Mega Code Archive
Dynamicarrays
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In Delphi 4 and higher, you can use a dynamic array:
var
MyArray: array of Integer;
Note the lack of bounds. Use the SetLength procedure as you would on an
AnsiString:
SetLength(MyArray, 20);
MyArray now behaves the same as an array[0..19] of Integer. The lower
bound of a dynamic array is always 0.
If you do not have Delphi4, then you can use pointers instead. Declare
some types like this:
type
PInteger = ^Integer; // Windows.pas may aready define this one.
PIntegerArray = ^TIntegerArray;
TIntegerArray = array[0..0] of Integer;
But this type only has 1 element! Not to worry, the compiler will never
notice (except in one circumstance, which I'll mention later). Declare
your array variable like this:
var
MyArray: PIntegerArray;
Before you can use it, you must allocate some memory:
GetMem(MyArray, 20 * SizeOf(Integer));
Ths allocates 80 bytes of memory, room for 20 Integers. You can access
this like you would a normal array:
for i := 0 to 19 do writeln(MyArray[i]);
You may have to dereference the pointer (MyArray^[i])--I use D4, and
dereferencing is optional there. You'll have to keep track of the array
size yourself since Length won't work. Don't forget to call FreeMem when
you're finished with your array.
With the above type declaration, you won't be able to access the second
element like this: MyArray[1]; the compiler will balk since the array
index is out of bounds. There are two ways to fix that. One is to assign
the index to a variable and use that, as shown in the for loop above. The
ther solution is to change the type declaration slightly:
TIntegerArray = array[0..1] of Integer;
The upper bound doesn't really matter since you never declare any variable
of type TIntegerArray, only PIntegerArray. Some people will declare an
upper bound of 32767, but I think that's overkill. Just set the bound as
high as you need since you only need it to make the compiler happy when
you access elements at constant offsets.
Wondering about that PInteger declaration? I almost forgot about it too.
There is a fast way of walking through an array that doesn't require an
offset counter all the time.
var
MyElement: PInteger;
begin
MyElement := MyArray; // This works since they're both pointers
n := 20;
while n > 0 do begin
writeln(MyElement^);
Inc(MyElement);
Dec(n);
end;
end;
This example doesn't exactly show off the advantages since it still
requires the n counter to know when to stop, but it shows the key point.
You can use the Inc and Dec operations on pointers and instead of just
adding 1, the compiler will increase the pointer address by the size of
what it points to. This is probably used most often in string (PChar)
manipulation since you know you've reached the end of the array when
MyElement^ = #0, meaning you never need to know the actual length of the
array.
***************************************************************************************************
Creating "Dynamic" Arrays?
Is it possible to re-size arrays at runtime?
I came across an example of how to do one. The author is Ben Licht, who presents an
age-old Pascal method of creating a dynamic array.
The trick is using a pointer to an array with a size of 1, then allocating memory for the
pointer by multiplying the number of items you want in the array by the size of the array
type.
Here's a sample unit I've adapted from his example:
{This unit demonstrates how to implement a dynArray}
unit U;
interface
uses
SysUtils, WinTypes, WinProcs, Classes, Controls, Forms, Dialogs, StdCtrls;
type
TResizeArr = array[0..0] of string;
PResizeArr = ^TResizeArr;
TForm1 = class(TForm)
Button1: TButton;
procedure Button1Click(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
procedure DefineDynArray(var h : THandle; {Handle to mem pointer}
NumElements : LongInt; {Number of items in array}
var PArr : PResizeArr); {Pointer to array struct}
procedure TestDynArray;
implementation
{$R *.DFM}
{============================================================================
Procedure that defines the dynarray. Note that the THandle and Pointer to
the array are passed by reference. This enables them to be defined outside
the scope of this procedure.
============================================================================}
procedure DefineDynArray(var h : THandle; {Handle to mem pointer}
NumElements : LongInt; {Number of items in array}
var PArr : PResizeArr); {Pointer to array struct}
begin
{Allocate Windows Global Heap memory}
h := GlobalAlloc(GMEM_FIXED, NumElements * sizeof(TResizeArr));
PArr := GlobalLock(h);
end;
{============================================================================
Procedure that uses the DefineDynArray proc. This is pretty useless, but
provides a good example of how you can access the elements of the 'array'
once the array is defined.
============================================================================}
procedure TestDynArray;
var
MyArray : PResizeArr;
I : Integer;
str : String;
h : THandle;
begin
str := '';
DefineDynArray(h, 10, MyArray); {Define the 'array'}
for I := 0 to 9 do
MyArray^[I] := IntToStr(I);
for I := 0 to 9 do
str := str + MyArray^[I] + ',';
ShowMessage(str);
GlobalUnlock(h); {Must make a call to unlock the memory, then}
GlobalFree(h); {free the memory and invalidate the handle}
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
TestDynArray;
end;
end.
This is a perfect example of one of those programming things that take hours to figure
out, but turn out to be amazingly simple. But I should point out that it might just be
simpler to use a TList, which does all of the above, but has methods to insert and delete
items. It's only limited by the amount of memory you have.
Copyright © 1997 Brendan V. Delumpa All Rights Reserved
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