Step by step

Folks,

Some times when we need to hold stripped informations, like socket or audio buffers, we need to create a chained list that control all the information sequentially and provide a navigation on the holded data only by stepping forward(next) or stepping back(prior).

A structure of a Chained List is divided on 2 main variables: Data and Next(or Prior, as needed), for every Item on the Chain you have the Data(Pointer, value or someting you have to hold) and a reference for the next or prior item on the list. Chained list don’t provide a direct access for items, so you can’t access a item without navigate on others. Chained Lists can be improved, but the main goal is provide a near infinite list that have the best performance of all others techniques.

As you know now, Chained lists don’t hold indexs for items (don’t have a direct access) and that avoid re-index procedures that consume a big time when you’ve a large list. When you need to remove a item in a Chained list that have a predecessor and a antecessor, you’ll have to make the first improvement on the chain structure to hold the Prior and the Next item for each Item you have, and thats provide you a confortable way to manipulate items without writing a lot of code and variables.

A basic structure of a Chained List on Delphi

 Delphi | 
 copy code |
?
 
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uses ChainedListCtrl;
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type
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  TfrmMain = class(TForm)
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    edtWord: TEdit;
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    btnAdd: TButton;
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    lblPhrase: TLabel;
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    btnClean: TButton;
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    procedure FormCreate(Sender: TObject);
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    procedure FormDestroy(Sender: TObject);
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    procedure btnAddClick(Sender: TObject);
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    procedure btnCleanClick(Sender: TObject);
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  private
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    { Private declarations }
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  public
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    Phrase: TChainedList;
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    procedure UpdateList;
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  end;
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var
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  frmMain: TfrmMain;
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implementation
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{$R *.dfm}
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{ Note: Create the Chain List Control Object
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}
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procedure TfrmMain.FormCreate(Sender: TObject);
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begin
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  Phrase := TChainedList.Create;
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end;
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{ Note: Destroy the Chain Object
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}
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procedure TfrmMain.FormDestroy(Sender: TObject);
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begin
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  Phrase.Free;
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end;
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{ Note: Update the Label to hold all the words
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        included on the Chained list
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}
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procedure TfrmMain.UpdateList;
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var
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  sPhrase: string;
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begin
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  sPhrase := '';
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  Phrase.First;
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  while Phrase.Current <> nil do
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  begin
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    if sPhrase <> '' then
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      sPhrase := sPhrase + ' ';
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    sPhrase := sPhrase + PChar(Phrase.Current^);
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    Phrase.Next;
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  end;
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  lblPhrase.Caption := sPhrase;
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end;
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{ Note: Add the text on the Chain
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   p.s: StrNew create a copy of the string that will be managed
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        out of the garbage collector(str refcount structure)
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}
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procedure TfrmMain.btnAddClick(Sender: TObject);
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begin
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  Phrase.Add.Current^ := StrNew(PChar(edtWord.Text));
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  UpdateList;
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end;
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{ Note: Clean the Chain list
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   p.s: StrDispose release the memory of the create string
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        to avoid memory leaks
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}
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procedure TfrmMain.btnCleanClick(Sender: TObject);
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begin
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  Phrase.Last;
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  while Phrase.Current <> nil do
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    Phrase.Remove(@StrDispose);
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  UpdateList;
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end;

I wrote some comments on the code to refers the goals of each method, but some things have to be said:
1st. Many improviments can be done on a Chained List, but some times a improvement can be bad to the performance result so keep it on the mind when you change some thing.
2nd. Chained List as the name says, chained, so don’t try to put a index control on it because can be a big mistake.
3rd. Use it when you need a great performance, not when you only need to hold some common data. A Indexed List control will be better and easy to manipulate then a Chained List.

Feel free to use and ask some thing.

Hugs for all

Tags: Chain List, Chained list, Delphi, TChainedList

This entry was posted on Friday, June 19th, 2009 at 10:54 am and is filed under articles. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.