TweetFollow Us on Twitter

Draw Towers
Volume Number:1
Issue Number:7
Column Tag:STRUCTURED PRogramming in Modula-2

Draw the Towers of Hanoi

By John Bogan

This month we will explore three items. First, we will continue to introduce the elements of Software Engineering in a historical perspective. Second, we will discuss why Modula-2 is not a hackers’ language and finally, we will look at a sample program that uses the Mac ROM to draw the starting position for the Towers of Hanoi.

A New Direction

Last month we saw that Software Engineering is capable of being abused as well as being able to provide important insights into the process of building good computer software. By the late 1960’s the Software Engineers had, in effect, dictated that COBOL would be the primary language of the FORTUNE 1000 probably until the end of the century. Most microcomputer programmers faced with the prospect of coding in COBOL would shudder in terror at the thought.

In 1968 Software Engineering took a turn for the better when a famous and well regarded European computer scientist lit a fire under the COBOL and FORTRAN programming community, a fire whose embers still smolder and flare up to this day. The scientist was E.W. Dijkstra and the arson was committed in the Communications of the ACM with a letter entitled “GOTO Statement Considered Harmful”. In this letter Dijkstra observed that after having read a multitude of programs in a variety of languages that the quality of a program was inversely proportional to the number of GOTO statements in that program. The graph below illustrates this discovery.

The idea is simple ... jumping around a program with branching statements leads to unreadable program texts (known in the trade as spaghetti code) which are next to impossible to debug or for a third party to pick up and read. Since ALGOL-60 (the European’s favorite language) has advanced control structures which permits GOTOless programming and FORTRAN doesn’t, the battle lines were drawn. Letter after letter poured into the journals feeding the flames.

Finally in 1972 in an effort to quell the controversy Dijkstra together with Dahl and Hoare published a book on just how to write high quality programs without using the dangerous GOTO statement. This book, Structured Programming, estab- lished once and for all that GOTOs are redundant. Every sequential program- ming task can be accomplished with a combination of three constructs.

• sequence:

BEGIN ... s1 ... s2 ... s3 ... END

• iteration:

WHILE c1 DO ... s1 ... ENDWHILE

• conditional:

IF c1 THEN s1 ENDIF

If acceptance in the curriculum of the worlds’ Universities’ Computer Science Departments is a valid measure then Structured Programming is an overwhelming success. Only those programmers corrupted by traditional BASIC or Assembler still grasp at the past and argue the merits of the GOTO. Meanwhile the course of Software Engineering was changed forever. An example of this change is Modula-2. This language is rich in structured control statements and does not support the GOTO at all. There are no statement labels in Modula-2 and while it is possible to write poor code in Modula it is impossible to write spaghetti code.

The structured control statements supported by Modula are the statement sequence, the WHILE ... DO, the REPEAT ... UNTIL, the FOR ... TO ... BY ... DO, the LOOP ... EXIT, the IF ... THEN ... ELSIF, the CASE ... OF and the WITH ... DO statements.

What is Structured?

In some ways this is a very difficult question to answer. For adherents of the Structured Techniques the concept of “Structured” is very much like the Bible is to Jerry Falwell. It is the guiding light, the one true path to paradise, the blessed and final word on how to think about solving complex logical problems.

A slightly more dispassionate view might produce the following definition of structured - a philosophy for solving problems which attempts to conserve scarce resources by arriving at the perfect solution in the fewest attempts by following a plan.

Why Plan When You Can Hack?

The idea of a plan is very important in understanding the Structured Techniques, Software Engineering and Modula-2. It also illustrates why Modula-2 is not particularly well suited for hacking. Most hackers I have known use the technique of incremental discovery or trial by error. In other words programs just grow from line 1 until the last bell and whistle is debugged. Assembly language and to a lesser extent C are well suited for this type of programming. Modula-2 most definitely is not. As we will see in future columns the quaility of a Modula-2 program is dependent on the quality of the detailed planning that occurs before the first line of code is written. In many ways this dependence on upfront planning is a distinct disadvantage for learning a new and unique system like the Mac. So many of the techniques peculiar to the Mac (such as the entire user interface or resources or Quickdraw) are best approached and mastered by trial and error hacks. When you combine this reality with the compile-link-execute overhead of Modula-2 it should be obvious why Modula-2 is not particularly suited to casual hacking. A good strategy for making the best use of Modula-2 would be to learn the Mac with Apple’s interpreted Pascal and then to translate these programs into the much faster Modula-2. As we progress in these columns we will see just how close Modula and Pascal are to each other so this suggestion won’t seem so painful. The bottom line is that Modula-2 is not a language for the seat-of-the-pants hacker.

This Month’s Code

The piece of Modula-2 code that follows is primarily useful because it shows how to access the Mac ROM on a 128K machine. The Quickdraw calls used are SetRect, PaintRect and PaintRoundRect. You should be aware that the method of specifying ROM calls is different for a 512K box. Also it should be noted that the data types VHSelect, Point and Rect could have been imported blindly instead of spelled out but then their internal structures would have been hidden and the topic of information hiding in Modula-2 is an advanced and complex issue.

MODULE Hanoi;
   (* build starting position for Hanoi Towers *)

   FROM Terminal IMPORT ClearScreen;
   FROM InOut IMPORT WriteString, ReadCard, WriteLn;
  
   (* data structures for Quickdraw calls *)
   TYPE 
      VHSelect = (v,h);
      
      Point = RECORD
                 CASE INTEGER OF
     0: v: INTEGER;
        h: INTEGER;
        
    |1: vh: ARRAY VHSelect OF INTEGER;
  END; (* CASE *)
       END; (* RECORD *)
       
      Rect = RECORD
                CASE INTEGER OF
    0: top: INTEGER;
       left: INTEGER;
       bottom: INTEGER;
       right: INTEGER;
       
   |1: topLeft: Point;
       botRight: Point;
  END; (* CASE *)
              END; (* RECORD *)
       
   CONST
     CX = 355B;
     QuickDraw1ModNum = 2; (* absolute module number 
        of QuickDraw1 *)
   VAR
      r: Rect; NumDisks: CARDINAL;
      
   PROCEDURE SetRect (VAR r: Rect; left,top,right,bottom: INTEGER);
      CODE CX; QuickDraw1ModNum; 51 END SetRect;
   
   PROCEDURE PaintRect  (r: Rect);
      CODE CX; QuickDraw1ModNum; 62 END PaintRect;
   
   PROCEDURE PaintRoundRect(r: Rect; ovWd, ovHt: INTEGER);
      CODE CX; QuickDraw1ModNum; 67 END PaintRoundRect;
   
   PROCEDURE DrawBase;
      CONST 
  BaseLeft = 36;
  BaseTop = 261;
  BaseRight = 476;
  BaseBottom = 270;
   BEGIN 
      SetRect(r,BaseLeft,BaseTop,BaseRight,BaseBottom);
      PaintRect(r);
   END DrawBase;
   
   PROCEDURE DrawPosts;
      CONST
         PostTop = 144;
  PostBottom = 261;
  PostWidth = 6;
  HalfPostWidth = PostWidth DIV 2;
  PostPosition = 128;
      VAR
         n, PostLeft, PostRight: INTEGER;
   BEGIN
      n:=1;
      WHILE n <= 3 DO
  PostLeft := (PostPosition * n) - HalfPostWidth;
  PostRight := PostLeft + PostWidth;
  SetRect(r,PostLeft,PostTop,PostRight,PostBottom);
  PaintRect(r);
  n:= n + 1;
      END; (* WHILE *)
   END DrawPosts;
   
   PROCEDURE DrawVarDisks(numberofdisks: CARDINAL);
      CONST
         bigdiskleft = 128 - 60;
  bigdisktop = 261 - 12;
  bigdiskright = 128 + 60;
  bigdiskbottom = 261;
  deltalength = 5;
  deltadepth = 12;
      VAR leftedge, topedge, rightedge, bottomedge: INTEGER;
          i: CARDINAL;
   BEGIN
      IF (numberofdisks > 2) AND (numberofdisks < 10)
         THEN
     leftedge := bigdiskleft; topedge := bigdisktop;
     rightedge := bigdiskright; bottomedge := bigdiskbottom;
     SetRect(r,leftedge,topedge,rightedge,bottomedge);
     PaintRoundRect(r,40,40);
     FOR i := 1 TO numberofdisks - 1 DO
        leftedge := leftedge + deltalength;
        topedge := topedge - deltadepth;
        rightedge := rightedge - deltalength;
        bottomedge := bottomedge - deltadepth;
        SetRect(r,leftedge,topedge,rightedge,bottomedge);
        PaintRoundRect(r,40,40);
     END; (* FOR *)
  END; (* IF *)
   END DrawVarDisks;
   
   PROCEDURE GetInput(VAR NDisks: CARDINAL);
   BEGIN
      ClearScreen;
      WriteString(“Enter number of disks (between 3 to 9)”);
      WriteLn;
      WriteString(“To quit - enter number out of range”);
      ReadCard(NDisks);
      ClearScreen;
   END GetInput;
   
   PROCEDURE InitGraphics(NumberofDisks: CARDINAL);
   BEGIN
      DrawBase;
      DrawPosts;
      DrawVarDisks(NumberofDisks);
   END InitGraphics;
   
   PROCEDURE ExecuteTowers;
   VAR Delay: CARDINAL;
   BEGIN
      FOR Delay := 1 TO 30000 DO  END; (* FOR *)
   END ExecuteTowers;
   
BEGIN
   GetInput(NumDisks);
   WHILE (NumDisks >= 3) AND (NumDisks <= 9) DO
      InitGraphics(NumDisks);
      ExecuteTowers;
      GetInput(NumDisks);
   END; (* WHILE *)
END Hanoi.
     

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

The beginner's guide to Warbits
Warbits is a turn-based strategy that's clearly inspired by Nintendo's Advance Wars series. Since turn-based strategy games can be kind of tricky to dive into, see below for a few tips to help you in the beginning. Positioning is crucial [Read... | Read more »
How to upgrade your character in Spellsp...
So you’ve mastered the basics of Spellspire. By which I mean you’ve realised it’s all about spelling things in a spire. What next? Well you’re going to need to figure out how to toughen up your character. It’s all well and good being able to spell... | Read more »
5 slither.io mash-ups we'd love to...
If there's one thing that slither.io has proved, it's that the addictive gameplay of Agar.io can be transplanted onto basically anything and it will still be good fun. It wouldn't be surprising if we saw other developers jumping on the bandwagon,... | Read more »
How to navigate the terrain in Sky Charm...
Sky Charms is a whimsical match-'em up adventure that uses creative level design to really ramp up the difficulty. [Read more] | Read more »
Victorious Knight (Games)
Victorious Knight 1.3 Device: iOS Universal Category: Games Price: $1.99, Version: 1.3 (iTunes) Description: New challenges awaits you! Experience fresh RPG experience with a unique combat mechanic, packed with high quality 3D... | Read more »
Agent Gumball - Roguelike Spy Game (Gam...
Agent Gumball - Roguelike Spy Game 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: Someone’s been spying on Gumball. What the what?! Two can play at that game! GO UNDERCOVERSneak past enemy... | Read more »
Runaway Toad (Games)
Runaway Toad 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: It ain’t easy bein’ green! Tap, hold, and swipe to help Toad hop to safety in this gorgeous new action game from the creators of... | Read more »
PsyCard (Games)
PsyCard 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: From the makers och Card City Nights, Progress To 100 and Ittle Dew PSYCARD is a minesweeper-like game set in a cozy cyberpunk... | Read more »
Sago Mini Robot Party (Education)
Sago Mini Robot Party 1.0 Device: iOS Universal Category: Education Price: $2.99, Version: 1.0 (iTunes) Description: -- Children's Technology Review Editor's Choice -- | Read more »
Egz – The Origin of the Universe (Games...
Egz – The Origin of the Universe 1.0.2 Device: iOS Universal Category: Games Price: $3.99, Version: 1.0.2 (iTunes) Description: ►►► Special offer until 2nd may : get the game at 2.99€ instead of 3.99€ ! ◄◄◄ Egz is a mesmerizing mix... | Read more »

Price Scanner via MacPrices.net

Apple price trackers, updated continuously
Scan our Apple Price Trackers for the latest information on sales, bundles, and availability on systems from Apple’s authorized internet/catalog resellers. We update the trackers continuously: - 15″... Read more
SanDisk Half-Terabyte SSD Optimized for Every...
SanDisk Corporation has announced the SanDisk Z410 SSD, a cost-competitive, half-terabyte solid state drive (SSD) that enables manufacturers to design for a broad range of desktop PCs and laptops.... Read more
Churchill Downs Racetrack Selects VenueNext t...
Churchill Downs Racetrack has announced an agreement with VenueNext to implement its technology platform for the start of Churchill Downs 2016 Spring Meet, which includes the 142nd running of the... Read more
Record 700 Million Pounds of CE Recycled in 2...
The Consumer Technology Association (CTA) reports that a record-setting 700 million pounds of consumer electronics (CE) have been recycled under the eCycling Leadership Initiative (ELI). According to... Read more
Price drops on clearance 12-inch Retina MacBo...
B&H Photo has dropped prices on leftover 2015 12″ Retina MacBooks with models now available starting at $999. Shipping is free, and B&H charges NY tax only: - 12″ 1.1GHz Gray Retina MacBook... Read more
15-inch Retina MacBook Pros available for $20...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $210 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1799 $200 off MSRP - 15″ 2.5GHz Retina... Read more
Target offers Apple Watch Sport for $50 off M...
Target has Apple Watch Sports on sale for $50 off MSRP for a limited time. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store prices may vary... Read more
Apple restocks Certified Refurbished Mac mini...
Apple has restocked Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon.com has the 15″ 2.2GHz Retina MacBook Pro on sale for $1699.99 including free shipping. Their price is $300 off MSRP, and it’s the lowest price available for this model from any reseller (and... Read more
Apple Beats Microsoft at Own Game; Amazon Pri...
First quarter seasonality combined with an overall disinterested customer base led to an annual decline of 14.7% in worldwide tablet shipments during the first quarter of 2016 (1Q16). Worldwide... Read more

Jobs Board

Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
Simply Mac *Apple* Specialist- Service Repa...
Simply Mac is the largest premier retailer of Apple products in the nation. In order to support our growing customer base, we are currently looking for a driven Read more
*Apple* Retail - Multiple Positions - Apple,...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
Restaurant Manager (Neighborhood Captain) - A...
…in every aspect of daily operation. WHY YOU'LL LIKE IT: You'll be the Big Apple . You'll solve problems. You'll get to show your ability to handle the stress and Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.