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

BBEdit 11.1.1 - Powerful text and HTML e...
BBEdit is the leading professional HTML and text editor for the Mac. Specifically crafted in response to the needs of Web authors and software developers, this award-winning product provides a... Read more
CrossOver 14.1.3 - Run Windows apps on y...
CrossOver can get your Windows productivity applications and PC games up and running on your Mac quickly and easily. CrossOver runs the Windows software that you need on Mac at home, in the office,... Read more
Little Snitch 3.5.3 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
OmniGraffle Pro 6.2.3 - Create diagrams,...
OmniGraffle Pro helps you draw beautiful diagrams, family trees, flow charts, org charts, layouts, and (mathematically speaking) any other directed or non-directed graphs. We've had people use... Read more
OmniFocus 2.2 - GTD task manager with iO...
OmniFocus helps you manage your tasks the way that you want, freeing you to focus your attention on the things that matter to you most. Capturing tasks and ideas is always a keyboard shortcut away in... Read more
Cocktail 8.4 - General maintenance and o...
Cocktail is a general purpose utility for OS X that lets you clean, repair and optimize your Mac. It is a powerful digital toolset that helps hundreds of thousands of Mac users around the world get... Read more
PDFKey Pro 4.3 - Edit and print password...
PDFKey Pro can unlock PDF documents protected for printing and copying when you've forgotten your password. It can now also protect your PDF files with a password to prevent unauthorized access and/... Read more
Kodi 15.0.beta1 - Powerful media center...
Kodi (was XBMC) is an award-winning free and open-source (GPL) software media player and entertainment hub that can be installed on Linux, OS X, Windows, iOS, and Android, featuring a 10-foot user... Read more
DiskCatalogMaker 6.4.12 - Catalog your d...
DiskCatalogMaker is a simple disk management tool which catalogs disks. Simple, light-weight, and fast. Finder-like intuitive look and feel. Super-fast search algorithm. Can compress catalog data... Read more
Macs Fan Control 1.3.0.0 - Monitor and c...
Macs Fan Control allows you to monitor and control almost any aspect of your computer's fans, with support for controlling fan speed, temperature sensors pane, menu-bar icon, and autostart with... Read more

Moleskine Timepage – Calendar for iCloud...
Moleskine Timepage – Calendar for iCloud, Google & Exchange 1.0 Device: iOS iPhone Category: Productivity Price: $4.99, Version: 1.0 (iTunes) Description: The most elegant calendar for your pocket and wrist, Timepage is a... | Read more »
QuizUp Gets Social in its New Update
Plain Vanilla Corp has released a new and improved version of their popular trivia game, QuizUp. The app now emphasizes social play so you can challenge friends from all over the world. [Read more] | Read more »
The Deep (Games)
The Deep 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: Swipe Controls Delve into the deep in this retro rogue-like! Swipe to move your diver around and keep away from the enemies as you... | Read more »
Battle of Gods: Ascension (Games)
Battle of Gods: Ascension 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: TURN-BASED TACTICAL COMBATFight tactical battles against the forces of Hades! In Battle of Gods: Ascension you play... | Read more »
Shadowmatic's Latest Update Adds a...
Shadowmatic's shadowy shadow-ness is getting a little shadowy-er thanks to a recent update that adds an Arcade Mode. [Read more] | Read more »
Sunrise Calendar and Slack Have Assimila...
Wunderlist is perhaps one of the most populat and beloved productivity apps on the App Store - and now it's gone and incorporated itself into other useful services like Sunrise Calendar and Slack. [Read more] | Read more »
Crossy Road Devs Hipster Whale are Bring...
Hipster Whale, the minds behind the rather popular (and rather great) Crossy Road, have teamed-up with Bandai Namco to create PAC-MAN 256: an absolutely bonkers looking maze runner chaser thing. | Read more »
Meet the New Spotify Music
Spotify Music  has a lot going on. They're introducing 3 new modes to serve all your musical needs, with the "Now" start page  gives you curated playlists based on your particular tastes. As you listen the app will learn more about your tastes and... | Read more »
What the Apple Watch Gets Right, and Wha...
| Read more »
Celebrate PAC-MAN's 35th Birthday W...
BANDAI NAMCO Entertainment America is celebrating PAC-MAN's 35th anniversary by releasing updates for PAC-MAN and PAC-MAN Lite for iOS. [Read more] | Read more »

Price Scanner via MacPrices.net

Apple refurbished 2014 13-inch Retina MacBook...
The Apple Store has Apple Certified Refurbished 2014 13″ Retina MacBook Pros available for up to $400 off original MSRP, starting at $979. An Apple one-year warranty is included with each model, and... Read more
What Would the ideal Apple Productivity Platf...
For the past four years I’ve kept a foot in both the Mac and iPad camps respectively. my daily computing hours divided about 50/50 between the two devices with remarkable consistency. However, there’... Read more
PageMeUp 1.2.1 Ten Dollar Page Layout Applica...
Paris, France-based Softobe, an OS X software development company, has announced that their PageMeUp v. 1.2.1, is available on the Mac App Store for $9.99. The license can be installed on up to 5... Read more
Eight New Products For USB Type-C Application...
Fresco Logic, specialists in advanced connectivity technologies and ICs, has introduced two new product families targeting the Type-C connector recently introduced across a number of consumer... Read more
Scripps National Spelling Bee Launches Buzzwo...
Scripps National Spelling Bee fans can monitor the action at the 2015 Spelling Bee with the new Buzzworthy app for iOS, Android and Windows mobile devices. The free Buzzworthy app provides friendly... Read more
13-inch 2.5GHz MacBook Pro on sale for $120 o...
B&H Photo has the 13″ 2.5GHz MacBook Pro on sale for $979 including free shipping plus NY sales tax only. Their price is $120 off MSRP, and it’s the lowest price for this model (except for Apple’... Read more
27-inch 3.3GHz 5K iMac on sale for $1899, $10...
B&H Photo has the new 27″ 3.3GHz 5K iMac on sale for $1899.99 including free shipping plus NY tax only. Their price is $100 off MSRP. Read more
Save up to $50 on iPad Air 2, NY tax only, fr...
B&H Photo has iPad Air 2s on sale for up to $50 off MSRP including free shipping plus NY sales tax only: - 16GB iPad Air 2 WiFi: $469 $30 off - 64GB iPad Air 2 WiFi: $549.99 $50 off - 128GB iPad... Read more
Updated Mac Price Trackers
We’ve updated our Mac Price Trackers with the latest information on prices, bundles, and availability on systems from Apple’s authorized internet/catalog resellers: - 15″ MacBook Pros - 13″ MacBook... Read more
New 13-inch 2.9GHz Retina MacBook Pro on sale...
B&H Photo has the 13″ 2.9GHz/512GB Retina MacBook Pro on sale for $1699.99 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price for this model from... Read more

Jobs Board

*Apple* Solutions Consultant - Retail Sales...
**Job Summary** As an Apple Solutions Consultant (ASC) you are the link between our customers and our products. Your role is to drive the Apple business in a retail Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
Engineering Manager for *Apple* Maps on the...
…the Maps App Team get to take part in just about any new feature in Apple Maps, often contributing a majority of the feature work. In our day-to-day engineering work, we Read more
Senior Software Engineer - *Apple* SIM - Ap...
Changing the world is all in a day039s work at Apple . If you love innovation, here039s your chance to make a career of it. You039ll work hard. But the job comes with Read more
Lead *Apple* Solutions Consultant - Retail...
**Job Summary** Job Summary The Lead ASC is an Apple employee who serves as the Apple business manager and influencer in a hyper-business critical Reseller's store Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.