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Active Forums => QB64 Discussion => Topic started by: TempodiBasic on April 08, 2021, 09:13:47 pm

Title: I think to get a bug with passing for value to a function or sub
Post by: TempodiBasic on April 08, 2021, 09:13:47 pm

Hi
while I was attempting to solve a Rosetta Code challange (task)
I fall on this issue...
please copy paste run this code

Code: QB64: [Select]

1. Dim As Integer a
2. Print " Demo with number"
3. a = 8
4.  
5. Half ((a)) ' here it doesn't modify a
6. Print a, Half(a) 'expected   8 , 4
7. Half (a) 'here it modifies a
8. Print a, Half(a) ' expected 4 , 2
9.  
10. Dim As String b
11.  
12. Print " Demo with string"
13. b = "I Love QB64!" ' 12 characters
14.  
15. Print b;
16. HalfString ((b)) ' this doesn't modify b
17. Print "--"; b;
18. HalfString (b) ' this modify b
19. Print "--"; b
20. End
21.  
22. Sub Half (num As Integer)
23.     num = num / 2
24. End Sub
25.  
26. Function Half (num As Integer)
27.     Half = num / 2
28. End Function
29.  
30. Sub HalfString (stri As String)
31.     stri = Left$(stri, Int(Len(stri) / 2))
32. End Sub

As you can see from my comments after passing a number for value I got no modification of it also in the following normal calling of SUB.
Moreover using a string variable and calling the SUB that chunks it at middle the variable has been modified also if passed fpr value.

Your feedbacks.

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: bplus on April 08, 2021, 10:21:28 pm

Don't know why string arguments enclosed with () are being modified in main code, my understanding is that is passing ByVal so they wouldn't be altered in main code.

Really confusing to have a SUB and Function by same name so I changed case on H for Sub, IDE shows difference.

Code: QB64: [Select]

1. Dim As Integer a
2. Print " Demo with number"
3. a = 8
4.  
5. half ((a)) ' here it doesn't modify a
6. Print a, Half(a) 'expected   8 , 4
7. half a 'here it >>>> really <<<< modifies a
8. Print a, Half(a) ' expected 4 , 2
9.  
10. Dim As String b
11.  
12. Print " Demo with string"
13. b = "I Love QB64!" ' 12 characters
14.  
15. Print b
16. HalfString ((b)) ' this modify b >>> it shouldn't but does apparently
17. Print "-"; b
18. HalfString (b) ' this modify b  >>> don't know why ?
19. Print "--"; b
20. End
21.  
22. Sub half (num As Integer)
23.     num = num / 2
24. End Sub
25.  
26. Function Half (num As Integer)
27.     Half = num / 2
28. End Function
29.  
30. Sub HalfString (stri As String)
31.     stri = Left$(stri, Int(Len(stri) / 2))
32. End Sub
33.  
34.  

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: bplus on April 09, 2021, 01:00:23 am

Holy crap! Even this is changing b$ !!!

Code: QB64: [Select]

1. Print " Demo with string"
2. b$ = "I Love QB64!" ' 12 characters
3.  
4. Print b$
5. HalfString ((b$ + "")) ' this modify b >>> it shouldn't but does apparently
6. Print "-"; b$
7. HalfString b$ + "" ' this modify b  >>> don't know why ?
8. Print "--"; b$
9.  
10. Sub HalfString (stri As String)
11.     stri = Left$(stri, Int(Len(stri) / 2))
12. End Sub
13.  
14.  

This is what is expected:

Code: QB64: [Select]

1. Print " Demo with string"
2. b$ = "I Love QB64!" ' 12 characters
3.  
4. Print b$
5. HalfString ((b$ + " ")) '
6. Print "-"; b$
7. HalfString b$ + " "
8. Print "--"; b$
9.  
10.  
11. Sub HalfString (stri As String)
12.     stri = Left$(stri, Int(Len(stri) / 2))
13. End Sub

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: RhoSigma on April 09, 2021, 02:15:35 am

I think, strings and entire arrays are always passed by reference no matter what.

It seems logic to me, think of passing a big string or array, maybe with 1000s of chars or elements, by value? - Wouldn't it in worst case not simply blow up the stack (stackoverflow), especially if done so in many nested SUB/FUNCs or even in a recursivly called function?

I've no idea how much stack space is available for a program assigned by the OS at startup time of a program, nor I know if there is some mechanism which can automatically increase the stack size at runtime if required.

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: bplus on April 09, 2021, 02:28:32 am

Yeah, I did not find anywhere in Wiki any notes where wrapping in () forces ByVal, and yes strings could take up allot of storage specially in recursion. So maybe it works only on numbers.

But I was surprised I could not force ByVal by adding a Null to the string in the call to sub.

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: TempodiBasic on April 09, 2021, 04:12:57 am

Hi
I'm sure and agree about not so good practice to try to pass a string as value and not as reference.
If we see at wiki page of ByVal it says that STRING must be passed as reference.

About recursion and stack issue I partially agree because it is real the limit of the stack of memory, but the same is for dynamic arrays
in this case the coder must set a limit to the expansion of the size of the memory used for the task.

However I have used so little times the passing by value and now I'm discovering how QB64 manages it.
And my philosophycal question is: is it the same in QB45?
As soon I have just a little time I'll test and I'll report here the results.

Thanks for feedbacks

PS How to find the () argument about passing by value in the wiki? I am not so clever with setting search in database!

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: bplus on April 09, 2021, 02:51:07 pm

Quote from: TempodiBasic on April 09, 2021, 04:12:57 am

Hi
I'm sure and agree about not so good practice to try to pass a string as value and not as reference.
If we see at wiki page of ByVal it says that STRING must be passed as reference.

About recursion and stack issue I partially agree because it is real the limit of the stack of memory, but the same is for dynamic arrays
in this case the coder must set a limit to the expansion of the size of the memory used for the task.

However I have used so little times the passing by value and now I'm discovering how QB64 manages it.
And my philosophycal question is: is it the same in QB45?
As soon I have just a little time I'll test and I'll report here the results.

Thanks for feedbacks

PS How to find the () argument about passing by value in the wiki? I am not so clever with setting search in database!

ByVal, the only reference I found in Wiki for ByVal concerned DECLARE LIBARY, using library, possible from another PL. I scoured Sub and Function and extra article on Subs for the mention of () wrap to pass ByVal instead of By Reference, nothing!

As I recall QB4.5 was ByVal because I remember being surprised to learn QB64 passed everything By Reference by default. In other Basic's I learned to pass arrays by reference to save stack or storage or processing time as ByVal passing was default for them.

So question remains when/where I learned trick of using () to pass numbers (only) by value. Don't think I'd even recommend doing that. I recommend making copy of passed variable if you intend on manipulating the value in the procedure and want it the same when exit procedure.

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: TempodiBasic on April 10, 2021, 05:37:59 pm

Hi
I have done some attempts in QB under Dosbox!
So I have discovered this: https://www.qb64.org/forum/index.php?topic=3798.0;topicseen (https://www.qb64.org/forum/index.php?topic=3798.0;topicseen) in QB45 a function and a sub cannot have the same name, in QB64 it is possible!

And more and more
the same code  with those little adjustments  (declaration of model of SUB FUNCTION at the beginning of the code, different names for SUB and Function) gives these results...
  [ This attachment cannot be displayed inline in 'Print Page' view ]  
so with number seems that QB64 follows QB45 exactly  both passing the variable for value both for reference ,
instead with the string variable Qb64 acts in opposite way in respect of QB45, in fact this last doesn't change the string variable  whiile QB64  modifies the string variable.

Your thinkings?

Title: Re: I think to get a bug with passing for value to a function or sub
Post by: Pete on April 10, 2021, 10:17:48 pm

Quote from: RhoSigma on April 09, 2021, 02:15:35 am

I think, strings and entire arrays are always passed by reference no matter what.

It seems logic to me, think of passing a big string or array, maybe with 1000s of chars or elements, by value? - Wouldn't it in worst case not simply blow up the stack (stackoverflow), especially if done so in many nested SUB/FUNCs or even in a recursivly called function?

I've no idea how much stack space is available for a program assigned by the OS at startup time of a program, nor I know if there is some mechanism which can automatically increase the stack size at runtime if required.

QBasic could set the stack space using: CLEAR , , stack

I recall needing CLEAR , , 2000 to get my office program to run without an out of stack space error.

Pete

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