怎样在“有类型文件”进行删除？

Question

做了个类似通讯录的小程序，用的是“有类型文件”实现了读，写可是不知道怎样删除记录？

Answer 1

一个小程序，有两个按钮，分别是新增和保存。现在想增加“删除”按钮，可是怎么删除呢？
-------------------------------------------------------------------------------unit Unit1;interfaceuses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, ComCtrls, StdCtrls, ExtCtrls;type
  TForm1 = class(TForm)
    numberedit: TLabeledEdit;
    nameedit: TLabeledEdit;
    phoneedit: TLabeledEdit;
    addressedit: TLabeledEdit;
    choosenumber: TUpDown;
    newbutton: TButton;
    savebutton: TButton;
    genderlabel: TLabel;
    malebtn: TRadioButton;
    femalebtn: TRadioButton;
    Label1: TLabel;
    birthdayedit: TDateTimePicker;
    procedure FormCreate(Sender: TObject);
    procedure FormClose(Sender: TObject; var Action: TCloseAction);
    procedure choosenumberChanging(Sender: TObject;
      var AllowChange: Boolean);
    procedure newbuttonClick(Sender: TObject);
    procedure savebuttonClick(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;
  type
  tfriendrec=packed record
  no:byte;
  name:string[10];
  gender:bool;
  birthday:tdatetime;
  phone:string[15];
  address:string[50];
  end;
var
  Form1: TForm1;
   friendrec:tfriendrec;
   friendfile:file of tfriendrec;
implementation{$R *.dfm}procedure TForm1.FormCreate(Sender: TObject);
begin
assignfile(friendfile,extractfilepath(application.exename)+'ylg.dat');
reset(friendfile);
choosenumber.Max:=filesize(friendfile)-1;end;procedure TForm1.FormClose(Sender: TObject; var Action: TCloseAction);
begin
closefile(friendfile);
end;procedure TForm1.choosenumberChanging(Sender: TObject;
  var AllowChange: Boolean);
begin
numberedit.Text:=inttostr(choosenumber.Position);
seek(friendfile,choosenumber.Position);
read(friendfile,friendrec);
nameedit.Text:=friendrec.name;
phoneedit.Text:=friendrec.phone;
addressedit.Text:=friendrec.address;
birthdayedit.DateTime:=friendrec.birthday;
if friendrec.gender then
malebtn.Checked:=true
else
femalebtn.Checked:=true;
end;procedure TForm1.newbuttonClick(Sender: TObject);
begin
seek(friendfile,filesize(friendfile));
choosenumber.max:=choosenumber.max+1;
choosenumber.Position:=filesize(friendfile);
numberedit.Text:=inttostr(filesize(friendfile));
choosenumber.enabled:=false;
numberedit.Enabled:=false;
nameedit.Text:='';
phoneedit.Text:='';
addressedit.Text:='';
end;procedure TForm1.savebuttonClick(Sender: TObject);
begin
with friendrec do
begin
no:=choosenumber.Position;
name:=nameedit.Text;
gender:=malebtn.Checked;
birthday:=birthdayedit.DateTime;
phone:=phoneedit.Text;
address:=addressedit.text;
end;
write(friendfile,friendrec);
choosenumber.Enabled:=true;
numberedit.Enabled:=true;
end;end.