unit Unit1;

{$mode objfpc}{$H+}

interface

uses

  Classes, SysUtils, FileUtil, Forms, Controls, Graphics, Dialogs, StdCtrls;

type

  { TForm1 }

  TForm1 = class(TForm)

    Edit1: TEdit;

    Label1: TLabel;

    procedure Edit1Change(Sender: TObject);

  private

    { private declarations }

  public

    { public declarations }

  end;

var

  Form1: TForm1;

implementation

{$R *.lfm}

{ TForm1 }

procedure TForm1.Edit1Change(Sender: TObject);

var

  i: integer;

  a, b: single;

  c, sign: char;

  digits: string;

  line: string;

  result: string;

begin

  digits := '';

  sign := ' ';

  a := 0;

  b := 0;

  line := Edit1.Text;

  try

    for i := 1 to length(line) do begin

      c := line[i];

      if (c='+') or (c='-') or (c='*') or (c='/') then begin

        a := StrToFloat(digits);

        digits := '';

        sign := c;

      end else

      if c = '=' then begin

        b := StrToFloat(digits);

        digits := '';

        result := '';

        if sign='+' then result := FloatToStr(a + b);

        if sign='-' then result := FloatToStr(a - b);

        if sign='*' then result := FloatToStr(a * b);

        if sign='/' then result := FloatToStr(a / b);

        Edit1.Text := copy(line, 1, i) + result;

      end else begin

        digits := digits + c;

      end;

    end;

  except end;

end;

end.