Possible bug with range checking in StrUtils.IfThen function

[BSaidus]

Hello.
Testing the following code in attached project raise an exception as shown in the screenshot.
The code

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Button1: TButton;
16.     cbParentIfs: TComboBox;
17.     Edit1: TEdit;
18.     procedure Button1Click(Sender: TObject);
19.   private
20.  
21.   public
22.  
23.   end;
24.  
25. var
26.   Form1: TForm1;
27.  
28. implementation
29.  
30. uses
31.   StrUtils
32.   ;
33.  
34. CONST
35.  CEth: array[0..4] of string = ('01','02','03','04','05');
36.  
37. {$R *.lfm}
38.  
39. { TForm1 }
40.  
41. procedure TForm1.Button1Click(Sender: TObject);
42. var
43.   lIndex: Integer;
44. begin
45.   lIndex := cbParentIfs.ItemIndex;
46.   Edit1.Text := IfThen( lIndex>=1, CEth[lIndex-1], 'tralllla');
47. end;
48.  
49. end.
50.  
51.  

PS: fpc 3.2.3 fixes, lazarus 2.2.7 fixes

[Bart]

IIRC then all parameters of a function call will be evaluated.
So the expression CEth[lIndex-1] will be evaluated wether or not lIndex >= 1.

The fact that the function is called IfThen does not imply that boolean shortcutting is applied to it's parameter evaluation...

Bart

[BSaidus]

IIRC then all parameters of a function call will be evaluated.
So the expression CEth[lIndex-1] will be evaluated wether or not lIndex >= 1.

The fact that the function is called IfThen does not imply that boolean shortcutting is applied to it's parameter evaluation...

Bart

Thanks for your answer.
Looking inside the code of IfThen in StrUtils, it do not looks (evaluate) the ATrue value if the condition is not sitisfied.

Code: Pascal  [Select][+][-]

1. function IfThen(AValue: Boolean; const ATrue: string; const AFalse: string): string;
2.  
3. begin
4.   if avalue then
5.     result:=atrue
6.   else
7.     result:=afalse;
8. end;  
9.  

[Fibonacci]

Your code didnt work for me, I had to use specialize:

Code: Pascal  [Select][+][-]

1. Edit1.Text := specialize IfThen<String>(lIndex>=1, CEth[lIndex-1], 'tralllla');

FPC trunk, no problems, no range errors

[BSaidus]

Your code didnt work for me, I had to use specialize:

Code: Pascal  [Select][+][-]

1. Edit1.Text := specialize IfThen<String>(lIndex>=1, CEth[lIndex-1], 'tralllla');

FPC trunk, no problems, no range errors

Did you try to compile it in full debug mode ??

[Fibonacci]

Yes

Also managed to run your code this way

Code: Pascal  [Select][+][-]

1. Edit1.Text := StrUtils.IfThen( lIndex>=1, CEth[lIndex-1], 'tralllla')

No errors, no crashes, TRUNK

[BSaidus]

Strange, same problem with : Lazarus 3.0RC1 (rev fe49fef4) FPC 3.3.1 i386-win32-win32/win64

[Fibonacci]

I compiled your project (.zip file) and well, there is range check error.

Ok, so both strings are known before the boolean is checked. Proof:

Code: Pascal  [Select][+][-]

1. function nono: string;
2. begin
3.   showmessage('hello');
4.   result := 'false';
5. end;
6.  
7. procedure TForm1.Button1Click(Sender: TObject);
8. begin
9.   Edit1.Text := IfThen(false, nono, 'hi');
10. end;

I guess since this is an array, or in case above a function, it cant be passed by ref.

Somehow when I created my form and just copy-pasted ur code there wasnt this error.

[Чебурашка]

Hello.
Testing the following code in attached project raise an exception as shown in the screenshot.
The code

Code: Pascal  [Select][+][-]

1. unit Unit1;
2.  
3. {$mode objfpc}{$H+}
4.  
5. interface
6.  
7. uses
8.   Classes, SysUtils, Forms, Controls, Graphics, Dialogs, StdCtrls;
9.  
10. type
11.  
12.   { TForm1 }
13.  
14.   TForm1 = class(TForm)
15.     Button1: TButton;
16.     cbParentIfs: TComboBox;
17.     Edit1: TEdit;
18.     procedure Button1Click(Sender: TObject);
19.   private
20.  
21.   public
22.  
23.   end;
24.  
25. var
26.   Form1: TForm1;
27.  
28. implementation
29.  
30. uses
31.   StrUtils
32.   ;
33.  
34. CONST
35.  CEth: array[0..4] of string = ('01','02','03','04','05');
36.  
37. {$R *.lfm}
38.  
39. { TForm1 }
40.  
41. procedure TForm1.Button1Click(Sender: TObject);
42. var
43.   lIndex: Integer;
44. begin
45.   lIndex := cbParentIfs.ItemIndex;
46.   Edit1.Text := IfThen( lIndex>=1, CEth[lIndex-1], 'tralllla');
47. end;
48.  
49. end.
50.  
51.  

PS: fpc 3.2.3 fixes, lazarus 2.2.7 fixes

The error is not inside the IfThen, but is before it is called, during the preparation of the stack for the function call.

https://forum.lazarus.freepascal.org/index.php/topic,59197.0.html

[Bart]

Looking inside the code of IfThen in StrUtils, it do not looks (evaluate) the ATrue value if the condition is not sitisfied.

That has nothing to do with it.
The parameters are evaluated at the time you call the function.
Say you have a function like this:

Code: Pascal  [Select][+][-]

1. function foo(x,y: integer): integer;

Then you call it like

Code: Pascal  [Select][+][-]

1. z := foo(1 div 0, 100);

Now, you would not be surprised that you get an exception (div by zero).
Suppose you also have a function like this:

Code: Pascal  [Select][+][-]

1. function bar(a,b: integer): integer;
2. begin
3.   result := a div b;
4. end;

And then you do this:

Code: Pascal  [Select][+][-]

1.   z := foo(1, bar(1,0));

This will of course also raise an exception

Now this

Code: Pascal  [Select][+][-]

1. function foo(b: boolean; x,y: integer): integer;
2. ...
3.   z:= foo(False, 1, bar(1,0));

This will crash regardless of the implementation of foo (so even if the imeplementation of foo is completely empty).

Bart

[Thaddy]

Your code didnt work for me, I had to use specialize:

Code: Pascal  [Select][+][-]

1. Edit1.Text := specialize IfThen<String>(lIndex>=1, CEth[lIndex-1], 'tralllla');

FPC trunk, no problems, no range errors

That is because the generic ifthen is in sysutils not in strutils. I wrote the generic version.

[BSaidus]

Thank you for all the answers,
I'll use the
   IF ... THEN ... ELSE
Better for me.

[Чебурашка]

Thank you for all the answers,
I'll use the
   IF ... THEN ... ELSE
Better for me.

The difference is that if then else end works differently: first you evaluate the condition and then you jump to the true or the false branch, and execute only the one you jump to.

The ifthen instead evaluates all the 3 values supplied to it, then you enter into the function and return the true or the false one depending on the condition.

Except for the name which is similar, the two processes have nothing in common.

[Paolo]

So, the question mow is : "Ifthen" is useful for what ?

[Bart]

So, the question mow is : "Ifthen" is useful for what ?

So you can do something like

Code: Pascal  [Select][+][-]

1. x := ifthen(a>b,c,d)

which requires less typing then the conventional

Code: Pascal  [Select][+][-]

1.   if a>b then
2.     x:=c
3.   else
4.     x:=d;

IMO it's for NARGS.
Make it less readable, so you, the programmer, look more like a wizard.

Bart