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

Minecraft 1.20.2 - Popular sandbox build...
Minecraft allows players to build constructions out of textured cubes in a 3D procedurally generated world. Other activities in the game include exploration, gathering resources, crafting, and combat... Read more
HoudahSpot 6.4.1 - Advanced file-search...
HoudahSpot is a versatile desktop search tool. Use HoudahSpot to locate hard-to-find files and keep frequently used files within reach. HoudahSpot is a productivity tool. It is the hub where all the... Read more
coconutBattery 3.9.14 - Displays info ab...
With coconutBattery you're always aware of your current battery health. It shows you live information about your battery such as how often it was charged and how is the current maximum capacity in... Read more
Keynote 13.2 - Apple's presentation...
Easily create gorgeous presentations with the all-new Keynote, featuring powerful yet easy-to-use tools and dazzling effects that will make you a very hard act to follow. The Theme Chooser lets you... Read more
Apple Pages 13.2 - Apple's word pro...
Apple Pages is a powerful word processor that gives you everything you need to create documents that look beautiful. And read beautifully. It lets you work seamlessly between Mac and iOS devices, and... Read more
Numbers 13.2 - Apple's spreadsheet...
With Apple Numbers, sophisticated spreadsheets are just the start. The whole sheet is your canvas. Just add dramatic interactive charts, tables, and images that paint a revealing picture of your data... Read more
Ableton Live 11.3.11 - Record music usin...
Ableton Live lets you create and record music on your Mac. Use digital instruments, pre-recorded sounds, and sampled loops to arrange, produce, and perform your music like never before. Ableton Live... Read more
Affinity Photo 2.2.0 - Digital editing f...
Affinity Photo - redefines the boundaries for professional photo editing software for the Mac. With a meticulous focus on workflow it offers sophisticated tools for enhancing, editing and retouching... Read more
SpamSieve 3.0 - Robust spam filter for m...
SpamSieve is a robust spam filter for major email clients that uses powerful Bayesian spam filtering. SpamSieve understands what your spam looks like in order to block it all, but also learns what... Read more
WhatsApp 2.2338.12 - Desktop client for...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more

Latest Forum Discussions

See All

Square Enix commemorates one of its grea...
One of the most criminally underused properties in the Square Enix roster is undoubtedly Parasite Eve, a fantastic fusion of Resident Evil and Final Fantasy that deserved far more than two PlayStation One Games and a PSP follow-up. Now, however,... | Read more »
Resident Evil Village for iPhone 15 Pro...
During its TGS 2023 stream, Capcom showcased the Following upcoming ports revealed during the Apple iPhone 15 event. Capcom also announced pricing for the mobile (and macOS in the case of the former) ports of Resident Evil 4 Remake and Resident Evil... | Read more »
The iPhone 15 Episode – The TouchArcade...
After a 3 week hiatus The TouchArcade Show returns with another action-packed episode! Well, maybe not so much “action-packed" as it is “packed with talk about the iPhone 15 Pro". Eli, being in a time zone 3 hours ahead of me, as well as being smart... | Read more »
TouchArcade Game of the Week: ‘DERE Veng...
Developer Appsir Games have been putting out genre-defying titles on mobile (and other platforms) for a number of years now, and this week marks the release of their magnum opus DERE Vengeance which has been many years in the making. In fact, if the... | Read more »
SwitchArcade Round-Up: Reviews Featuring...
Hello gentle readers, and welcome to the SwitchArcade Round-Up for September 22nd, 2023. I’ve had a good night’s sleep, and though my body aches down to the last bit of sinew and meat, I’m at least thinking straight again. We’ve got a lot to look at... | Read more »
TGS 2023: Level-5 Celebrates 25 Years Wi...
Back when I first started covering the Tokyo Game Show for TouchArcade, prolific RPG producer Level-5 could always be counted on for a fairly big booth with a blend of mobile and console games on offer. At recent shows, the company’s presence has... | Read more »
TGS 2023: ‘Final Fantasy’ & ‘Dragon...
Square Enix usually has one of the bigger, more attention-grabbing booths at the Tokyo Game Show, and this year was no different in that sense. The line-ups to play pretty much anything there were among the lengthiest of the show, and there were... | Read more »
Valve Says To Not Expect a Faster Steam...
With the big 20% off discount for the Steam Deck available to celebrate Steam’s 20th anniversary, Valve had a good presence at TGS 2023 with interviews and more. | Read more »
‘Honkai Impact 3rd Part 2’ Revealed at T...
At TGS 2023, HoYoverse had a big presence with new trailers for the usual suspects, but I didn’t expect a big announcement for Honkai Impact 3rd (Free). | Read more »
‘Junkworld’ Is Out Now As This Week’s Ne...
Epic post-apocalyptic tower-defense experience Junkworld () from Ironhide Games is out now on Apple Arcade worldwide. We’ve been covering it for a while now, and even through its soft launches before, but it has returned as an Apple Arcade... | Read more »

Price Scanner via MacPrices.net

New low price: 13″ M2 MacBook Pro for $1049,...
Amazon has the Space Gray 13″ MacBook Pro with an Apple M2 CPU and 256GB of storage in stock and on sale today for $250 off MSRP. Their price is the lowest we’ve seen for this configuration from any... Read more
Apple AirPods 2 with USB-C now in stock and o...
Amazon has Apple’s 2023 AirPods Pro with USB-C now in stock and on sale for $199.99 including free shipping. Their price is $50 off MSRP, and it’s currently the lowest price available for new AirPods... Read more
New low prices: Apple’s 15″ M2 MacBook Airs w...
Amazon has 15″ MacBook Airs with M2 CPUs and 512GB of storage in stock and on sale for $1249 shipped. That’s $250 off Apple’s MSRP, and it’s the lowest price available for these M2-powered MacBook... Read more
New low price: Clearance 16″ Apple MacBook Pr...
B&H Photo has clearance 16″ M1 Max MacBook Pros, 10-core CPU/32-core GPU/1TB SSD/Space Gray or Silver, in stock today for $2399 including free 1-2 day delivery to most US addresses. Their price... Read more
Switch to Red Pocket Mobile and get a new iPh...
Red Pocket Mobile has new Apple iPhone 15 and 15 Pro models on sale for $300 off MSRP when you switch and open up a new line of service. Red Pocket Mobile is a nationwide service using all the major... Read more
Apple continues to offer a $350 discount on 2...
Apple has Studio Display models available in their Certified Refurbished store for up to $350 off MSRP. Each display comes with Apple’s one-year warranty, with new glass and a case, and ships free.... Read more
Apple’s 16-inch MacBook Pros with M2 Pro CPUs...
Amazon is offering a $250 discount on new Apple 16-inch M2 Pro MacBook Pros for a limited time. Their prices are currently the lowest available for these models from any Apple retailer: – 16″ MacBook... Read more
Closeout Sale: Apple Watch Ultra with Green A...
Adorama haș the Apple Watch Ultra with a Green Alpine Loop on clearance sale for $699 including free shipping. Their price is $100 off original MSRP, and it’s the lowest price we’ve seen for an Apple... Read more
Use this promo code at Verizon to take $150 o...
Verizon is offering a $150 discount on cellular-capable Apple Watch Series 9 and Ultra 2 models for a limited time. Use code WATCH150 at checkout to take advantage of this offer. The fine print: “Up... Read more
New low price: Apple’s 10th generation iPads...
B&H Photo has the 10th generation 64GB WiFi iPad (Blue and Silver colors) in stock and on sale for $379 for a limited time. B&H’s price is $70 off Apple’s MSRP, and it’s the lowest price... Read more

Jobs Board

Housekeeper, *Apple* Valley Villa - Cassia...
Apple Valley Villa, part of a 4-star senior living community, is hiring entry-level Full-Time Housekeepers to join our team! We will train you for this position and Read more
Housekeeper, *Apple* Valley Village - Cassi...
Apple Valley Village Health Care Center, a 4-star rated senior care campus, is hiring a Part-Time Housekeeper to join our team! We will train you for this position! Read more
Optometrist- *Apple* Valley, CA- Target Opt...
Optometrist- Apple Valley, CA- Target Optical Date: Sep 23, 2023 Brand: Target Optical Location: Apple Valley, CA, US, 92308 **Requisition ID:** 796045 At Target Read more
Senior *Apple* iOS CNO Developer (Onsite) -...
…Offense and Defense Experts (CODEX) is in need of smart, motivated and self-driven Apple iOS CNO Developers to join our team to solve real-time cyber challenges. Read more
*Apple* Systems Administrator - JAMF - Activ...
…**Public Trust/Other Required:** None **Job Family:** Systems Administration **Skills:** Apple Platforms,Computer Servers,Jamf Pro **Experience:** 3 + years of Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.