Trunc() function

Question

look the follow code, why the result of Trunc function is different?

procedure TForm1.Button1Click(Sender: TObject);
var
  D: Double;
  E: Extended;
  I: Int64;
begin
  D := Frac(101 / 100) * 100;
  E := Frac(101 / 100) * 100;
  I := Trunc(D);
  ShowMessage('Trunc(Double): ' + IntToStr(I));  // Trunc(Double): 1
  I := Trunc(E);
  ShowMessage('Trunc(Extended): ' + IntToStr(I)); // Trunc(Extended): 0
end;

Answer 1

Formatting functions don't always display the actual numbers (data).
Real numbers and precision can be tricky.

Check out this code where I use more precision on what I want to see on the screen:

  D := Frac(101 / 100);
  E := Frac(101 / 100);
  ShowMessage(FloatToStrF(D, ffFixed, 15, 20));
  ShowMessage(FloatToStrF(E, ffFixed, 18, 20));

It appears that D is something like 0.010000000000 while E is like 0.00999999999.

Edit: Extended type has better precision than Double type. If we try to display the values of D and E with FloatToString() we'll probably get the same result, even though the actual values are not the same.