Illegal counter variable?

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Illegal counter variable?

Zaaphod
Can someone please tell me why this happens?

Var
   I : Longint = 0;
Begin
For I := 1 to 6 Do
   Writeln(I);
End.

This gives me :
initialize.pas(4,5) Error: Illegal counter variable
initialize.pas(6,4) Fatal: There were 1 errors compiling module, stopping
initialize.pas(0) Fatal: Compilation aborted

but this works fine:
Var
   I : Longint;
Begin
I := 0;
For I := 1 to 6 Do
   Writeln(I);
End.

Shouldn't these two programs be identical?  What difference does it make if I initialize the variable in the Var section?

James
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Re: Illegal counter variable?

A. Fortuny
Hi.

Index variable can neither be initialized nor computed. Read
https://wiki.lazarus.freepascal.org/FOR..DO

Antonio

Le 09/09/2019 à 11:30, James Richters a écrit :

> Can someone please tell me why this happens?
>
> Var
>     I : Longint = 0;
> Begin
> For I := 1 to 6 Do
>     Writeln(I);
> End.
>
> This gives me :
> initialize.pas(4,5) Error: Illegal counter variable
> initialize.pas(6,4) Fatal: There were 1 errors compiling module, stopping
> initialize.pas(0) Fatal: Compilation aborted
>
> but this works fine:
> Var
>     I : Longint;
> Begin
> I := 0;
> For I := 1 to 6 Do
>     Writeln(I);
> End.
>
> Shouldn't these two programs be identical?  What difference does it make if I initialize the variable in the Var section?
>
> James
> _______________________________________________
> fpc-pascal maillist  -  [hidden email]
> https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-pascal

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Re: Illegal counter variable?

Free Pascal - General mailing list
On Mon, 9 Sep 2019 11:44:05 +0200
SPRL AFOR <[hidden email]> wrote:

> Hi.
>
> Index variable can neither be initialized nor computed. Read
> https://wiki.lazarus.freepascal.org/FOR..DO

That page only talks about assigning the loop var *inside* the loop,
which is forbidden.

James question is about the difference between an initialized global
var and a non initialized global var.

My guess is that it has to do with the different exe/linker sections
where these vars are put. Hopefully some compiler guru can explain.

Mattias

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Re: Illegal counter variable?

Zaaphod
I don't understand what difference it makes if I assign I :=0 with the variable declaration or If I was using I for something else..  It's irrelevant what the condition of I was before the for loop, just as it's irrelevant that the status of a variable is before you assign it to something.    I just don't think this should produce any kind of an error,  I'm not doing anything technically wrong as far as I can tell.
I should be able to use I for a whole pile of things then re-cycle it in my for loop.. then use it for more stuff after.. the condition of I before or after the for loop is irrelevant, as long as I make sure I initialize it before using it.. everything should be fine.   I should be able to do this if I want:

Var
   I : Longint = 57;
Begin
Inc(I);
Writeln(I);
I:=50;
Inc(I);
Writeln(I);
For I := 1 to 6 Do
  Writeln(I);
I:=-10;
Inc(I);
Writeln(I);
End.

It should not matter what I was using I for before or after the for loop or if it was initialized or used for some other purpose before or after the loop.

James
-----Original Message-----
From: fpc-pascal <[hidden email]> On Behalf Of Mattias Gaertner via fpc-pascal
Sent: Monday, September 9, 2019 5:59 AM
To: [hidden email]
Cc: Mattias Gaertner <[hidden email]>
Subject: Re: [fpc-pascal] Illegal counter variable?

On Mon, 9 Sep 2019 11:44:05 +0200
SPRL AFOR <[hidden email]> wrote:

> Hi.
>
> Index variable can neither be initialized nor computed. Read
> https://wiki.lazarus.freepascal.org/FOR..DO

That page only talks about assigning the loop var *inside* the loop, which is forbidden.

James question is about the difference between an initialized global var and a non initialized global var.

My guess is that it has to do with the different exe/linker sections where these vars are put. Hopefully some compiler guru can explain.

Mattias

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Re: Illegal counter variable?

Zaaphod
In reply to this post by Free Pascal - General mailing list
>That page only talks about assigning the loop var *inside* the loop, which is forbidden.

You can re-assign the loop variable if you are in {$Mode TP} it works perfectly fine to re-assign the for variable inside the loop, and I use that extensively, I have TP units specifically so I can do that.  I really don't understand why it's forbidden if you aren't in TP mode...  programmers can take care of their own variables and don't need to be protected from themselves.  I have good reasons to want to change the for loop variable and never have a problem with it.  I know there are ways around it, but I prefer to just modify the for variable... it can be very powerful.

This works great!

{$MODE TP}
Var
   I:Byte;
Begin
  I:=57;
  For I := I to 100 do
    Begin
       If I=87 then
          I:=95;
       Write(I,' ');
    End;
End.

Output:
57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 95 96 97 98 99 100

James
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Re: Illegal counter variable?

martin_wynne
On 09/09/2019 13:38, James Richters wrote:

> Var
>     I:Byte;
> Begin
>    I:=57;
>    For I := I to 100 do
>      Begin
>         If I=87 then
>            I:=95;
>         Write(I,' ');
>      End;
> End.

Why not:

Var
     I:Byte;
Begin
    I:=57;
    For I := I to 100 do
      Begin
         If (I>86) And (I<95) then Continue;
         Write(I,' ');
      End;
End.

which is much easier to follow the logic.

cheers,

Martin.
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Re: Illegal counter variable?

DougC
The last messages have wandered off topic somewhat.

Can we stick to the original request, which was why the handling of the two declarations differed?

Doug C.



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Re: Illegal counter variable?

Zaaphod
In reply to this post by martin_wynne
If (I>86) And (I<95) then Continue;
What does continue do exactly?  Loop back to the beginning of the for loop right away?   I didn't even know there was such a command.. it's not mentioned here:
https://wiki.lazarus.freepascal.org/FOR..DO

only break is mentioned.

As I said, there are ways around it, and that was just a stupid example that proves it works in {$Mode TP}  I have some really complicated nested for loops and changing the loop variable is really useful.. also it allows you to skip cycling around in the loop.  I just don't see why having the limitation, there is no technical reason that the for loop couldn't change that I can see.. especially since it works in TP mode.   Yes, I can work around it.. change to a while loop instead where or something.. where you can change variables all you want... I just don't see the point of enforcing this no-changing-the-for-loop-variable rule...

Pascal doesn't have things like step... but if I modify the for variable myself I can make it act like it had step

{$Mode TP}
Var
   I:Byte;
Begin
  For I := 0 to 100 do
    Begin
       Write(I,' ');
       Inc(I,3);
    End;
End.
0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 76 80 84 88 92 96 100

Or

{$Mode TP}
Var
   I:Byte;
Begin
  For I := 101 downto 0 do
    Begin
       Dec(I);
       Write(I,' ');
    End;
End.
100 98 96 94 92 90 88 86 84 82 80 78 76 74 72 70 68 66 64 62 60 58 56 54 52 50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0

Yes lots of ways to do these things... but could always do it with for loops with Pascal before.. and they work fine this way in FPC if you are in TP mode, so why not just allow it all the time?... just to have the flexibility to do what you want.    It would just be nice to get everything to work in all my units all the time.. but if I use TP mode, then I can't do things like use case statements with strings.

James

-----Original Message-----
From: fpc-pascal <[hidden email]> On Behalf Of Martin Wynne
Sent: Monday, September 9, 2019 8:53 AM
To: [hidden email]
Subject: Re: [fpc-pascal] Illegal counter variable?

On 09/09/2019 13:38, James Richters wrote:

> Var
>     I:Byte;
> Begin
>    I:=57;
>    For I := I to 100 do
>      Begin
>         If I=87 then
>            I:=95;
>         Write(I,' ');
>      End;
> End.

Why not:

Var
     I:Byte;
Begin
    I:=57;
    For I := I to 100 do
      Begin
         If (I>86) And (I<95) then Continue;
         Write(I,' ');
      End;
End.

which is much easier to follow the logic.

cheers,

Martin.
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Re: Illegal counter variable?

Bernd Oppolzer

Am 09.09.2019 um 16:11 schrieb James Richters:
> I just don't see why having the limitation, there is no technical reason that
> the for loop couldn't change that I can see.. especially since it works in TP mode.


The original reason why some Pascal implementations had this limitation:

for performance or optimization reasons, the loop control variable was
transferred to a register at the beginning of the loop, and changing the
variable (at its storage location) inside the loop simply had no effect,
because the variable was not fetched from there again during loop
execution.
Worse: maybe, to make read accesses to the loop control variable valid
inside the loop, they are prepared by storing the control register value
into the loop control variable, thus turning changes to the loop control
variable useless.

Forbidding (write) accesses to the loop control variable allows for many
aggressive optimization strategies around loops.

Maybe today such limitations seem too restrictive.

Kind regards

Bernd

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Re: Illegal counter variable?

Jonas Maebe-3
In reply to this post by Zaaphod
On 2019-09-09 11:30, James Richters wrote:

> Can someone please tell me why this happens?
>
> Var
>    I : Longint = 0;
> Begin
> For I := 1 to 6 Do
>    Writeln(I);
> End.
>
> This gives me :
> initialize.pas(4,5) Error: Illegal counter variable
> initialize.pas(6,4) Fatal: There were 1 errors compiling module,
> stopping
> initialize.pas(0) Fatal: Compilation aborted

"var i : longint = 0;" is internally handled using the same code path as
"const i : longint = 0", and typed constants cannot be used as counter
variables. This is indeed probably a bug.


Jonas
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Re: Illegal counter variable?

martin_wynne
In reply to this post by Zaaphod
On 09/09/2019 15:11, James Richters wrote:
> If (I>86) And (I<95) then Continue;
> What does continue do exactly?  Loop back to the beginning of the for loop right away?

Hi James,

Yes in effect -- it jumps forward to the test at the end of a loop. Very
useful.

See: https://www.freepascal.org/docs-html/rtl/system/continue.html

cheers,

Martin.
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Re: Illegal counter variable?

greim
AFAIK these is the standard behavior since the first PASCAL versions.
We must not change it. It prevents a lot of side effects, and PASCAL is NOT C without brackets!
Use while or repeat instead!

From Niklaus Wirths last 2004 Oberon manual:


t     "It is recommended that the for statement be used in simple cases only; in particular, no
components of the expressions determining the range must be affected by the repeated
statements, and, above all, the control variable itself must not be changed by the repeated
statements. The value of the control variable must be considered as undefined after the for
statement is terminated."

Kind Regards

Markus

On September 9, 2019, 5:20 PM GMT+2 [hidden email] wrote:

On 09/09/2019 15:11, James Richters wrote:

> If (I>86) And (I<95) then Continue;
> What does continue do exactly? Loop back to the beginning of the for loop right away?

Hi James,

Yes in effect -- it jumps forward to the test at the end of a loop. Very
useful.

See: https://www.freepascal.org/docs-html/rtl/system/continue.html

cheers,

Martin.
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Re: Illegal counter variable?

Zaaphod
In reply to this post by Bernd Oppolzer
Yes, today such limitations do seem too restrictive, I wonder if the reasons for the restrictions have become obsolete.  You would have to have a really slow computer with very limited resources to optimize loops to the point of reducing functionality like this, and the tendency with modern pc's is to have a larger library of much more powerful and more flexible tools, even if they are more complex and take more memory and resources.  For example Case statements used to only work with characters or integers,  but the modern version now also works with strings, very much added functionality for sure, but also would use more resources.... but we would all rather have the capability because even a raspberry pi is blazing fast and has ram to burn compared to our old 8086s  It seems silly to me that I can't so what I used to do with Turbo Pascal for DOS where I was limited to programs of a few hundred K, due to optimizations that just can't make much of a difference at all on modern computers.   I am happy that FPC has TP compatibility mode though!   It's just I get to a point where I eventually can't cross units since I can't have a circular unit reference.  So I have to choose for any given unit.. do I want my old for loops and change the control variables, or do I want cool new case statements... etc..  it would be nice to have {$Mode everything_the_way_I_want_it}

Since the compiler knows I was trying to change the variable inside the for loop, could it not just compile in not quite as efficient code (TP MODE for loop) when it detects this, and use the more efficient optimized code when it detects that it is able to use it?

James

-----Original Message-----
From: fpc-pascal <[hidden email]> On Behalf Of Bernd Oppolzer
Sent: Monday, September 9, 2019 10:46 AM
To: FPC-Pascal users discussions <[hidden email]>
Subject: Re: [fpc-pascal] Illegal counter variable?


Am 09.09.2019 um 16:11 schrieb James Richters:
> I just don't see why having the limitation, there is no technical
> reason that the for loop couldn't change that I can see.. especially since it works in TP mode.


The original reason why some Pascal implementations had this limitation:

for performance or optimization reasons, the loop control variable was transferred to a register at the beginning of the loop, and changing the variable (at its storage location) inside the loop simply had no effect, because the variable was not fetched from there again during loop execution.
Worse: maybe, to make read accesses to the loop control variable valid inside the loop, they are prepared by storing the control register value into the loop control variable, thus turning changes to the loop control variable useless.

Forbidding (write) accesses to the loop control variable allows for many aggressive optimization strategies around loops.

Maybe today such limitations seem too restrictive.

Kind regards

Bernd

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Re: Illegal counter variable?

Bart-48
In reply to this post by Jonas Maebe-3
On Mon, Sep 9, 2019 at 4:54 PM Jonas Maebe <[hidden email]> wrote:

> "var i : longint = 0;" is internally handled using the same code path as
> "const i : longint = 0", and typed constants cannot be used as counter
> variables. This is indeed probably a bug.

D7 does not allow this for global variables: "for loop variables must
be simple local variables".
In D7 you cannot initialize a local variable, so I cannot test that scenario.
Someone with a more recent Delphi should check if this sysnatx is allowed.

--
Bart
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Re: Illegal counter variable?

greim
In reply to this post by Zaaphod
James,

not every body is using a GHz machine. I am , for example, programming a 80186 in an embedded system with very limited speed an RAM.
But I understand thats not a general argument.

But look at MISRA C. Its a big set of rules for "real" save C programming, more or less now the standard in the automobile industry.
Also here are strict rules for counters in for loops.
See:

So what they are doing is writing Pascal programs in C... ;-) And they need huge and expensive and complex  tools to check the C code for
MISRA compatibility!
So we should NOT give up the safety of Pascal for the sloppiness of C!

Kind Regards

Markus

On September 9, 2019, 7:19 PM GMT+2 [hidden email] wrote:

Yes, today such limitations do seem too restrictive, I wonder if the reasons for the restrictions have become obsolete. You would have to have a really slow computer with very limited resources to optimize loops to the point of reducing functionality like this, and the tendency with modern pc's is to have a larger library of much more powerful and more flexible tools, even if they are more complex and take more memory and resources. For example Case statements used to only work with characters or integers, but the modern version now also works with strings, very much added functionality for sure, but also would use more resources.... but we would all rather have the capability because even a raspberry pi is blazing fast and has ram to burn compared to our old 8086s It seems silly to me that I can't so what I used to do with Turbo Pascal for DOS where I was limited to programs of a few hundred K, due to optimizations that just can't make much of a difference at all on modern computers. I am happy that FPC has TP compatibility mode though! It's just I get to a point where I eventually can't cross units since I can't have a circular unit reference. So I have to choose for any given unit.. do I want my old for loops and change the control variables, or do I want cool new case statements... etc.. it would be nice to have {$Mode everything_the_way_I_want_it}

Since the compiler knows I was trying to change the variable inside the for loop, could it not just compile in not quite as efficient code (TP MODE for loop) when it detects this, and use the more efficient optimized code when it detects that it is able to use it?

James

-----Original Message-----
From: fpc-pascal <[hidden email]> On Behalf Of Bernd Oppolzer
Sent: Monday, September 9, 2019 10:46 AM
To: FPC-Pascal users discussions <[hidden email]>
Subject: Re: [fpc-pascal] Illegal counter variable?

Am 09.09.2019 um 16:11 schrieb James Richters:

> I just don't see why having the limitation, there is no technical
> reason that the for loop couldn't change that I can see.. especially since it works in TP mode.

The original reason why some Pascal implementations had this limitation:

for performance or optimization reasons, the loop control variable was transferred to a register at the beginning of the loop, and changing the variable (at its storage location) inside the loop simply had no effect, because the variable was not fetched from there again during loop execution.
Worse: maybe, to make read accesses to the loop control variable valid inside the loop, they are prepared by storing the control register value into the loop control variable, thus turning changes to the loop control variable useless.

Forbidding (write) accesses to the loop control variable allows for many aggressive optimization strategies around loops.

Maybe today such limitations seem too restrictive.

Kind regards

Bernd

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Re: Illegal counter variable?

Free Pascal - General mailing list
In reply to this post by Bart-48
On Mon, 9 Sep 2019 19:27:49 +0200
Bart <[hidden email]> wrote:

> On Mon, Sep 9, 2019 at 4:54 PM Jonas Maebe <[hidden email]>
> wrote:
>
> > "var i : longint = 0;" is internally handled using the same code
> > path as "const i : longint = 0", and typed constants cannot be used
> > as counter variables. This is indeed probably a bug.  
>
> D7 does not allow this for global variables: "for loop variables must
> be simple local variables".
> In D7 you cannot initialize a local variable, so I cannot test that
> scenario. Someone with a more recent Delphi should check if this
> sysnatx is allowed.

Same in 10.3.

Mattias
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Re: Illegal counter variable?

Zaaphod
Maybe there's no technical reason it doesn't work... it's just set to not work to make it compatible with other pascal programs?

-----Original Message-----
From: fpc-pascal <[hidden email]> On Behalf Of Mattias Gaertner via fpc-pascal
Sent: Monday, September 9, 2019 1:32 PM
To: [hidden email]
Cc: Mattias Gaertner <[hidden email]>
Subject: Re: [fpc-pascal] Illegal counter variable?

On Mon, 9 Sep 2019 19:27:49 +0200
Bart <[hidden email]> wrote:

> On Mon, Sep 9, 2019 at 4:54 PM Jonas Maebe <[hidden email]>
> wrote:
>
> > "var i : longint = 0;" is internally handled using the same code
> > path as "const i : longint = 0", and typed constants cannot be used
> > as counter variables. This is indeed probably a bug.
>
> D7 does not allow this for global variables: "for loop variables must
> be simple local variables".
> In D7 you cannot initialize a local variable, so I cannot test that
> scenario. Someone with a more recent Delphi should check if this
> sysnatx is allowed.

Same in 10.3.

Mattias
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Re: Illegal counter variable?

Zaaphod
In reply to this post by greim

I’m trying to use it in a way that always worked in Turbo Pascal for DOS, I have no idea how to program in C,  I find Pascal much more clear and easy to follow and C just confuses me, so I never spent much time other than a few stray projects here and there on C…  I’ve always used pascal since the 80s, and in fact my units that change the for loop variable that run in {$Mode TP} are the original code from turbo pascal written in the 80s.. over the decades I’ve modified stuff all around it, but the actual code with for loops with changing control variables is the exact same code I was running on old 4.7MHz Dos machines with turbo pascal for DOS…  so for nearly 40 years I have never even known there was some rule against changing the control variable in for loops,  it’s always worked and I’ve always used it.. and it still works if I use {$Mode TP}  it may not work if I try to use Delphi or something else, and there may be some pascal version even older than turbo pascal for DOS that didn’t act this way, but from the entire scope of my programming experience, this is something that has just always worked and I never had a reason not to use it.

 

James

 

From: fpc-pascal <[hidden email]> On Behalf Of Markus Greim
Sent: Monday, September 9, 2019 1:32 PM
To: FPC-Pascal users discussions <[hidden email]>
Subject: Re: [fpc-pascal] Illegal counter variable?

 

James,

 

not every body is using a GHz machine. I am , for example, programming a 80186 in an embedded system with very limited speed an RAM.

But I understand thats not a general argument.

 

But look at MISRA C. Its a big set of rules for "real" save C programming, more or less now the standard in the automobile industry.

Also here are strict rules for counters in for loops.

See:

 

So what they are doing is writing Pascal programs in C... ;-) And they need huge and expensive and complex  tools to check the C code for

MISRA compatibility!

So we should NOT give up the safety of Pascal for the sloppiness of C!

 

Kind Regards

 

Markus



On September 9, 2019, 7:19 PM GMT+2 [hidden email] wrote:

Yes, today such limitations do seem too restrictive, I wonder if the reasons for the restrictions have become obsolete. You would have to have a really slow computer with very limited resources to optimize loops to the point of reducing functionality like this, and the tendency with modern pc's is to have a larger library of much more powerful and more flexible tools, even if they are more complex and take more memory and resources. For example Case statements used to only work with characters or integers, but the modern version now also works with strings, very much added functionality for sure, but also would use more resources.... but we would all rather have the capability because even a raspberry pi is blazing fast and has ram to burn compared to our old 8086s It seems silly to me that I can't so what I used to do with Turbo Pascal for DOS where I was limited to programs of a few hundred K, due to optimizations that just can't make much of a difference at all on modern computers. I am happy that FPC has TP compatibility mode though! It's just I get to a point where I eventually can't cross units since I can't have a circular unit reference. So I have to choose for any given unit.. do I want my old for loops and change the control variables, or do I want cool new case statements... etc.. it would be nice to have {$Mode everything_the_way_I_want_it}

Since the compiler knows I was trying to change the variable inside the for loop, could it not just compile in not quite as efficient code (TP MODE for loop) when it detects this, and use the more efficient optimized code when it detects that it is able to use it?

James

-----Original Message-----
From: fpc-pascal <[hidden email]> On Behalf Of Bernd Oppolzer
Sent: Monday, September 9, 2019 10:46 AM
To: FPC-Pascal users discussions <[hidden email]>
Subject: Re: [fpc-pascal] Illegal counter variable?

Am 09.09.2019 um 16:11 schrieb James Richters:


> I just don't see why having the limitation, there is no technical
> reason that the for loop couldn't change that I can see.. especially since it works in TP mode.

The original reason why some Pascal implementations had this limitation:

for performance or optimization reasons, the loop control variable was transferred to a register at the beginning of the loop, and changing the variable (at its storage location) inside the loop simply had no effect, because the variable was not fetched from there again during loop execution.
Worse: maybe, to make read accesses to the loop control variable valid inside the loop, they are prepared by storing the control register value into the loop control variable, thus turning changes to the loop control variable useless.

Forbidding (write) accesses to the loop control variable allows for many aggressive optimization strategies around loops.

Maybe today such limitations seem too restrictive.

Kind regards

Bernd

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Re: Illegal counter variable?

wkitty42
In reply to this post by Zaaphod
On 9/9/19 10:11 AM, James Richters wrote:
> Pascal doesn't have things like step...

hunh??? i don't think that's right but i'm just catching up after several 10+
hours days of $job...

i know that i've written code in the past that did use something to step X
numbers per run through the look and it did not involved manually incrementing
the loop var...


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Re: Illegal counter variable?

Ralf Quint
On 9/10/2019 4:26 PM, [hidden email] wrote:

> On 9/9/19 10:11 AM, James Richters wrote:
>> Pascal doesn't have things like step...
>
> hunh??? i don't think that's right but i'm just catching up after
> several 10+ hours days of $job...
>
> i know that i've written code in the past that did use something to
> step X numbers per run through the look and it did not involved
> manually incrementing the loop var...
>
>
I am not aware of any Pascal implementation that does have a STEP
parameter for FOR loops, and I am programming in Pascal at least as long
as you... ;-)

The closest would be the FOR..IN variant, which works in Delphi (D2006?
and later) and is AFAIK supported in FPC by now. But that would be
iterating over a set, not increment an integer/cardinal variable in an
increment other than 1...

Ralf

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