[SOLVED] Array of Tpoint as proc parameter corrupted data
lazarus / 39-Graphics

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whips · 2014-01-13 13:05

Hello
I am fighting third night with this code without success.
I need pass vector data for drawing procedure drawtest
When is vector  element rendered on Canvas is totaly corrupted. mostly
values array[3..7] index are wrong.

Points are random values. What I am doing wrong?

My array is small can be alocated on stack.
I dont want use dynamic heap and free it
( i wish keep this on compiller)

How to correctly pass small array of Tpoints or record (x,y:word) if
will be longint Tpoint problem?
Code: [Select]type

  { TForm1 }

  TForm1 = class(TForm)
    button1: TButton;
    procedure FormCreate(Sender: TObject);
    procedure Button1Click(Sender: TObject);
  private
         procedure drawtest( var el : array of tpoint);
    { private declarations }
  public
    { public declarations }
  end;

Button1Click
Code: [Select]procedure Button1Click(Sender: TObject);
var
  horny  : array[1..7] of Tpoint;
    c : byte;
begin
   ...
   ...
   ...
   horny[1] := Point(  54,26  );
   horny[2] := Point(  253,26 );
   horny[3] := Point(  260,33 );
   horny[4] := Point(  261,35 ); // from here point is bad in proc.
   horny[5] := Point(  220,72 );
   horny[6] := Point(  81,72  );
   drawtest(horny);
   ....
   ....
   { other vector should look like this
   secondvector[1] := Point(  540,6  );
   secondvector[2] := Point(  20,6 );
   drawtest(secondvector);}
Drawtest
Code: [Select] procedure TForm1.drawtest(  var el : array of tpoint);
 var c: byte;
      el2 : array[1..7] of Tpoint; /reserver for move/copy
 begin

   canvas.Line(el[1].x,el[1].y, el[2].x,el[2].y);
   canvas.Line(el[2].x,el[2].y, el[3].x,el[3].y);
  canvas.Line(el[3].x,el[3].y, el[4].x,el[4].y);
   canvas.Line(el[4].x,el[4].y, el[5].x,el[5].y);
   canvas.Line(el[5].x,el[5].y, el[6].x,el[6].y);
   canvas.Line(el[6].x,el[6].y, el[7].x,el[7].y);
   canvas.Line(el[7].x,el[7].y, el[1].x,el[1].y); 
 end;

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howardpc · 2014-01-13 14:09

You declare drawtest as a procedure taking a parameter of type

Code: [Select]array of TPoint

but when you call it you pass a different (static) array

Code: [Select] horny  : array[1..7] of Tpoint;

Because you have unwisely (and uneccesarily) also declared the
parameter as var, the compiler does not warn you about the mismatched
types.
You cannot mix dynamic and static arrays in this fashion. Decide which
type to use and stick to it. If you go for a dynamic array you also
have to initialise its length before use, such as:

Code: [Select] SetLength(dynamicArray, 7);

If you declared FHorny as a private field of the desired type in the
TForm1 class, your DrawTest procedure would not need any parameters at
all, and could work directly with its private data, which you set in
the button click event.

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whips · 2014-01-14 04:16

Thank you for your reply,

If I want to use static array without using encapsuling data into
class.

How I should right modify procedure declaration ?
I have about 20 variables with statis data (line vectors)I wish to
know how to right pass static array to function/procedure without OOP.

Thank you.

PS: horny means in my lang upper  or top (its not english horny)

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taazz · 2014-01-14 05:46

Change var to const and you should be good to go. egCode:
[Select]procedure drawtest(Const el : array of tpoint);. Keep in mind
that el will always be zero(0) based regardless of how your static
array is declared.

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whips · 2014-01-14 06:28

I am not sure if I understand right but I changed it
from var to const.
Still same problem wrong data passed to procedure
Code: [Select] procedure TForm1.drawtest(  const el : array of
tpoint);
 begin
   canvas.Line(el[1].x,el[1].y, el[2].x,el[2].y);
   canvas.Line(el[2].x,el[2].y, el[3].x,el[3].y);
   canvas.Line(el[3].x,el[3].y, el[4].x,el[4].y);
   canvas.Line(el[4].x,el[4].y, el[5].x,el[5].y);
   canvas.Line(el[5].x,el[5].y, el[6].x,el[6].y);
   canvas.Line(el[6].x,el[6].y, el[7].x,el[7].y);
   canvas.Line(el[7].x,el[7].y, el[1].x,el[1].y);
 end;

I am calling it this way

Code: [Select]procedure TForm1.DrawClick(Sender: TObject);
var
  upper : array[1..7] of Tpoint;

begin
   upper[1] := Point(  54, 26 );
   upper[2] := Point(  253,26 );
   upper[3] := Point(  260,33 );
   upper[4] := Point(  261,35 );
   upper[5] := Point(  220,72 );
   upper[6] := Point(  81, 72 );   
   drawtest(upper);
   ......
   ......
end;

is producing this wrong result

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User137 · 2014-01-14 07:50

Like taazz said, the parameter array el is zero based. If you pass
array 1..7 to it, it will become 0..6.

Easiest way to fix that with your current code is change

Code: [Select]  upper : array[1..7] of Tpoint;to
Code: [Select]  upper : array[0..7] of Tpoint;You'll be allocating 1
unnecessary array index (0), but you don't have to modify anything
else then.

Or proper solution would be to use indices 0..6 in drawtest().

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Leledumbo · 2014-01-14 08:05

Make a good use of Low() and High(), they are there to help you forget
about literal array index.

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whips · 2014-01-14 08:12

Oh,,, I understand now
I cannot fix that this way zero index contain
special point calculated from other I am using it for
coordinates when rotate whole vertor object.

Ok now is working i rewrited drawtest
and object is rendered fine now.

Thank you.