Newton Polinomial.

Here's code for Newton's divided differences interpolation polynomial (quite mouthful huh, :) ).

The purpose of this method is to create a function (polynomial) that passes through given set of data points.




I read data point from several edit box.

procedure TForm1.Button3Click(Sender: TObject);
var i:integer;
begin
  for i:=0 to n do begin
    x[i]:=strToFloat(kx[i].Text);
    y[i]:=strToFloat(ky[i].Text);
  end;
  xc:=strToFloat(kxc.Text);
  yc:=fn(xc);
  kyc.Text:=floatToStr(yc);
  gambarNewton;
end;

.
kx and ky is tEdit created when button1 is clicked

procedure TForm1.Button1Click(Sender: TObject);
var i:integer;
begin
  button2.Enabled:=true;
  button3.Enabled:=true;
  button4.Enabled:=true;
  button5.Enabled:=true;

  n:=strToInt(edit1.Text);
  kxc:=tEdit.Create(form1);       kyc:=tEdit.Create(form1);
  kxc.Parent:=form1;              kyc.Parent:=form1;
  kxc.Left:=36;                   kyc.Left:=72;
  kxc.Width:=36;                  kyc.Width:=36;
  kxc.Text:='0,5';

  for i:=0 to n do begin
    kx[i]:=tEdit.Create(form1);     ky[i]:=tEdit.Create(form1);
    kx[i].Parent:=form1;            ky[i].Parent:=form1;
    kx[i].Top:=36+36*i;             ky[i].Top:=36+36*i;
    kx[i].Left:=36;                 ky[i].Left:=72;
    kx[i].Width:=36;                ky[i].Width:=36;
    kx[i].Text:=intToStr(i);        ky[i].Text:=intToStr(i);
  end;
end;

xc is x coordinate where the corresponding y (yc) is obtained using Newton method by calling it

yc=fn(xc)

function tform1.fn(xs:real):real;
var i:integer;fs:real;
begin
  fs:=0;
  for i:=0 to n do begin
    fs:=fs+b(i,0)*c(xs,i);
  end;
  fn:=fs;
end;

the fn function call the two other function. The b function, a recursive contain divided difference like this

function tform1.b(i,j:integer):real;
begin
  if i=0 then b:=y[0]
    else if (i-j)=1 then
      b:=(y[i]-y[j])/(x[i]-x[j])
        else
          b:=(b(i,j+1)-b(i-1,j))/(x[i]-x[j]);
end;

and c function, a recursive function (or you could rewrite it using simple for command)

function tform1.c(xs:real;i:integer):real;
begin
  if i=0 then c:=1
    else c:=(xs-x[i-1])*c(xs,i-1);
end;

and finally, draw the data and the function on image1

fprocedure tform1.gambarNewton;
var i,x0,y0:integer;px,py:real;
begin
  x0:=image1.Width div 2;               y0:=image1.Height div 2;
  image1.Canvas.Brush.Color:=clLime;
  image1.Canvas.Rectangle(0,0,image1.Width,image1.Height);
  image1.Canvas.Brush.Color:=clWhite;
  image1.Canvas.Pen.Color:=clBlack;
  image1.Canvas.MoveTo(0,y0);           image1.Canvas.LineTo(image1.Width,y0);
  image1.Canvas.MoveTo(x0,0);           image1.Canvas.LineTo(x0,image1.Height);
  for i:=-300 to 300 do begin
    px:=i/skala;                          py:=skala*fn(px);
    image1.Canvas.Pixels[x0+i,y0-round(py)]:=clGreen;
  end;
  for i:=0 to n do begin
    px:=x0+skala*x[i];                        py:=y0-skala*y[i];
    image1.Canvas.Ellipse(round(px)-7,round(py)-7,round(px)+7,round(py)+7);
  end;
  px:=x0+skala*xc;                        py:=y0-skala*yc;
  image1.Canvas.Brush.Color:=clred;
  image1.Canvas.Ellipse(round(px)-7,round(py)-7,round(px)+7,round(py)+7);
  image1.Canvas.Brush.Color:=clwhite;
end;