OpenEuphoria

Re: 3.0.3 - type boolean

• Posted by Juergen Luethje <j.lue at gmx.de> May 12, 2007
• 679 views

Alex Caracatsanis wrote:

[type boolean]

> A serious, newbie question: why is this a useful type to check?
> 
> Thank you

In programs often so called "flags" are used. The name refers to the
fact, that (somewhat simplified) a real flag either is hoisted or not.
Similarly, variables used as flags are either TRUE or not.
The smallest possible flag is a bit, which either is set (= 1) or not
set (= 0). So e.g. an integer variable in Euphoria consists of 31 bits
("flags"), which are independent of each other and can be manipulated
individually. Well, this is a good subject for scaring beginners ... 
   It is easier to think of a whole variable as a flag, which is
either hoisted (TRUE) or not (FALSE). A variable that can only hold the
values TRUE or FALSE is called a boolean variable -- in honor of
George Boole <http://en.wikipedia.org/wiki/Boole>.

Here is a simple example. In the following program we have two nested
loops, and when a special condition is met, we want the program to leave
_both_ loops. Euphoria's exit statement only exits the innermost loop.
We can use a boolean variable to achieve our goal:

constant
   FALSE = 0,
   TRUE  = not FALSE

integer x, y
boolean exitFlag

x = 5
y = 9
exitFlag = FALSE

for i = 1 to 10 do
   for k = 7 to 12 do
      if i = x and k = y then
         exitFlag = TRUE          -- set flag to remind ourselves
         exit                     -- exit inner loop
      end if
   end for
   if exitFlag = TRUE then
      exit                        -- exit outer loop
   end if
end for

? x
? y

The above program only runs if you have previously defined a "boolean"
data type in your program. You can also declare
   integer exitFlag
instead, but variables that are boolean by their "nature", from a
logical point of view, should better be declared as boolean. In Euphoria
this is done by a user-defined type. This has the advantage of both better
readability of the code and easier debugging, see also
<http://rapideuphoria.com/refman_2.htm#43>.

Did this answer your question?

Regards,
   Juergen