2011-11-10



The path of my studies passed some theoretical  computer  science
basics  just by.  Well, that made it easier for me then, but more
difficult and worse (!) now.

Yesterday and today,  I  learned  to  implement  a  finate  state
machine,  the  hard  way. I had to teach me almost everything my-
self. If you have the chance, then learn it when it is tought  to
you.

Some remarks:  Draw the right graph first.  Implement it with one
large  loop, having the one simple gettoken() function.  Although
I don't like the switch syntax in C, you  best  take  two  nested
switch  statements.   Use ungettoken() and return to the starting
state, if you find a non-matching token.

Well, it took about 200 lines of state machine to match  and  sum
up  signed  decimal  integer  and floating point numbers (also in
scientific notation).


To draw the graph for a troff document, I used pic.  There should
be some preprocessor for pic to ease this task, but I found none.
The Dragon Book includes some  nice  graphs,  probably  generated
with  pic  macros.  I should find out how they created them.  The
other possibility would be graphviz, I think.  Unfortunately, I'm
not  matured  using it. I would have needed too much time to find
out how to use it. Hence, I did it in a poor man's  version  with
pic.  Here are the sources:

     .PS
     circlerad = .15

     S0: circle "\fBS0\fP" at .2,-.6
     S1: circle "\fBS1\fP" at 1,0
     S2: circle "\fBS2\fP" at 2.5,.2
     S3: circle "\fBS3\fP" at 2,-.4
     S4: circle "\fBS4\fP" at 3,-.5
     S5: circle "\fBS5\fP" at 3,-1.3
     S6: circle "\fBS6\fP" at 2,-1.3
     S7: circle "\fBS7\fP" at 1,-1.5

     arrow <- left from S0.w
     box invis "\fIStart\fP"

     arrow "       + -" from S0 to S1 chop
     arrow ". " above from S0 to S3 chop
     arc -> cw from S0.n to S2.nw rad .7
     "0-9" at .3,.1

     arrow "0-9 " above from S1 to S2 chop
     arrow "." above from S1 to S3 chop

     arrow " ." above from S2 to S4 chop
     arc -> cw from S2.e to S5.e rad .4
     "E e" at 3.4,.1
     spline -> from S2.ne right .3 up .3 then left up .1 then \
             to S2.n "              0-9" above

     arrow "0-9  " above from S3 to S4 chop

     arrow "E e" from S4 to S5 chop
     spline -> from S4.e right .3 then up .4 left .1 then to S4.ne
     "0-9" at 3.3,-.6

     arrow "+ -" below from S5 to S6 chop
     arc -> cw from S5.sw to S7.se rad 2
     "0-9" at 2.7,-1.7

     arrow "  0-9" below from S6 to S7 chop

     spline -> from S7.w left .2 then down .2 right .1 then \
             to S7.sw "0-9          " above
     .PE



http://marmaro.de/lue/        markus schnalke <meillo@marmaro.de>