Questions regardling Structures, overlays and arrays

TSEYFARTH · Joined: 01 Jul 2006 Posts: 1054

Hello all,

I have a couple of questions to which I am hoping someone might be able to offer a suggestion. They are:
1. Multi-Dimensional arrays

I am trying to create a multi-dimensional structure to work as a buffer in a form such as this VB6 Code:

MWS · Joined: 22 Aug 2009 Posts: 2262

1. Bascom knows basically only one-dimensional arrays. More dimensions have to be set up and handled by user code. The only two-dimensional built-in array I know, would be an array of strings. There the index is the first dimension, while the addressed string itself is the second dimension. But one needs to use then string functions, while one wants to work maybe with a byte array. The terminator byte is also a waste then and I feel it too complicated. Before using such workarounds, I'd have already written code, to handle these multiple dimensions and project it onto one large byte array.

2. An overlay uses the same memory space of one variable, by laying over another variable on it. That's where the name comes from. The overlaying variable can be of a different type than the overlaid one.
The shown sample makes little sense, while it can be done. However, I miss the meaning of overlaying a single string with length of 5 chars (included terminator) onto a word array of 20 words.
Overlay does not create a structure, nor can it be repeated by any parameters.

3. No.

albertsm · Joined: 09 Apr 2004 Posts: 5921 Location: Holland

just want to add that in the next release arrays can have multiple dimensions like a(1,2,3,4,5).

TSEYFARTH · Joined: 01 Jul 2006 Posts: 1054

Hi all,

@ Mark - that will be a really cool addition!
@MWS the overlay is for a single word length, not all of them. It is for an array holding 20 descrete addresses each of 2 bytes.

For example,
Tx_Cmd_Wireless_Addr(1) = &hCDEF
The string is to hold the 4 characters 'CDEF' in this case.

Tim

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

You have a serious misunderstanding if you think a 4 character string overlayed onto a word will hold the characters 'CDEF' when the value of the word is &HCDEF.

TSEYFARTH · Joined: 01 Jul 2006 Posts: 1054

HI Adrian,

That was exactly why I posted the question! Normally the overlay would cover the entire 80 bytes (+1) for the string overlay. So how to do it using just 1 element of the word array??? The only other way I could think of was to have a temp word variable, and overlay that. But that also makes for a lot of manipulation. So was looking for the correct and better way.

Tim

AdrianJ · Joined: 16 Jan 2006 Posts: 2483 Location: Queensland

No Tim,

You still have it wrong. If you have a word ( 2 bytes ) with value &hCDEF, then there is no direct way using an overlay that you will ever see the representation of its value as a (4 byte) string 'CDEF'. The only way to get that is to do

Dim MyString as string * 4
Dim myWord as word

MyString = hexval(myWord )

which invokes a routine to convert a word value into its (hex) string representation.

Think of the degenerate case where the word value is &h0000 ( ie binary zero ). Then any sort of overlay would put two nulls ( binary zero ) into the first two bytes of the string. But no string can ever contain a null, since that is the string ender character.
The string representation of a binary zero word would be '0000', which is 4 ASCII characters of value 48 decimal ( &h30 ). But Bascom would suppress the leading zeroes of this, and only show the string as '0', ie one character of decimal value 48.

Get it ?

TSEYFARTH · Joined: 01 Jul 2006 Posts: 1054

Hi Adrian,

I believe I can get a handle on it. Thank you for your time and detailed explanation/education.

Tim