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# How to perform a Shell Sort

Shell Sort is really just an extension of Insertion Sort, with two observations in mind:

• Insertion Sort is efficient if the input is "almost sorted".
• Insertion Sort is inefficient, on average, because it moves values just one position at a time.

Shell Sort is similar to Insertion Sort, but it works by taking "bigger steps" as it rearranges values, gradually decreasing the step size down towards one. In the end, Shell Sort performs a regular Insertion Sort but by then, the array of data is guaranteed to be "almost sorted".

Consider a small value that is initially stored in the wrong end of the array. Using Insertion Sort, it will take roughly N comparisons and exchanges to move this value all the way to the other end of the array. With Shell Sort, we'll first move values using giant step sizes, so a small value will move a long way towards it's final position, with just a few comparisons and exchanges.

```procedure SortShell(var aSort: array of Word);
var
iI, iJ, iK,
iSize: Integer;
wTemp: Word;
begin
iSize := High(aSort);
iK := iSize shr 1;
while iK > 0 do begin
for iI := 0 to iSize - iK do begin
iJ := iI;
while (iJ >= 0) and (aSort[iJ] > aSort[iJ + iK]) do begin
wTemp := aSort[iJ];
aSort[iJ] := aSort[iJ + iK];
aSort[iJ + iK] := wTemp;
if iJ > iK then
Dec(iJ, iK)
else
iJ := 0
end;
end;
iK := iK shr 1;
end;
end;```

A ready made project containing this demo code is available. View the project.

This demo creates a list of random numbers and displays them in a list box. It then sorts the numbers using SortShell and displays the sorted list in a second list box.

Create a new Delphi VCL project and drop two TListBox controls on it. Also create an OnCreate event handler for the form. Name the form "Form1" and save the form unit as `Unit1.pas`.

Code Unit1 as follows:

```unit Unit1;

interface

uses
SysUtils, Forms, Classes, Controls, StdCtrls;

type
TForm1 = class(TForm)
ListBox1: TListBox;
ListBox2: TListBox;
procedure FormCreate(Sender: TObject);
end;

var
Form1: TForm1;

implementation

{\$R *.dfm}

// Routine from tip
procedure SortShell(var aSort: array of Word);
var
iI, iJ, iK,
iSize: Integer;
wTemp: Word;
begin
iSize := High(aSort);
iK := iSize shr 1;
while iK > 0 do begin
for iI := 0 to iSize - iK do begin
iJ := iI;
while (iJ >= 0) and (aSort[iJ] > aSort[iJ + iK]) do begin
wTemp := aSort[iJ];
aSort[iJ] := aSort[iJ + iK];
aSort[iJ + iK] := wTemp;
if iJ > iK then
Dec(iJ, iK)
else
iJ := 0
end;
end;
iK := iK shr 1;
end;
end;

procedure TForm1.FormCreate(Sender: TObject);

procedure AddToLB(const Nums: array of Word; LB: TListBox);
var
I: Integer;
begin
for I := Low(Nums) to High(Nums) do
end;

var
I: Integer;
Nums: array[1..50] of Word;
begin
// Create list of random numbers and display it
Randomize;
for I := Low(Nums) to High(Nums) do
Nums[I] := Random(1000);