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- From: ir230@sdcc6.ucsd.edu (john wavrik)
- Newsgroups: comp.lang.forth
- Subject: Style
- Message-ID: <12149@sdcc6.ucsd.edu>
- Date: 3 Aug 90 03:49:06 GMT
- Organization: University of California, San Diego
- Wil Baden has started a discussion of Forth style. I dug out the
- following example which I once used in a Forth course. The task is
- simple enough that it can be easily understood -- and difficult enough
- so that style matters. It was chosen so that the issue would be coding
- (rather than improvement in algorithm, knowledge of math, etc.)
- TASK:  Write a program in which the computer uses binary search to
-        guess a user's "secret" number.  The user thinks of a secret
-        number, an integer from 1 to 100. The computer makes a guess
-        to which the user must respond by typing LOW, HIGH or RIGHT.
-        This is repeated until the computer guesses the number.
- I don't remember what version of Forth we were using when this was
- written -- but it shouldn't matter unless you plan to actually run it.
- We used the following words from a string package (all strings are
- counted with a 1-byte count and are represented by their address):
- $VARIABLE    ( n -- )  defining word for string variables.
-              <n> $VARIABLE X  allocates storage for a counted string
-              of length n. When X executes the address of the string is
-              put on the stack.
- $"           ( -- $addr ) the following text up to a delimiting " is
-              stored as a counted string. The address of the string is
-              put on the stack.
- IN$          ( -- $addr ) accept a string from the keyboard until <cr>.
-              The input is stored as a counted string whose address is
-              put on the stack.
- $COMPARE     ( $1 $2 -- -1|0|1 ) compares counted strings. Returns -1
-              if $1 comes before $2 in lexicographic order, 1 if it
-              comes after, and 0 if the strings are equal.
- $!           ( $ addr -- ) copy counted string at address $ to addr.
-              Typical usage  $" HELLO" X $!  where X is a string
-              variable.
- Here is a solution to the problem which shows why some people think
- Forth is a write-only language:
- ( GUESSING GAME PROGRAM                                 1  )
-$VARIABLE A
- :  GO  BEGIN 0 101 BEGIN  2DUP + 2/  DUP  CR ." I GUESS "  . CR BEGIN
- ." IS THAT HIGH, LOW, OR RIGHT " IN$ A $! A $" HIGH" $COMPARE  0=  IF
- SWAP DROP 0 1 ELSE A $" LOW" $COMPARE 0= IF ROT DROP  SWAP  0 1 ELSE A
- $" RIGHT" $COMPARE 0= IF CR ." I  GOT IT!!" DROP DROP DROP BEGIN CR ."
- PLAY AGAIN (Y OR N)" KEY DUP 78 = IF DROP  1  1 1 1 ELSE 89 = IF 0 1 1
-ELSE 0 THEN  THEN  UNTIL ELSE CR 0 THEN THEN THEN UNTIL UNTIL UNTIL ;
- ( Why spend hundreds of dollars for a program to obfuscate your source
- code?  You can do it yourself for less in Forth! )
- Actually, something like this can be done in any language. I did it by
- removing the indentation from the following program. The following is
- bad style also. It is a transcription to Forth of a program which the
- student originally wrote in Pascal (this stuff comes from several
- years back when Pascal was the "in" language). I don't think this is
- what Wirth had in mind when he invented Pascal -- but it seems typical
- of the way many people write Pascal and 'C':
- ( GUESSING GAME PROGRAM                                 2A )
-$VARIABLE ANSWER
- : GAME   BEGIN 0 101
-           BEGIN  2DUP + 2/  DUP
-                  CR ." I GUESS " .  CR
-                BEGIN  ." IS THAT HIGH, LOW, OR RIGHT "
-                       IN$  ANSWER $!
-                       ANSWER $" HIGH" $COMPARE 0=
-                       IF   SWAP DROP 0 1
-                       ELSE ANSWER $" LOW" $COMPARE 0=
-                         IF   ROT DROP SWAP 0 1
-                         ELSE  ANSWER $" RIGHT" $COMPARE 0=
-                             IF CR ." I GOT IT!!"  DROP DROP
-                                DROP
-                                BEGIN CR ." PLAY AGAIN (Y OR N)"
-                                      KEY DUP 78 =    -->
- ( GUESSING GAME PROGRAM                                 2B )
-                                      IF   DROP 1 1 1 1
-                                      ELSE  89 =
-                                         IF  0 1 1 1
-                                         ELSE 0
-                                         THEN
-                                      THEN
-                                UNTIL
-                            ELSE  CR 0
-                            THEN
-                         THEN
-                       THEN
-                UNTIL
-           UNTIL
-         UNTIL ;
- [I notice in reproducing it that the programmer forgot to deal with the
- case in which the user types in something other than HIGH, LOW or
- RIGHT.]
- The guessing game program is logically more complex than it might
- first appear. You might want to see what you can do with it before
- reading the code below.
- =======================================================================
- ( GUESSING GAME PROGRAM                                 3A )
- VARIABLE LL   VARIABLE HH   ( low and high for current range )
- VARIABLE GG                 ( current guess )
- : GUESS  CR  ." I guess "
-         LL  @  HH  @  + 2/     ( guess  midpoint  of  range  )
-         DUP    GG   !    .     ( and store guess  )
-         ." is that LOW, HIGH, or RIGHT" ;
- : START  0 LL !  101 HH !  GUESS  ;
- :  HIGH    GG @ HH !  GUESS   ;  ( take lower half of  range )
- :  LOW     GG @ LL !  GUESS   ;  ( take upper half of  range )
- :  RIGHT   CR ." Wow, I got it!!"  ;
- In this version, the complexity of nested control structures has been
- elminated by factoring. The Forth interpreter has been made part of
- the game (LOW and HIGH are not strings decoded by the program -- but
- are now Forth words). The low and high bounds and the guess were put
- in variables rather than left on the stack: if the user puts in a bad
- response, the Forth interpreter will trap the error -- but also will
- clear the stack. Putting the numbers in variables allows the game to
- continue after an error.
- While I won't claim that this version is the ultimate, I do feel that
- someone can read the code and understand what is going on. I do not
- feel this is true of either of the earlier versions.
- ======================
- To Wil Baden and others who may be interested in this:
-     I recently was asked by someone why I was using Forth to do work
- in Computer Algebra rather than Scheme or a Computer Algebra system
- (like MACSYMA, Maple, Mathematica, etc). I answered that, among other
- things, I feel that it is easier to write clear code in Forth. To
- prove my point, I found a computer algebra project for which I have
- code written "by the masters" (by Ableson and Sussman^2 in Scheme, and
- by David Stoutemyer in MuSIMP -- the implementation language for the
- MuMATH computer algebra system he created). In both cases the authors
- were explaining to students how to do the project -- so I must assume
- they tried to make their work a model of clarity. I think that the
- improvement of the Forth version over the other is substantial. The
- difference would be visible to anyone who knows the three languages
- and a bit about symbol manipulation. [The comparison is too long to
- post, but I would be willing to email it -- and I would welcome
- detailed comments by others who are interested in using Forth in
- mathematics]
-    I believe that, potentially, Forth can be quite clear -- just as I
- think, potentially, that Forth can be quite portable. In both cases
- the barriers are not in limitations of the language.
-                                                   John J Wavrik
-              jjwavrik@ucsd.edu                    Dept of Math  C-012
-                                                   Univ of Calif - San Diego
-                                                   La Jolla, CA  92093
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