the documentation state they should be
allocated
by Dim(Rows, Columns)?

I want an outsiders opinion because I don't think any of the VB
programmers I
work with really know the answer.

Any explanation would be greatly appreciated.
Signed,

A confused C/C++ programmer.


I don't think any "official" documentation explicitly defines which
subscript is Rows and which one is Columns. This is, IMO, where your problem
(or subject for discussion) lies. You, as an author, will define the meaning
of each subscript (or index, or dimension). C and VB use opposite layouts
for arrays, which is true. I don't see a problem here as long as you know
this fact. None of these two layouts, as others mentioned, has definite
advantages comparing to another one. So if you want to have the same memory
layout in both languages, then you just need to switch "meaning" of
subscripts to the opposite - if you think that the first dimension in C is a
row, then you should think that the first subscript in VB is a column.

For example, I personally, often prefer to think that the second dimension
is Rows, when array is to be redimensioned with Preserve keyword, because I
more often need to add or remove Rows without loosing existing data, and it
is not that often when I need to change number of columns. In cases, where
Preserve is not used, then meaning of each subscript is more matter of
personal taste and/or organizational policies and/or context of given task.

One more thing to mention - be aware that SafeArrayBound elements of
SafeArray descriptor are placed in the order, which is an opposite to how
subscripts are defined in VB code. So the second dimension in VB's
declaration of 2D array is presented by the first SafeArrayBound element.

BTW. As presented and as mentioned by MichaelC, you are actually comparing
1D array in C with 2D array in VB, which is somewhat incorrect. You could
define and populate 1D array in VB in virtually the same way and this would
eliminate the "heated discussions" in your company before they started - you
would compare apples with apples and not different fruits, as you are doing
right now.

Dmitriy.




.

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