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Article #116 (145 is last):
Newsgroups: freenet.sci.comp.atari.library
From: aa700@cleveland.Freenet.Edu (Michael Current)
Subject: Boolean Logic in BASIC
Posted-By: xx004 (aa700 - Michael Current)
Reply-To: aa700@cleveland.Freenet.Edu (Michael Current)
Date: Tue Jan 28 22:39:37 1992


Reprinted from the A.C.E.C. BBS (614)-471-8559

-----------------------------------
      Boolean Logic in BASIC
-----------------------------------

Boolean logic is a very useful, but
often unused, function of most
BASIC languages.  Boolean logic is
a true/false comparison operation.
Because it only returns a true or
false value, it returns a 1 or a 0.

If you enter PRINT 1=1 the computer
will print "1" on the screen.  Why?
Because 1=1 is a true statement.
The value 1 does equal the value 1.
A true statement is identified by
a 1.  If you enter PRINT 2=3 the
computer will print a "0" on the
screen.  This is becase 2=3 is NOT
a true statement.  A false
comparison will return a value of
0.

You can also make variable
assignments with boolean logic. If
you say A=1=1 then A=1 because 1=1
would return a 1.  A short table
might be helpful at this point:

Statement  Result
---------  ------
A=1=1      A=1
A=1=2      A=0
A=2=1      A=0
A=2=2      A=1

This alone is useful in programming
but we are allowed to use even more
complex boolean logic.  Right now
we can say A=B=C.  But what about
A=B=C=D?  Or A=B=C=D=E?  How does
that work?  DOES it work?  The
answer is yes, it does work.

Long boolean operations such as
this can be confusing unless you
break it down.  So that is what we
will do.  A=B=C=D is the same as
A=B=(C=D).  Lets try a few:

B=1, C=1, D=0: A=1=(1=0) so
  A=1=(0) so A=1=0 so A=0

B=0, C=0, D=0: A=0=(0=0) so
  A=0=(1) so A=0=1 so A=0

B=0, C=0, D=1: A=0=(0=1) so
  A=0=(0) so A=0=0 so A=1

It is really simple.  Just evaluate
the comparison within the
paranthesese first, just like a
normal problem.  In the last
example, the comparison in the
parans is (0=1).  This is a false
statement so we can put a 0 in the
place of the parans.  Still quite
easy, right?

Now how about A=B=C=D=E?  Getting a
bit more complex here.  What this
amounts to is A=B=(C=(D=E)).  Let's
try a few:

B=1, C=0, D=1, E=0: A=1=(0=(1=0))so
  A=1=(0=0) so A=1=(1) so A=1=1 so
  A=1

B=0, C=1, D=0, E=1: A=0=(1=(0=1))so
  A=0=(1=0) so A=0=(0) so A=0=0 so
  A=1

As you can see, it is a little more
complex but the idea is the same.
Just do the parans first and then
work out to the next set of parans.
Again, just like normal math
operations: Parans come first.

So it is very simple:

Statement     Equivalent To
------------- ---------------------
A=B=C=D       A=B=(C=D)
A=B=C=D=E     A=B=(C=(D=E))
A=B=C=D=E=F   A=B=(C=(D=(E=F)))
A=B=C=D=E=F=G A=B=(C=(D=(E=(F=G))))

I think you can see the pattern
here.

It should be noted that while
simple boolean logic like A=B=C is
useful, complex boolean logic like
A=B=C=D=E=F is really too complex
to be useful in a real programming
enviroment.  In most cases there
would be an easier, more memory
efficient way to accomplish the
same thing.  But knowing how to use
boolean logic will be very useful.
Once you have completely mastered
it you will find yourself using it
often.

                    --Craig Steiner



-- 
 Michael Current, Cleveland Free-Net 8-bit Atari SIGOp   -->>  go atari8  <<--
   The Cleveland Free-Net Atari SIG is the Central Atari Information Network
      Internet: currentm@carleton.edu / UUCP: ...!umn-cs!ccnfld!currentm
      BITNET: currentm%carleton.edu@interbit / Cleveland Free-Net: aa700





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