Weird problem with real variable

[Christian Knudsen]

Sorry about the vague topic title, but I have no idea what's going on here and was therefor not able to give a more precise topic title. Anyway...

I'm programming in FreePascal 2.2.2 on Win32 and have run into a strange issue with a real variable. I have a procedure called PlaceLightsource like this:

PROCEDURE PlaceLightSource(Const LightSourceNumber : Byte; Const Radius, HotSpot, X, Y, Red, Green, Blue : Real; Const Life : SmallInt);
BEGIN
   LightSource[LightSourceNumber].AbsoluteX := X;
   LightSource[LightSourceNumber].AbsoluteY := Y;
   LightSource[LightSourceNumber].Active := T;
   LightSource[LightSourceNumber].HotSpot := HotSpot;
   LightSource[LightSourceNumber].Life := Life;
   LightSource[LightSourceNumber].Radius := Radius;
   LightSource[LightSourceNumber].R := Red;
   LightSource[LightSourceNumber].G := Green;
   LightSource[LightSourceNumber].B := Blue;
   PlaceLightSourceReferencesOnTiles(LightSourceNumber);
   RecalculateTileLighting := T;
END;

I then call this procedure like this:

   PlaceLightSource(I, 1.5, 0, X, Y, 0.5, 0.5, 0.3, -1);

So, LightSource[I].Radius should now be set to 1.5, LightSource[I].R to 0.5, LightSource[I].G to 0.5, and LightSource[I].B to 0.3. If I get the program to print the values of these variables to the screen, it checks out fine.

However, when I call this piece of code...

   IF (LightSource[I].Radius = 1.5) AND (LightSource[I].R = 0.5) AND (LightSource[I].G = 0.5) AND (LightSource[I].B = 0.3) THEN BEGIN
      Character[CharacterNumber].LightSource := 0;
      LightSource[I].Active := F;
      RemoveLightSourceReferencesOnTiles(I);
      RecalculateTileLighting := T;
   END;

...it never gets past the IF check. Weird. Even more weird, if I comment out the "AND (LightSource[I].B = 0.3)", it does everything it's supposed to do. So I double checked that LightSource[I].B = 0.3 by printing the value to the screen again, and it IS 0.3. Now, if I set it to 0.5, it also works fine!

I'm completely stumped by this. Is anybody aware of some things to look out for when checking the value of real variables?

[Darthman]

Try to avoiding this kind of check. Because 0.3 can be not exactly 0.3. It can be 0.30000000001 or 0.29999999999.
So you can check like this: if round(b*10)=3 (dont like it) or like this: > 0.29 and < 0.31 (for example).

[chronozphere]

Make a special equals() function, like this:

const
  //A treshold for float comparison
  EPSILON = 1e-5;

implementation

function Equals(const a, b: Single): Boolean;
begin
  Result := (a - b < EPSILON);
end;

Make similar functions to compare vectors and colors.

[KidPaddle]

Small changes, cause you forgot function call ABS(a - b)

const
  //A treshold for float comparison
  EPSILON = 1e-5;

implementation

function Equals(const a, b: Single): Boolean;
begin
  Result := (abs(a - b) < EPSILON);
end;

Thomas

[Christian Knudsen]

Interesting! And thanks for the replies!

So, when you assign the value 0.3 to a real, you can't be 100% sure that it actually gets the precise value? That seems a bit odd.


EDIT: Still doesn't work. I tried this:

   IF (Trunc(LightSource[I].Radius * 10) = 15) AND (Trunc(LightSource[I].R * 10) = 5)
   AND (Trunc(LightSource[I].G * 10) = 5) AND (Trunc(LightSource[I].B * 10) = 3) THEN BEGIN
      Character[CharacterNumber].LightSource := 0;
      LightSource[I].Active := F;
      RemoveLightSourceReferencesOnTiles(I);
      RecalculateTileLighting := T;
   END;

And it will still only go into the IF block if I comment out the "AND (Trunc(LightSource[I].B * 10) = 3)" bit.


ANOTHER EDIT: It seems that Trunc(LightSource[I].B * 10) returns 2, so 0.3 must actually be something just below 0.3, like 0.299999999999, or something...

[User137]

You can use Round() instead of Trunc() so 2.7 would round upwards to 3 instead.

[chronozphere]

Small changes, cause you forgot function call ABS(a - b)

const
  //A treshold for float comparison
  EPSILON = 1e-5;

implementation

function Equals(const a, b: Single): Boolean;
begin
  Result := (abs(a - b) < EPSILON);
end;

Thomas

Argh.. that's indeed a stupid mistake. Thanks for the heads up.