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                         MORE PARSING by Bob Wiber                              
                                                                                
Hello,  before we begin I need to tell you that we will  be  using  the  program
listing  that  was in last months GAZZETTE,  the program that was in the library
file has the name of 'ROOMS.BAS'.  We will be talking line numbers and you  will
probably  need  a listing handy to make much sense out of this months ramblings,
so I will wait here while you get a listing.                                    
                                                                                
Ok,  now we start to take the program apart and follow  a  command  through  the
parser  and try to figure out how the thing works.  Also I'll try to point out a
couple of places that need some special attention.  Let's begin with the program
lines between 100 and 900.                                                      
 
This is the area in which the arrays are dimensioned and loaded, and some of the
special values are  set  into  place.  Along  with  this  section  is  the  area 
beginning with line 9300,  this area will expand as we go along adding things to 
the bare bones of our adventure game.  Right now we only have the data that will 
allow us to move around our "world", and at least know the name of the room that 
we are in at the time.  Later we will expand the 9300+ area  to  load  in  other 
information  that  will  be needed to run a successful game,  but for now I will 
leave the explanation for another article (well,  I have to  have  something  to 
drag this out with).  

One  point  to notice in the loading of the verb and word arrays,  we don't do a 
FOR-NEXT loop - rather we go until we find "<<END>>".  This allows  us  to  edit
the  word  lists  without having to count the number of words in them and change 
the length of the loop.  The only thing that we need to watch is that the  array 
is  large  enough to handle the lists - in our case we can have 150 words in the 
verb array and 250 words in the other array with no problem, and with the memory 
that is basically standard in most machines today a small array like that is  NO 
PROBLEM (after all, I write these things on a PC JR with 128K).  One other thing 
to  note  is that we have changed the 'name' of the WORDS to TYPE,  not really a 
large point but a confusing one if you don't catch it right away.  

Ok,  we have set up the arrays and loaded all of the various background elements 
that control our world - now what?  At this point we are at line 910 and we find
that  the name of the current room is displayed on the screen (which room we are 
in is set in line 110 with L=4). Then it tries to decide if EXTRA is true, which 
it is not at this time (it's workings will be described later).  So, it's off to 
line 9000 and the real workhorse around here -- the PARSER  (see  it  even  says 
so).  Here's  where I really get into problems and will have to have Aaron check 
my work,  he wrote it and he is the whiz with strings (and graphics and figuring 
out programs and - well, enough) all I know is that I can make it do what I want 
by putting in the commands.  Anyway, we may all learn something here ---

Line 9005 asks us for our command, and if you don't like the ">" then change the 
prompt  in the input,  but remember - your player has to see it at every move so
the cute "WHAT NOW STUPID?" stuff wears thin real soon (in fact,  I have a  game 
that  I  haven't finished playing simply because of the prompt lines).  Whatever 
you choose to do with the prompt let's use  the  line  "GO  TO  THE  NORTH  THEN 
SOUTH".  With  that  safely  entered into the A$ we are off to line 10000.  This 
little sequence (line numbers 10000-10020) takes all of our input and makes sure 
that it is in capital letters, so we can type in our commands in upper, lower or 
a combination of the two cases.  With that safely out of the way  we  return  to 
line 9007 and the main bulk of the parser.  

The  parser  starts  off  by  checking  to  see if the command that was input is 
smaller then three letters long or is a "G",  if so a lot of  the  work  can  be
eliminated.  Other  checks that are performed at this time let the computer know 
if the command that has been entered is multiple or not,  and in the case of our 
example  it  is.  This  is  signified by the use of "THEN" or by using a period, 
with this information the computer cuts off the rest of  the  command  line  and 
assigns  it  to a holding string (A1$) and sets EXTRA to 1.  From this point the 
parser checks to see if we are using "G" or"AGAIN" which tells it to repeat  the 
last  command,  i.e.  the command "GO NORTH.  G THEN GET THE CAT.  S THEN NW" is 
perfectly legal and will be fully complied with - if you haven't made a  mistake 
(if  there  is no cat then you won't get it,  but you will end up where you told 
the computer to put you - so it is important to know what you really want to  do 
before using the multiple command feature).

At  this point the parser just starts to examine the first command and to cut it 
into the individual words that make it up.  In our example command at this point 
we would have set EXTRA, assigned "SOUTH" TO A1$, and would have split the first 
command into four individual words.  The parser,  as with all of the others that 
I am aware of, assigns the FIRST word to the verb and lets the others be as they 
may with the noun usually being the last word in the command.  

Having  done  all of the above the computer returns to the program lines between 
1010 and 1080 with a small detour or two.  At this point all  we  have  is  four 
words  and  the knowledge that we still have some commands to be worked on.  The
following lines really  only  go  through  the  word  strings  that  the  Parser
generated  and  compare  them  to the word lists,  the verb is done first (lines
9200-9220) and if no verb is present the  computer  complains  that  it  doesn't
understand the sentence.

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