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.