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AppGameKit Classic Chat / When is an integer not an integer

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PopsHugo
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Posted: 13th Feb 2015 12:03
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I have a problem. The code is below.

Why is the compiler not accepting the integer icx in lines 17 onward?

Pops Hugo

tables.agc

16 icx as integer = 200
17 global backgroundicon as integer = icx
18 global clickanywhereicon as integer = icx : icx = icx + 1
19 global colorcrazeehicon as integer = icx : icx = icx + 1

Error messages:
Compilation failed.
tables:agc:17 error: Variable default value must be an integer literal or constant
tables:agc:18 error: Variable default value must be an integer literal or constant
tables:agc:19 error: Variable default value must be an integer literal or constant

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Van B
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Posted: 13th Feb 2015 13:14
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Someone will probably come along and correct this, as I'm not completely sure - but I think that you have to treat a Global as a changing constant - so you can't assign it with another variable, you'd have to assign an actual numeric value instead when declaring the global.

Like...

global backgroundicon as integer
backgroundicon = icx

I don't think AppGameKit is designed to use a variable in global deceleration, just an actual value or nothing at all. As I said though, not 100% sure, it's just something I do regardless.

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unlikely
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Posted: 13th Feb 2015 16:29 Edited at: 13th Feb 2015 16:32
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I don't think it has to do with global scope. I believe the compiler doesn't want you to assign a typed variable with a non-literal at declaration. You can, however, do this with a dynamically typed variable...

e.g.
F as integer = 5
G = F // works
H as integer = F // Does not work (H is "statically" typed)

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Markus
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Posted: 13th Feb 2015 16:35
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the compiler is not perfect ...

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unlikely
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Posted: 13th Feb 2015 16:37
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I think this is reasonable... At least the compiler lets you know what's going on and how to fix it.

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Markus
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Posted: 13th Feb 2015 16:58
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thats all stumbling blocks, the compiler should understand this,
also because its a basic language.
there is no error in it.

@unlikely
what means "statically" typed ???

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unlikely
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Posted: 13th Feb 2015 17:05 Edited at: 13th Feb 2015 17:11
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Basically:
Static typing: The compiler knows, right off at compile-time, what type the variable is. You specified what it is at declaration, and it cannot change. (C/++, Java, etc.)

Dynamic typing: The compiler doesn't necessarily know what type the variable is until run-time... (Most scripting languages, and others.)

There are more intricacies in the differences, but that's it in a nutshell.

The reason I wrote "statically" typed, is because I don't actually know if the AppGameKit compiler does dynamic typing.

EDIT:
And I realized that the:
F = foo
declaration is actually the same as:
F as integer
F = foo
As I think if you do not use # or $, it assumes integer (so not dynamic at all, and no reason to disallow F as integer = g!)
I think you're right Markus, the compiler should handle this!

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Paul Johnston
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Posted: 13th Feb 2015 17:20
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Quote: "global backgroundicon as integer = icx"


Assigning default values to variables is done at compile time, this allows you to declare the variable in an include file and know that it will have the desired value without the program having to reach the line where you declared it. As such the default value must be a value, rather than a variable, so that the compiler can write it into the bytecode. Then if you declare

backgroundicon will equal 5 from the very first line of your program until you change it.

You can do

but the program would have to reach this line, and only after it was run would your variable equal icx.
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JimHawkins
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Posted: 13th Feb 2015 18:07
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Paul's explanation is entirely sensible.

Onwards and sometimes upwards
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unlikely
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Posted: 13th Feb 2015 18:10
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Yep, makes sense. Thanks Paul.

What about:



vs.


Why does the first not work and the second work? They are the same, right?

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Paul Johnston
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Posted: 13th Feb 2015 18:23
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Quote: "Why does the first not work and the second work? They are the same, right?"


The first one is a declaration, which is read by the compiler, the second is an assignment which is read at run time.
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unlikely
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Posted: 13th Feb 2015 18:24 Edited at: 13th Feb 2015 18:25
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But the second one works if the variable has not been declared, so it is a declaration and assignment, like the first, no?

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Paul Johnston
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Posted: 13th Feb 2015 18:31
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True, but then the variable doesn't have a default value until that line is reached. Whereas the first one declares to the entire program that X is equal to, say, 5 until it is assigned another value.

When you use

without declaring x, the compiler inserts a hidden declaration for you of

to the start of the program
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BatVink
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Posted: 13th Feb 2015 18:33 Edited at: 13th Feb 2015 18:34
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[EDIT] posted while Paul was posting

The second one is a runtime declaration. It works it out when the program runs. At this point it knows what the value in variable is.

The first - with as integer - tells the compiler to evaluate the variable. Thus, it can only manage values that can be determined before the program is run. The compiler cannot know what the value in variable is.

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unlikely
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Posted: 13th Feb 2015 19:09 Edited at: 14th Feb 2015 02:18
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Quote: "without declaring x, the compiler inserts a hidden declaration for you of

x as integer = 0

to the start of the program "


Ah, okay. There's the key! Thanks Paul!

(Coming from using Python, with its dynamic typing, this explanation clears things up a lot for me.)

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paulrobson
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Posted: 13th Feb 2015 20:01
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The compiler does not understand constants in the same way that C does. So you cannot write things like:



if you ask it to calculate 2*3+7*9 it will do it, manually. A modern C compiler will say, ah, all those are constants so that is actually 69, AGK2 sees it as a non-constant. When AGK2 says constant it means constant
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Markus
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Posted: 13th Feb 2015 20:23 Edited at: 13th Feb 2015 20:27
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this
global backgroundicon as integer = icx
can be intern this or not?
global backgroundicon as integer = 0: backgroundicon = icx
its the same syntax like this we can write
global backgroundicon as integer
backgroundicon = icx
so why the compiler can not? (it shows a message instead translate it)

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BatVink
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Posted: 13th Feb 2015 20:52
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Quote: "A modern C compiler will say, ah, all those are constants so that is actually 69, AGK2 sees it as a non-constant"


I used to work with a language called RPG on the IBM midrange systems. We used flags which were '0' or '1', so the program code was always checking for '1'. But whenever you said

IF FLAG *EQ 1

It needed a permanent memory slot for the number 1 for the lifetime of the program. Every reference to '1' was a new variable (even though it was constant!) Frequently we ran out of memory for this reason, especially when stringing together many open programs. So we had to declare a 1 byte variable, set it to 1, and use this variable everywhere in the program. Then it only took up 1 byte for all instances.

Oh how times have changed!

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Paul Johnston
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Posted: 14th Feb 2015 00:55
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Quote: "global backgroundicon as integer = 0: backgroundicon = icx"


You can do that but it will be split into two separate lines, so backgroundicon will be 0 from the start of your program until it reaches that line at which point it will change to equal icx.
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Markus
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Posted: 14th Feb 2015 01:17 Edited at: 14th Feb 2015 01:18
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i mean the compiler can do this without show a error message.
16 icx as integer = 200
17 global backgroundicon as integer = icx <<< Compilation failed. tables:agc:17 error: Variable default value must be an integer literal or constant

compile / define with default values
icx as integer = 200
global backgroundicon as integer = 0

at row 17 ...
backgroundicon = icx

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Paul Johnston
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Posted: 14th Feb 2015 02:06
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Technically if the compiler was intelligent enough it could see that icx has a default value of 200 and apply the same to backgroundicon, but unfortunately it isn't that smart and only sees that icx is a variable that could have any value, so doesn't know what value to write to the bytecode.
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Markus
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Posted: 14th Feb 2015 19:36
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i think for variables there should be default the value 0 or 0.0 or ""
except constants.
means only at runtime if the app run in the line of code it do the assignment, not before.

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