Dynamic allocation with array of record

[Sylvain Fortin]

Hi everybody,

A simple question for you. I want to create a dynamic array of record
with Delphi 2 and I can't find out how to do it. In C I just have to use
calloc and the size of the array and use it exactly as a regular array.

I need a way to do the same thing with Delphi 2 and I don't want to use
TList or something like that. I tried with GetMem but it was impossible
to access each element of the array. An example follow.

Type
test = record
x,y :Integer;
end;
...

Var
list :^test;
size :Integer;
begin
size := 10;
GetMem(list, size);
list[1]^.x := 1; // not working
list[1]^.y := 2; // not working
end;

All the help will be appreciated.
Thank You

Sylvain Fortin

[Wayne Niddery]

Sylvain Fortin wrote in message <34F9D47E...@videotron.ca>...

>A simple question for you. I want to create a dynamic array of record
>with Delphi 2 and I can't find out how to do it. In C I just have to use
>calloc and the size of the array and use it exactly as a regular array.

Whereas in C/C++, you can define a pointer to a type (e.g. ^test) and then
have that pointer point to an *array* of that type, in Pascal, you need to
define an *array type*.

>
>Type
> test = record
> x,y :Integer;
> end;

TestArray = array [0..9] of Test;
PTestArray = ^TestArray;

var
List: PTestArray;
begin
GetMem(list, sizeof(TestArray));
list^[0].x := 1;
list^[0].y := 2;
list^[1].x := 1;
list^[1].y := 2;

* Wayne Niddery - WinWright Consulting
* Host of RADBooks at http://home.ican.net/~wniddery/RADBooks.html
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* ...remove X when replying...

[Wayne Niddery]

-----Message forwarded from private email-----
From: Sylvain Fortin
Date: Sunday, March 01, 1998 7:39 PM
Subject: Re: Dynamic allocation with array of record

>Hi Wayne,
>
> In your example the memory is allocated dynamically but the array isn't
>dynamic because it can only have a constant value (array [0..9]). I want
>to be able to create an array of 1,15,20,111, ... Everything depend on
>the size of my database.
>
>Is it possible with Delphi?
>
>Thank You very much
>
>Sylvain Fortin

[Wayne Niddery]

>-----Message forwarded from private email-----
>From: Sylvain Fortin
>Date: Sunday, March 01, 1998 7:39 PM
>Subject: Re: Dynamic allocation with array of record
>
>Hi Wayne,
>
> In your example the memory is allocated dynamically but the array isn't
>dynamic because it can only have a constant value (array [0..9]). I want
>to be able to create an array of 1,15,20,111, ... Everything depend on
>the size of my database.
>
>Is it possible with Delphi?

Yes. At first, the method may seem a bit awkward for someone coming from
C/C++ (at least it was to me), but it's not really. Same example as before
with appropriate changes:

Type
test = record
x,y :Integer;
end;

const
// max records that can fit in 2 gig
MaxTest = MaxInt div sizeof(Test) - 1;

type
// array can hold MaxTest records
TestArray = array [0..MaxTest] of Test;
PTestArray = ^TestArray;

var
List: PTestArray;
NumTest: integer;
begin
// allocate memory just for what's needed
NumTest := Random(MaxTest) + 1;
GetMem(list, sizeof(Test) * NumTest);

list^[0].x := 1;
list^[0].y := 2;
list^[1].x := 1;
list^[1].y := 2;

* Wayne Niddery - WinWright Consulting

[jo...@enteract.com]

In article <34F9D47E...@videotron.ca>, Sylvain Fortin
<sylv...@videotron.ca> wrote: > > Hi everybody, > > A simple question for

you. I want to create a dynamic array of record > with Delphi 2 and I can't
find out how to do it. In C I just have to use > calloc and the size of the

array and use it exactly as a regular array. > > I need a way to do the same

thing with Delphi 2 and I don't want to use > TList or something like that. I
tried with GetMem but it was impossible > to access each element of the

array. An example follow. > > Type > test = record > x,y :Integer; > end;

> ... > > Var > list :^test; > size :Integer; > begin > size := 10; >
GetMem(list, size); > list[1]^.x := 1; // not working > list[1]^.y := 2; //
not working > end; > > All the help will be appreciated. > Thank You > >

Sylvain Fortin > Hi; In response to your inquiry pertaining to dynamic
arrays, the best vehicle Ive found is to use a Tlist. Here is an example of
how to build a TList (it expands dynamically automatically) and add objects
to the list. Instead of creating a record, you create a class. Hope this
helps Joe T unit Unit1; interface uses Windows, Messages, SysUtils,
Classes, Graphics, Controls, Forms, Dialogs, StdCtrls; type TForm1 =
class(TForm) Button1: TButton; Edit1: TEdit; Button2: TButton; Edit2:
TEdit; Edit3: TEdit; procedure Button1Click(Sender: TObject); procedure
FormClose(Sender: TObject; var Action: TCloseAction); private { Private
declarations } public { Public declarations } end; tIssues = class
count : integer; Desc : string; bAVail : boolean; // This means THIS
"Issue" is available for demo end; var Form1: TForm1; issueList:
Tlist; sDesc: string; scheck: tIssues;
issue: tIssues; implementation {$R *.DFM}
procedure TForm1.Button1Click(Sender: TObject); begin issue :=
tIssues.create; sCheck := tIssues.create; issue.count := 1; issue.Desc
:= 'Joes Desc'; issue.bAVail := TRUE; issueList := Tlist.Create;
issueList.Clear; issueList.Add(issue); issue :=
tIssues.create; issue.count := 1; issue.Desc := 'Joes Desc2';
issue.bAVail := TRUE; issueList.Add(issue); sCheck :=
issueList.Items[0]; Edit1.text := sCheck.Desc; sCheck :=
issueList.Items[1]; Edit2.text := sCheck.Desc; edit3.text :=
intTostr(issuelist.count); end; end.

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[Robert Lee]

Wayne's example is perfectly adequate, but I thought I'd chime in with a couple
of options to his example

If you always use a variable as in index (as opposed to the constants in
Wayne's example) you can define the array as:

type
TestArray=array[0..0] of Test;

I like this form better because it is more apparent that the array has no
actual size.

Secondly, you can address the items as

list[i].x

without the ^. The compiler will still perform the dereferencing and code is
more readable in my opinion.

Finally, as a C/C++ convertee, you may be interested to know that inc and dec
work on pointer types in a familiar way:

inc(TestPtr,2); // equivalent to TestPtr:=pointer(integer(TestPtr) +
2*sizeof(test));

This makes walking through an array with a pointer much easier, from a
readability standpoint.

Bob Lee

[Wayne Niddery]

Robert Lee wrote in message <35004CC9...@nwu.edu>...

>Wayne's example is perfectly adequate, but I thought I'd chime in with a
couple
>of options to his example
>
>If you always use a variable as in index (as opposed to the constants in
>Wayne's example) you can define the array as:
>
>type
> TestArray=array[0..0] of Test;

A caveat here is you need to turn off range-checking for this to work,
that's why I prefer the other way (a max value).