Ubound of a Index Pointer

[Edwin Knoppert]

I posted an example to use the array descriptor.

All that fear of incompatibility!
If it changes, you simply adapt to that compiler ain't it?
We are programmer's
This looks much of the same fear of using callbacks in VB.
Spooookkyyy!

On my site in the freetips help, you can find a copy of that source.


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[email protected]

[Michael Mattias]

I'm just wanting to make one function that I can call with
1 parameter that is a long or dword that will point to the
address of the array and I wish to find the ubound of that
pointer.

Let me spare you some angst: this cannot be done with the current release of the PowerBASIC Windows compilers.
(That is, in a factory-supported manner).

It can be done with the PB/DOS compilers using the internal functions ArrayCalc and ArrayInfo.

MCM

[Semen Matusovski]

Greg --
Probably, this code explains you better, how PB (and not only it) passes arrays.
Each array (like a string, for example), has a descriptor, which describes dimensions, location and so on.
When you use A(), PB simply transfers VarPtr to descriptor, which already exist before CALL.
So for performance this is equal to transfer scalar variable.

To demonstrate this, I included "hooligan action" and as you can see, LBound eats new "information".

   #Compile Exe
   ' DO NOT USE SUCH WAY
   Function Bounds1 (Ar() As Long) As Long
      ReDim Arr(0) As Dword At VarPtr(Ar())
      MsgBox Format$(Arr(6), "LBOUND = # ") + $CRLF + _
             Format$(Arr(7), "UBOUND = # ") + $CRLF + _
             Format$(Arr(0), "At = #"), , "Variant 1"
      Arr(6) = 1752 ' hooligan action
   End Function
   ' USE THIS WAY
   Function Bounds2 (Ar() As Long) As Long
      MsgBox Format$(LBound(Ar()), "LBOUND = # ") + $CRLF + _
             Format$(UBound(Ar()), "UBOUND = # ") + $CRLF + _
             Format$(VarPtr(Ar(LBound(Ar()))), "At = #"), , "Variant 2"
   End Function
   Function PbMain
      Dim A(705 To 1425) As Long
      Bounds1 A()
      Bounds2 A()
   End Function

[This message has been edited by Semen Matusovski (edited July 29, 2000).]

[Gregery D Engle]

Lance,

I guess I'm just wanting to use pointers because I assume it's
faster. The main reason I'm just simply wanting to pass
the dword address to the function is so its easier to call from
VB.

I don't really understand Semen's example. To me it looks like
common sense but you talk about descriptors?

I'm just wanting to make one function that I can call with
1 parameter that is a long or dword that will point to the
address of the array and I wish to find the ubound of that
pointer.



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-Greg

[Lance Edmonds]

I know I could simple pass the ubound to the function as well or
have it global but I really don't want to do that. Any ideas?

When you pass an array by it's descriptor (as Semen's example shows), PowerBASIC internally passes a single 32-bit pointer to the array's descriptor table - very little overhead, and your Sub/Function code can have complete control over the array. This is likely to be the most efficient method. It is possible to access the information by reading the descriptor table values, but these tables are intentionally undocumented as they may change from version to version of PowerBASIC.

Regardless, I don't see why is it so important to avoid using the methods that PowerBASIC currently provides to work with arrays in Subs/Functions. Maybe I'm missing something...?

As an aside, it is likely that a future version of PowerBASIC will include facilities to programmatically query array descriptor information, but that will likely require access to the array descriptor too.



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Lance
PowerBASIC Support
mailto:[email protected][email protected]</A>

[Semen Matusovski]

I prefer descriptors

   #Compile Exe
   Function Bounds (Ar() As Long) As Long
      MsgBox Format$(LBound(Ar()), "LBOUND = #  ") + $CRLF + _
             Format$(UBound(Ar()), "UBOUND = #  ") + $CRLF + _
             Format$(VarPtr(Ar(LBound(Ar()))), "At = #")
   End Function
   Function PbMain
      Dim A(5 To 25) As Long
      Bounds A()
   End Function

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[Gregery D Engle]

How do I find the ubound of an index pointer without passing it
to a function?

eg.

function example(address as dword) export as single
ArrayPTR = Address
Ubound& = <--------------------- How do I get this?
   for next a = 0 to ubound&
   ...
   ...

   next
end function

function pbmain() as long
   a! = example(VARPTR(ArrayIn(0))
end function

I know I could simple pass the ubound to the function as well or
have it global but I really don't want to do that. Any ideas?

Thanks

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-Greg