虽然很简单，但是我忙活了半个下午，才写出来的小计算器

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
  Dialogs, StdCtrls;

type
  TForm1 = class(TForm)
    Button1: TButton;
    Edit1: TEdit;
    Edit2: TEdit;
    Edit3: TEdit;
    Button2: TButton;
    Button3: TButton;
    Button4: TButton;
    procedure Button1Click(Sender: TObject);
    procedure Button2Click(Sender: TObject);
    procedure Button3Click(Sender: TObject);
    procedure Button4Click(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  x1 : integer;
  x2 : integer;
  x3 : integer;
  str1 : string;
  str2 : string;
  str3 : string;

implementation

{$R *.dfm}

//+
procedure TForm1.Button1Click(Sender: TObject);
begin
  if ( edit1.Text = '' ) OR ( edit2.Text = '' ) then
    messagebox(handle , '输入两个数啊' , '出错' , 0 )
  else
    begin
    str1 := edit1.Text;
    str2 := edit2.Text;
    x1 := strtoint(str1);
    x2 := strtoint(str2);
    x3 := x1 + x2 ;
    str3 := inttostr(x3);
    edit3.Text := str3 ;
    end;
end;

//-
procedure TForm1.Button2Click(Sender: TObject);
begin
  if ( edit1.Text = '' ) OR ( edit2.Text = '' ) then
    messagebox(handle , '输入两个数啊' , '出错' , 0 )
  else
    begin
    str1 := edit1.Text;
    str2 := edit2.Text;
    x1 := strtoint(str1);
    x2 := strtoint(str2);
    x3 := x1 - x2 ;
    str3 := inttostr(x3);
    edit3.Text := str3 ;
    end;
end;

//*
procedure TForm1.Button3Click(Sender: TObject);
begin
  if ( edit1.Text = '' ) OR ( edit2.Text = '' ) then
    messagebox(handle , '输入两个数啊' , '出错' , 0 )
  else
    begin
    str1 := edit1.Text;
    str2 := edit2.Text;
    x1 := strtoint(str1);
    x2 := strtoint(str2);
    x3 := x1 * x2 ;
    str3 := inttostr(x3);
    edit3.Text := str3 ;
    end;
end;

///
procedure TForm1.Button4Click(Sender: TObject);
begin
  if ( edit1.Text = '' ) OR ( edit2.Text = '' ) then
    messagebox(handle , '输入两个数啊' , '出错' , 0 )
  else
    begin
    str1 := edit1.Text;
    str2 := edit2.Text;
    x1 := strtoint(str1);
    x2 := strtoint(str2);
    x3 := round(x1 / x2) ;
    str3 := inttostr(x3);
    edit3.Text := str3 ;
    end;
end;

end.