Linked List Memory Table

unit Unit1;
 
 interface
 
 uses
   Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
   StdCtrls;
 
 type
   TMyObjectPtr = ^TMyObject;
   TMyObject = record
     First_Name: String[20];
     Last_Name: String[20];
     Next: TMyObjectPtr;
   end;
 
 type
   TForm1 = class(TForm)
     bSortByLastName: TButton;
     bDisplay: TButton;
     bPopulate: TButton;
     ListBox1: TListBox;
     bClear: TButton;
     procedure bSortByLastNameClick(Sender: TObject);
     procedure bPopulateClick(Sender: TObject);
     procedure bDisplayClick(Sender: TObject);
     procedure bClearClick(Sender: TObject);
   private
     { Private declarations }
   public
     { Public declarations }
   end;
 
 var
   Form1: TForm1;
   pStartOfList: TMyObjectPtr = nil;
 
 {List manipulation routines}
 procedure SortMyObjectListByLastName(var aStartOfList: TMyObjectPtr);
 function CreateMyObject(aFirstName, aLastName: String): TMyObjectPtr;
 procedure AppendMyObject(var aCurrentItem, aNewItem: TMyObjectPtr);
 procedure ClearMyObjectList(var aMyObject: TMyObjectPtr);
 procedure RemoveMyObject(var aStartOfList, aRemoveMe: TMyObjectPtr);
 function AreInAlphaOrder(aString1, aString2: String): Boolean;
 
 
 implementation
 
 {$R *.DFM}
 
 
 procedure TForm1.bClearClick(Sender: TObject);
 begin
   ClearMyObjectList(pStartOfList);
 end;
 
 procedure TForm1.bPopulateClick(Sender: TObject);
 var
   pNew: TMyObjectPtr;
 begin
   {Initialize the list with some static data}
   pNew := CreateMyObject('Suzy','Martinez');
   AppendMyObject(pStartOfList, pNew);
   pNew := CreateMyObject('John','Sanchez');
   AppendMyObject(pStartOfList, pNew);
   pNew := CreateMyObject('Mike','Rodriguez');
   AppendMyObject(pStartOfList, pNew);
   pNew := CreateMyObject('Mary','Sosa');
   AppendMyObject(pStartOfList, pNew);
   pNew := CreateMyObject('Betty','Hayek');
   AppendMyObject(pStartOfList, pNew);
   pNew := CreateMyObject('Luke','Smith');
   AppendMyObject(pStartOfList, pNew);
   pNew := CreateMyObject('John','Sosa');
   AppendMyObject(pStartOfList, pNew);
 end;
 
 procedure TForm1.bSortByLastNameClick(Sender: TObject);
 begin
   SortMyObjectListByLastName(pStartOfList);
 end;
 
 procedure TForm1.bDisplayClick(Sender: TObject);
 var
   pTemp: TMyObjectPtr;
 begin
   {Display the list items}
   ListBox1.Items.Clear;
   pTemp := pStartOfList;
   while pTemp <> nil do
   begin
     ListBox1.Items.Add(pTemp^.Last_Name + ', ' + pTemp.First_Name);
     pTemp := pTemp^.Next;
   end;
 end;
 
 procedure ClearMyObjectList(var aMyObject: TMyObjectPtr);
 var
   TempMyObject: TMyObjectPtr;
 begin
   {Free the memory used by the list items}
   TempMyObject := aMyObject;
   while aMyObject <> nil do
   begin
     aMyObject := aMyObject^.Next;
     Dispose(TempMyObject);
     TempMyObject := aMyObject;
   end;
 end;
 
 function CreateMyObject(aFirstName, aLastName: String): TMyObjectPtr;
 begin
   {Instantiate a new list item}
   new(result);
   result^.First_Name := aFirstName;
   result^.Last_Name := aLastName;
   result^.Next := nil;
 end;
 
 procedure SortMyObjectListByLastName(var aStartOfList: TMyObjectPtr);
 var
   aSortedListStart, aSearch, aBest: TMyObjectPtr;
 begin
   {Sort the list by the Last_Name "field"}
   aSortedListStart := nil;
   while (aStartOfList <> nil) do
   begin
     aSearch := aStartOfList;
     aBest := aSearch;
     while aSearch^.Next <> nil do
     begin
       if not AreInAlphaOrder(aBest^.Last_Name, aSearch^.Last_Name) then
         aBest := aSearch;
       aSearch := aSearch^.Next;
     end;
     RemoveMyObject(aStartOfList, aBest);
     AppendMyObject(aSortedListStart, aBest);
   end;
   aStartOfList := aSortedListStart;
 end;
 
 procedure AppendMyObject(var aCurrentItem, aNewItem: TMyObjectPtr);
 begin
   {Recursive function that appends the new item to the end of the list}
   if aCurrentItem = nil then
     aCurrentItem := aNewItem
   else
     AppendMyObject(aCurrentItem^.Next, aNewItem);
 end;
 
 procedure RemoveMyObject(var aStartOfList, aRemoveMe: TMyObjectPtr);
 var
   pTemp: TMyObjectPtr;
 begin
   {Removes a specific item from the list and collapses the empty spot.}
   pTemp := aStartOfList;
   if pTemp = aRemoveMe then
     aStartOfList := aStartOfList^.Next
   else
   begin
     while (pTemp^.Next <> aRemoveMe) and (pTemp^.Next <> nil) do
       pTemp := pTemp^.Next;
     if pTemp = nil then Exit; //Shouldn't ever happen 
    if pTemp^.Next = nil then Exit; //Shouldn't ever happen 
    pTemp^.Next := aRemoveMe^.Next;
   end;
   aRemoveMe^.Next := nil;
 end;
 
 function AreInAlphaOrder(aString1, aString2: String): Boolean;
 var
   i: Integer;
 begin
   {Returns True if aString1 should come before aString2 in an alphabetic ascending sort}
   Result := True;
 
   while Length(aString2) < Length(aString1) do  aString2 := aString2 + '!';
   while Length(aString1) < Length(aString2) do  aString1 := aString1 + '!';
 
   for i := 1 to Length(aString1) do
   begin
     if aString1[i] > aString2[i] then Result := False;
     if aString1[i] <> aString2[i] then break;
   end;
 end;
 
 end.

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