Great Ideas in Computer Science

Karel the Robot (continued)

            define-new-instruction <new instr name> as
               <instr>

        where:

        *    "define-new-instruction" and "as" are
            reserved words

          *    <new instr name> is the name you give
              to your new instruction

        *    <instr> can be:

                (a)    any of the 5 basic instructions
          OR  (b)    a sequence of basic or
                        previously-defined instructions,
                        separated by semicolons (";"),]
                        and surrounded by the reserved
                        words "begin" and "end"

        e.g., define-new-instruction changelocation as
                    move

        e.g., define-new-instruction changedirection as
                    turnleft

        e.g., define-new-instruction write as
                    putbeeper

         e.g., define-new-instruction erase as
                    pickbeeper

        e.g., define-new-instruction halt as
                    turnoff

        e.g., define-new-instruction tl as
                    turnleft

        e.g., define-new-instruction turnright as
                    begin
                        turnleft;
                        turnleft;
                        turnleft
                    end

NOTE:  In a real Karel program,

            *    the define-new-instruction command
                 goes after the
                 "beginning-of-program" reserved word,

            *    it must be separated from the next part
                 of the program (which will either be the
                 "beginning-of-execution" reserved word
                 or else another  "define-new-instruction")
                 by a semicolon

            *     you don't need a semicolon before the
                   reserved word "end", but the compiler
                    in our implementation will complain
                    if you leave it out :-)

    e.g., the program from Lecture Notes #5 could
            now be written more briefly as:

            beginning-of-program
        define-new-instruction turnright as
          begin
            turnleft;
            turnleft;
             turnleft
          end;
        beginning-of-execution
          move;
          turnright;
          move;
          pickbeeper;
          turnleft;
          turnoff
        end-of-execution
       end-of-program

    NOTE:  Karel will execute this program exactly as
                he executed the earlier version

                (in particular, he will continue to turnleft
                3 times):

                You cannot create new instructions using
                "define-new-instruction";

                you can only give a new name to old ones.

    NOTE:  From now on, when I describe worlds,
                I will write (A, S) to represent Ath Ave.
                and Sth St.

    e.g.,    Initial Situation:    Karel at origin, facing N;
                                          1 beeper each at:
                                                (2,2), (3,3), (4,4)

              Final Situation:        Karel at (4,4), facing N;
                                            no beepers at any
                                            corners

             long solution:
                (21 instructions, not counting reserved
                words or "turnoff")

                (note that each group of 7 instructions
                is the same as for the previous program!)

            beginning-of-program
         beginning-of-execution
           move;
           turnleft;
           turnleft;
           turnleft;
           move;
           pickbeeper;
           turnleft;

           move;
           turnleft;
           turnleft;
           turnleft;
           move;
           pickbeeper;
           turnleft;

           move;
           turnleft;
           turnleft;
           turnleft;
           move;
           pickbeeper;
           turnleft;

           turnoff
          end-of-execution
         end-of-program

            medium solution:
            (15 instructions)

            beginning-of-program
        define-new-instruction turnright as
          begin
            turnleft;
            turnleft;
            turnleft
          end;
        beginning-of-execution
          move;
          turnright;
          move;
          pickbeeper;
          turnleft;

          move;
          turnright;
          move;
          pickbeeper;
          turnleft;

          move;
          turnright;
          move;
          pickbeeper;
          turnleft;

          turnoff
 
        end-of-execution
       end-of-program

            short solution:
            (3 instructions!)

            beginning-of-program
        define-new-instruction turnright as
           begin
             turnleft;
             turnleft;
             turnleft
           end;
        define-new-instruction getbeeper as
           begin
             move;
             turnright;
             move;
             pickbeeper;
             turnleft
           end;
         beginning-of-execution
           getbeeper;
           getbeeper;
           getbeeper;
           turnoff
         end-of-execution
        end-of-program

NOTES:

 
*    The definition of "getbeeper" must come after
        the definition of "turnright", since "getbeeper"
        is defined in terms of "turnright"

*    The short program takes Karel just as long to
        execute as the long program (maybe a bit
        longer, since he has to look up the definitions
        of the new instructions each time he hears one)

*    But the short program is easier for us humans to
        type, to read, and to understand

*    It also shows the structure of the program more
         clearly than the longer programs.