In our continuing study of the spider, all packages and programs have assumed
that only one spider lives in the room. How could we emulate a situation in
which the room contains an entire village of spiders? As we saw in Chapter 7,
the spider package implements what we now know as an abstract data object. To
provide for multiple spiders, we need an abstract data type.
Program
10.19 gives a specification for an ADT package Spiders. This
package provides all the operations of Spider and also a type,
allowing a client program to declare spider variables. As a close reading will
show, additional operations are provided as well.
Program 10.19
PACKAGE Spiders IS
------------------------------------------------------------------------
--| This package provides procedures to emulate multiple
--| spiders. The spiders can move around
--| the screen drawing simple patterns.
--| Author: John Dalbey, Cal Poly San Luis Obispo, 1992
--| Adapted by: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------
TYPE Switch IS (On, Off);
TYPE Directions IS (North, East, South, West);
TYPE ScreenColors IS (Red, Blue, Green, Black);
MaxHeight: Positive := 20;
MaxWidth: Positive := 20;
SUBTYPE RoomHeight IS Positive RANGE 1..MaxHeight;
SUBTYPE RoomWidth IS Positive RANGE 1..MaxWidth;
TYPE Spider IS PRIVATE;
Hit_The_Wall: EXCEPTION;
PROCEDURE DrawRoom;
-- Pre: None
-- Post: Spider's room appears on the screen
PROCEDURE Start (Which: IN OUT Spider;
Row: IN RoomHeight; Col: IN RoomWidth;
WhichColor: IN ScreenColors;
WhichWay: IN Directions);
-- Pre: None
-- Post: Spider appears in the given position.
PROCEDURE Blue (Which: IN OUT Spider);
PROCEDURE Green (Which: IN OUT Spider);
PROCEDURE Red (Which: IN OUT Spider);
PROCEDURE Black (Which: IN OUT Spider);
-- Pre: None
-- Post: Change the color of ink with which the spider draws
-- On black-and-white screen, uses different characters
-- to represent the colors.
PROCEDURE Face (Which: IN OUT Spider; WhichWay: IN Directions);
-- Pre: WhichWay has been assigned a value
-- Post: Spider turns to face the given direction.
FUNCTION IsFacing (Which: Spider) RETURN Directions;
-- Pre: None
-- Post: Returns the direction the spider is facing.
PROCEDURE Step (Which: IN OUT Spider);
-- Pre: None
-- Post: Spider takes one step forward in the direction it is facing.
-- Raises: Hit_the_Wall is if spider tries to step into a wall.
PROCEDURE Jump (Which: IN OUT Spider; HowMany: IN Positive);
-- Pre: None
-- Post: Spider jumps forward the given number of steps, without
-- leaving tracks at each step.
-- Raises: Hit_the_Wall if spider tries to step into or over a wall.
PROCEDURE Right (Which: IN OUT Spider);
-- Pre: None
-- Post: Spider turns 90 degrees to the right.
FUNCTION AtWall (Which: Spider) RETURN Boolean;
-- Pre: None
-- Post: Returns True if the spider is standing next to a wall
-- (edge of the room) and facing it, and False otherwise.
FUNCTION NearWall (Which: Spider; HowMany: Positive) RETURN Boolean;
-- Pre: None
-- Post: Returns True if the spider is standing within
-- HowMany steps of a wall and facing it, and False otherwise.
PROCEDURE Quit;
-- Pre: None
-- Post: End the drawing
PROCEDURE Debug (Setting: Switch);
-- Pre: None
-- Post: Turns on or off single stepping through the program.
FUNCTION Debugging RETURN Switch;
-- Pre: None
-- Post: Returns on or Off depending on Debug setting
PRIVATE
TYPE Spider IS RECORD
Ink : ScreenColors;
Heading : Directions;
CurrentColumn : RoomWidth; -- spider's position
CurrentRow : RoomHeight; -- in the room.
END RECORD;
END Spiders;
To
give an idea of how the ADT package operates,
Program
10.20 shows several different spiders drawing boxes. We omit the package
body; you can write it as a modified version of the original
Spider body (
Program
7.11).
Program 10.20
WITH Spiders;
USE Spiders;
PROCEDURE Draw_Many_Boxes IS
------------------------------------------------------------------------
--| Draw several boxes with spiders - use loop
--| Author: Michael B. Feldman, The George Washington University
--| Last Modified: July 1995
------------------------------------------------------------------------
Spider1 : Spider;
Spider2 : Spider;
Spider3 : Spider;
BEGIN -- Draw_Many_Boxes
Debug(On);
DrawRoom;
Start (Which => Spider1, Row => 10, Col => 11,
WhichColor => Green, WhichWay => North);
Start (Which => Spider2, Row => 10, Col => 11,
WhichColor => Blue, WhichWay => North);
Right (Spider2);
Jump (Spider2, 4);
Start (Which => Spider3, Row => 10, Col => 11,
WhichColor => Red, WhichWay => North);
Right (Spider3);
Right (Spider3);
Right (Spider3);
Jump (Spider3, 3);
FOR Side IN 1..4 LOOP
FOR Count IN 1..3 LOOP
Step (Spider1);
END LOOP;
Right (Spider1);
END LOOP;
FOR Side IN 1..4 LOOP
FOR Count IN 1..4 LOOP
Step (Spider2);
END LOOP;
Right (Spider2);
END LOOP;
FOR Side IN 1..4 LOOP
FOR Count IN 1..5 LOOP
Step (Spider3);
END LOOP;
Right (Spider3);
END LOOP;
Quit;
END Draw_Many_Boxes;
Sample
Run
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Copyright © 1996 by Addison-Wesley Publishing Company, Inc.