Subj : Sets
To   : Sean Dennis
From : Scott Little
Date : Mon Oct 12 2009 05:45 pm

[ On 2009-10-09 at 11:31:50, Sean Dennis wrote to All ]

SD> Telegard (the BBS software I use) uses sets to do some
SD> "bit-twiddling" with certain settings on a user's record, such as if
SD> the user is "hidden", et al.

Sets are fancy bitfields.  A set is made of one or more values of an ordinal
type, which can be reduced to an integer and therefore a bit number.

eg. an enumerated (ordinal) type, specifying the days of the week:

 type
  Day = (Mon, Tue, Wed, Thu, Fri, Sat, Sun);
  Days = set of Day;

 var
  d: Days;

Then you can set one or more days:

 d := [Mon, Wed, Fri];

Since ord(Mon) = 0, ord(Wed) = 2, ord(Fri) = 4, the above will flip bits 0, 2
and 4 to on.  Byte(d) = 21.

The compiler will automatically size the set to accomodate all the potential
values.  Since there are seven days, sizeof(Days) = 1. A set of Char would be
32 bytes (256 possible values divided by 8 bits per byte).

You can also do set math, eg.

 if Mon in [Mon, Wed, Fri] - [Wed, Fri] then writeln('Wed and Fri are gone,
Mon is left');
 if not (Mon in [Mon, Wed, Fri] - [Mon]) then writeln('Mon is gone, Wed and
Fri are left');
 if Mon in [Mon, Wed, Fri] * [Mon, Fri] then writeln('Mon and Fri are common
to both sets');

The same can be done manually with bitwise operations, but sets are easier to
read.

Clear as mud? ;)


-- Scott Little  [fidonet#3:712/848 / [email protected]]

--- The government solution to a problem is usually as bad as the problem.
* Origin: What the hell are you waiting for? Christmas? (3:712/848)

You are viewing proxied material from cvs.synchro.net. The copyright of proxied material belongs to its original authors. Any comments or complaints in relation to proxied material should be directed to the original authors of the content concerned. Please see the disclaimer for more details.