PureBasic Forums - English

I'm wondering, why the following code ends with an error message: I mean something like : shouldn't produce errors either...
Always the first declared value will be taken and any following declaration will be ignored.
At least i know this handling from other languages.

look the word "constant" up in a dictionary

What you need is a variable


Oh well, I looked it up for you

1. [a] persistent in occurrence and unvarying in nature.

2. [a] continually recurring or continuing without interruption.

3. [n] a quantity that does not vary.

4. [a] steadfast in purpose or devotion or affection.

5. [n] a number representing a quantity assumed to have a fixed value in a specified mathematical context.

6. [a] uninterrupted in time and indefinitely long continuing.

I am talking about the Error-Message and the resulting impossibility to compile!

Nothing of your pasted Text says something like it is forbidden
(Well i guess thats because it isn't forbidden at all!)!
And there is of course a deeper sense for my question (not this micky-maus-code i've pasted)

btw.: None of my examples touch any of your rules, so what did you try to say me ?

I think what he was trying to say is that a constant has to be, well, constant. It has to be declared at runtine, not varied during the program which requires a variable.

> I am talking about the Error-Message and the resulting impossibility to compile!

You can only declare a constant ONCE in your code. That's why you're
getting errors. You can't change it later, otherwise it's not a constant.
It doesn't matter that the declaration is inside an If/Then loop, because
the app simply can't choose one until run, and can't run until compiled.

> Always the first declared value will be taken and any following declaration will be ignored.

How can it be ignored? It's impossible. Think about it, and you'll see. Look:
The PureBasic compiler sees #Debugging twice and doesn't know which
value to assign to it: #False or #True? How can it know, unless the app
is run? And how can it run unless it's been compiled? See the problem?

Hi HeX0R,

Constants are identified with a leading # symbol, and are fixed or constant values.

They cannot be assigned a variable value.

Changing "#Debugging" to "DebuggingVar" or similar would fix the problem.


Edit:

Darn, PB, when did you sneak that post in? Now this response is overkill!

Hexor,

If you look at the ASM for some code you'll notice that the ASM makes no reference to any Constant. Not one.

This is because all constants are resolved at compile time. If you use the constant #Five as 5 then in the compiled code every instance of #Five will have been replaced with 5.

Take a look at you're code:
At compile time how can the compiler possibly know the value of l$? It cannot, this variable is resolved at Runtime.

Lets examine some possible solutions.

Firstly, your own suggestion:

Always the first declared value will be taken and any following declaration will be ignored.

In this instance #Debugging will always be false because that is the first occurance of #Debugging in the text. Regardless of the command paramaters the #Debugging will always be #False.

This is clearly not what you wanted. The difference would be that now the fault is silent. You will have no warning that you're debugging mode does not work until you have deployed your executable and you really need it to fix a problem while you're customer is breathing down you're neck.

Instead we could have Constants evaluated at runtime. This will slow down you're code because now instead of 'mov eax, 5' you are using 'mov eax, [#lbl_five].

If you want to have conditional constants then use compiler directives:
http://www.purebasic.com/documentation/ ... tives.html

If you want values that are evaluated at Runtime, use variables.

Thx GedB for the wonderful explanation

I had no idea, that pb handles it this way.
As i said, i know from other languages things like defined(CONSTANT) or similar to check if a Constant is already declared.
Now i know, why such a command is missing in pb

Then we just forget about the request

HeXOR,

I'm glad you found my explanation helpful. When I go back and read a post I always worry that my attempts at clarity and brevity just come across as impolite and arrogant.

GedB wrote:HeXOR,

I'm glad you found my explanation helpful. When I go back and read a post I always worry that my attempts at clarity and brevity just come across as impolite and arrogant.

LOL, i always think so about my posts

That's why I use so many smilies!

LOL, i always think so about my posts
That's why I use so many smilies!

Me too!

HeX0R wrote:As i said, i know from other languages things like defined(CONSTANT) or similar to check if a Constant is already declared.
Now i know, why such a command is missing in pb :P

I think this has been requested by some people (to check if a constant has been defined) - hopefully it will appear in the next version of PureBasic :)

I think this has been requested by some people (to check if a constant has been defined) - hopefully it will appear in the next version of PureBasic

Would that be as a Compiler Directive?

GedB wrote:

I think this has been requested by some people (to check if a constant has been defined) - hopefully it will appear in the next version of PureBasic :)

Would that be as a Compiler Directive?

It would have to be otherwise there would be no point in it. You wouldn't be able to do something like:

If defined(#MY_CONST)

at runtime, since all constants are replaced in the code with the value they represent. Chances are it would be done something like

CompilerIfDefined #MY_CONST

Although perhaps it would be more useful to have a general "defined" operator, so that you could use it on structures, variables (have they been specifically declared at this point in the code?), constants, etc. There must already be code for doing this since the compiler checks for stuff like that.

I think I've missed the point. What would be the advantage of a compiler directive to tell you whether a constant is already defined? Surely you could just run with debug on and it would tell you.