Re: Extracting data from an XML to put into a constant


"ekkehard.horner" <ekkehard.horner@xxxxxxxx> wrote in message
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Paul Randall schrieb:
"ekkehard.horner" <ekkehard.horner@xxxxxxxx> wrote in message
news:47e9256b$0$4852$9b4e6d93@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Paul Randall schrieb:
"Bob Barrows [MVP]" <reb01501@xxxxxxxxxxxxxxx> wrote in message
news:%23n413wojIHA.5660@xxxxxxxxxxxxxxxxxxxxxxx
wills.jason@xxxxxxxxx wrote:
what i need to do is to put the <FolderID> into a constant
within my
script,
Well, you cannot set a contant's value at runtime: constants can
only be
set at compile time, which means you can only use literal values
when
defining the constants. You have to set the value to a variable
instead.
[...]
'you cannot ...' seems incorrect to me. I thought a script could
build strings containing almost any sequence of valid VBScript
statements and use the Execute or ExecuteGlobal statements to do
almost anything that can be done during normal execution of
predefined scripts.
[...]
-Paul Randall
While it's not wrong/technically correct to say that you can
create
constants at runtime by Execute[Global] strings like
[...]
works as expected. But users/advocates of such hacks will come to
grieve
(and deserve it). The problem with 'on the fly Consts' is that you
can't
use them where you would like to use them most:

ExecuteGlobal "Const cnUB = 9"
WScript.Echo cnUB
On Error Resume Next
Dim aFix( cnUB )
WScript.Echo Err.Description
On Error GoTo 0

this will break at *compile time*, because "Dim aFix( cnUB )" is
compiled
(fails to compile) before 'ExecuteGlobal "Const cnUB = 9"' is
executed.
[...]
Thank you for posting sample code to demonstrate the problem. But
it actually demonstrates a problem that has nothing to do with
Execute[Global]. The following script causes the same error.

Const cnUB = 9
WScript.Echo cnUB
On Error Resume Next
Dim aFix( cnUB )
WScript.Echo Err.Description
On Error GoTo 0

The Dim statement expects an integer constant which is not quite
the same as a Constant with an integer value.

I would like to see sample code that demonstrates problems with
using Execute[Global].

-Paul Randall

Thanks, Paul, for pointing out my error. I forgot/repressed that
VBScript
doesn't accept even 'real' Consts when dimming a fixed array. But
the
special treatment of 'real' Consts ('moving up' to start of scope)
that
isn't emulated by Execute-Consts can be seen from this code

Dim sSecs1 : sSecs1 = 10 * cnSecsPerMin
Dim sSecs2 : sSecs2 = 10 * cnSecsPerHour
ExecuteGlobal "cnSecsPerMin = 60"
Const cnSecsPerHour = 3600
WScript.Echo "sSecs1:", sSecs1
WScript.Echo "sSecs2:", sSecs2

and output:

cscript execconst.vbs
sSecs1: 0
sSecs2: 36000

In my opinion, compiling/executing code generated/loaded at runtime
(Exec[Global] and Eval in VBScript) is a valuable feature of
scripting
languages - suited for special purposes and with their own set of
problems (e.g. security). I use ExecGlobal regularly to load
'libraries'
(from a CPP host application, where the src attribute of a script
tag
can't be used as in .wsf, .html, or .hta files), but I'd consider
creating
variables and/or constants by this method as a second best practice
(to
be replaced by using Dictionaries or Classes with restricted access
to
memebers most of the time).

OK, now I understand what you are trying to demonstrate; I think this
script demonstrates it better:

'Start of code
MsgBox "PrescanConstant = " & PrescanConstant & vbCrLf & _
"GlobalExecConstant = " & GlobalExecConstant & vbCrLf & _
"OrdinaryVariable = " & OrdinaryVariable

Dim GlobalExecConstant
ExecuteGlobal "GlobalExecConstant = 2"
MsgBox "PrescanConstant = " & PrescanConstant & vbCrLf & _
"GlobalExecConstant = " & GlobalExecConstant & vbCrLf & _
"OrdinaryVariable = " & OrdinaryVariable

Dim OrdinaryVariable: OrdinaryVariable = 3
MsgBox "PrescanConstant = " & PrescanConstant & vbCrLf & _
"GlobalExecConstant = " & GlobalExecConstant & vbCrLf & _
"OrdinaryVariable = " & OrdinaryVariable

Const PrescanConstant = 1
'End of code

Like subroutine and function definitions, the value of constants is
done during the prescan of a script. This means that the subroutine
or function can be called from the first line of code, and that the
value of a constant can also be used from the first line of code.
Ordinary variables and constants defined with execute statements have
no value until their defining statement is executed at run time.

But sometimes, the a constant's value is difficult to come by when you
write the code, but a simple subroutine can determine the names of the
constants and their values. This is what I do in do in the
DefineConstants routine I posted last year; it access the typlib to
get the info and uses ExecuteGlobal to do the definitions. So an
ExecuteGlobal statement executed prior to the constant's being used is
a good solution, especially for a person of limited resources who just
wants to make use of objects already installed on their computer.

-Paul Randall


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