[PESTS]

Так просто не сделаешь нужна писать парсер!
Вот готовая:

Код:

function Calculate(SMyExpression: string; digits: Byte): string;

  // Calculate a simple expression 

// Supported are:  Real Numbers, parenthesis 

var

  z: Char;

  ipos: Integer;

 

  function StrToReal(chaine: string): Real;

  var

    r: Real;

    Pos: Integer;

  begin

    Val(chaine, r, Pos);

    if Pos > 0 then Val(Copy(chaine, 1, Pos - 1), r, Pos);

    Result := r;

  end;

 

  function RealToStr(inreal: Extended; digits: Byte): string;

  var

    S: string;

  begin

    Str(inreal: 0: digits, S);

    realToStr := S;

  end;

 

  procedure NextChar;

  var

    s: string;

  begin

    if ipos > Length(SMyExpression) then

    begin

      z := #9;

      Exit;

    end

    else

    begin

      s := Copy(SMyExpression, ipos, 1);

      z := s[1];

      Inc(ipos);

    end;

    if z = ' ' then nextchar;

  end;

 

  function Expression: Real;

  var

    w: Real;

 

    function Factor: Real;

    var

      ws: string;

    begin

      Nextchar;

      if z in ['0'..'9'] then

      begin

        ws := '';

        repeat

          ws := ws + z;

          nextchar

        until not (z in ['0'..'9', '.']);

        Factor := StrToReal(ws);

      end

      else if z = '(' then

      begin

        Factor := Expression;

        nextchar

      end

      else if z = '+' then Factor := +Factor

      else if Z = '-' then Factor := -Factor;

    end;

 

    function Term: Real;

    var

      W: Real;

    begin

      W := Factor;

      while Z in ['*', '/'] do

        if z = '*' then w := w * Factor

      else

        w := w / Factor;

      Term := w;

    end;

  begin

    w := term;

    while z in ['+', '-'] do

      if z = '+' then w := w + term

    else

      w := w - term;

    Expression := w;

  end;

begin

  ipos   := 1;

  Result := RealToStr(Expression, digits);

end;