OpenEuphoria

Re: +Help!

• Posted by David Cuny <dcuny at DSS.CA.GOV> May 13, 1998
• 822 views

(I think) Luis Campos wrote:

>        -- mouse press?
>        clicked =3D and_bits( mouse[1], LEFT_DOWN ) ----->???????
>        -- some one knows the use of and_bits() in here????????

The 'and_bits' is needed because there can be more than one mouse event =
at a time. For example, the mouse could move AND the left button could =
be clicked. The flags are ORd together. In this case, the returned =
result would be:

   MOVE AND LEFT_DOWN

which equals=20

   1 AND 2

which is, of course, 3. In this case, the test:

   if mouse[1] =3D LEFT_DOWN

would not detect the mouse click event, because 2 !=3D 3. If you look at =
the flags, they are set up in powers of 2:

   global constant
      MOVE =3D 1,
      LEFT_DOWN =3D 2,
      LEFT_UP =3D 4,
      RIGHT_DOWN =3D 8
      RIGHT_UP =3D 16,
      MIDDLE_DOWN =3D 32,
      MIDDLE_UP =3D 64

This is not by accident - you store all the flags into a single byte, by =
setting the _bits_ in the flag:

   1 =3D bit 0 =3D 2^0
   2 =3D bit 1 =3D 2^1
   4 =3D bit 2 =3D 2^2
   8 =3D bit 3 =3D 2^3
  16 =3D bit 4 =3D 2^4
  32 =3D bit 5 =3D 2^5
  64 =3D bit 6 =3D 2^6

To look at a _single_ bit in the byte, you AND the bits together. So the =
test:

   if and_bits( mouse[1], LEFT_DOWN )

works because

   3 AND 1

is equal to 1, and a non-zero result is treated as true.

Note that 'and_bits' is different than 'and'; 'and' returns true if both =
values are non-zero. It works like this:

   function and( object o1, object o2 )
      if o1 then
         if o2 then
            return 1
         end if
      end if
      return 0
   end function

So the test:

   -- WRONG!
   if mouse[1] and LEFT_DOWN then

tests to see if both values are non-zero. It doesn't do actually do a =
bit-by-bit comparison of the values.

Hope this helps!

-- David Cuny