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

Adobe Animate CC 2018 18.0.1.115 - Anima...
Animate CC 2018 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Flash Professional customer). Animate CC 2018 (was Flash CC) lets you... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
Carbon Copy Cloner 5.0.5 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Bartender 3.0.32 - Organize your menu-ba...
Bartender lets you organize your menu-bar apps by hiding them, rearranging them, or moving them to Bartender's Bar. You can display the full menu bar, set options to have menu-bar items show in the... Read more
Adobe Lightroom Classic CC 7.1 - Import,...
Adobe Lightroom is available as part of Adobe Creative Cloud for as little as $9.99/month bundled with Photoshop CC as part of the photography package. Lightroom 6 is also available for purchase as a... Read more
Ortelius 2.0.8 - Vector drawing app espe...
Ortelius is a full-featured vector drawing application especially for map design. Draw directly with features such as roads, rivers, coastlines, buildings, symbols and contours. Ortelius is known for... Read more
Tunnelblick 3.7.4b - GUI for OpenVPN.
Tunnelblick is a free, open source graphic user interface for OpenVPN on OS X. It provides easy control of OpenVPN client and/or server connections. It comes as a ready-to-use application with all... Read more
Carbon Copy Cloner 5.0.5 - Easy-to-use b...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Postbox 5.0.22 - Powerful and flexible e...
Postbox is a new email application that helps you organize your work life and get stuff done. It has all the elegance and simplicity of Apple Mail, but with more power and flexibility to manage even... Read more

Latest Forum Discussions

See All

Amazing Katamari Damacy guide - beginner...
Amazing Katamari Damacy brings the bizarro world of the original games to mobile and shifts them into an endless format that's just as addictive as the PlayStation entries. Your goal is still to roll as much random stuff as you possibly can, though... | Read more »
Portal Knights guide - crafting tips and...
In Portal Knights, you're only as strong as the items you have at your disposal. This sandbox adventure is all about crafting and building up the next big thing. Whether you're an avid explorer or collector, crafting will likely play a large part... | Read more »
The best deals on the App Store this wee...
A new week means new discounts on the App Store. This week's deals run the gamut of action-adventure titles, puzzle games, and one of the best narrative adventure series out there. If you're looking to fill out your mobile gaming library on a... | Read more »
What you need to know about Animal Cross...
We hope you've been hard at work on collecting all of those holiday items in Animal Crossing: Pocket Camp, because you're about to get a whole new list of fun things to do as the game receives its first big update sometime soon. There are a lot of... | Read more »
Reigns: Her Majesty guide - how to use e...
Ruling a kingdom isn't easy--doubly so for a queen whose every decision is questioned by the other factions seeking a slice of power. Reigns: Her Majesty builds on the original game's swipey tactics, adding items that you can use to move the story... | Read more »
The best new games we played this week -...
Friday has crept up on us once again, so it's time to honor the best new games we've played over the past few days. This past week was a pretty exciting one, with the debut of lots of beautiful new indies and some familiar faces returning to the... | Read more »
Portal Knights guide- beginner tips and...
Portal Knights is finally making the jump to iOS and Android, and it's already climbing the ranks to become the next big MMO experience on mobile. This sprawling sandbox game will let you pursue any adventure you wish, whether you want to sling... | Read more »
Reigns: Her Majesty guide - how to swipe...
Reigns: Her Majesty is storming the App Store this week, bringing more tinder-esque kingdom building to eager players everywhere. If you've played the original Reigns, you'll know that leading a kingdom is never easy. It's a careful balancing act... | Read more »
Getting Over It (Games)
Getting Over It 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: A game I madeFor a certain kind of person To hurt them. • Climb up an enormous mountain with nothing but a hammer and a pot.•... | Read more »
Reigns: Her Majesty (Games)
Reigns: Her Majesty 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: | Read more »

Price Scanner via MacPrices.net

Apple Watch Series 2, Certified Refurbished,...
Apple has Certified Refurbished Apple Watch Nike+ Series 2s, 42mm Space Gray Aluminum Case with Anthracite/Black Nike Sport Bands, available for $249 (38mm) or $279 (42mm). The 38mm model was out of... Read more
Apple offers Certified Refurbished 2016 12″ R...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $949. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
B&H drops price on 13″ 256GB MacBook Air...
B&H has the 13″ 1.8GHz/256GB Apple MacBook Air (MQD42LL/A) now on sale for $1079 including free shipping plus NY & NJ sales tax only. Their price is $120 off MSRP, and it’s the lowest price... Read more
Holiday sale: 9″ iPads starting at $299, take...
MacMall has 9″ WiFi iPads on sale for $30 off including free shipping: – 9″ 32GB WiFi iPad: $299 – 9″ 128GB WiFi iPad: $399 Read more
Green Monday deal: 15″ 2.8GHz MacBook Pro on...
B&H Photo has the 15″ 2.8GHz Space Gray MacBook Pro on sale for $250 off MSRP for today only as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax for NY... Read more
Green Monday sale: B&H offers 12″ Apple i...
B&H Photo has 12″ iPad Pros on sale for up to $150 off MSRP as part of their Green Monday/Holiday sale. Shipping is free, and B&H charges sales tax in NY & NJ only: – 12″ 64GB WiFi iPad... Read more
Holiday deal: 21″ and 27″ Apple iMacs on sale...
MacMall has 2017 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP. Shipping is free: – 21″ 2.3GHz iMac: $999 $100 off MSRP – 21″ 3.0GHz iMac: $1199 $100 off MSRP – 21″ 3.4GHz iMac: $1379 $120... Read more
Holiday deal: Apple Mac minis for up to $150...
MacMall has Mac minis on sale for up to $100 off MSRP, each including free shipping: – 1.4GHz Mac mini: $399 $100 off MSRP – 2.6GHz Mac mini: $599 $100 off MSRP – 2.8GHz Mac mini: $949 $50 off MSRP... Read more
Beats by Dr. Dre – BeatsX Earphones on sale f...
Best Buy has BeatsX Earphones on sale for $109, $40 off, on their online store. Sale price for online orders only. Choose free store pickup, if available, or choose free shipping. Read more
10″ 64GB WiFi Apple iPad Pros on sale for $59...
MacMall has 10.5″ 64GB Apple iPad Pros on sale for $599 including free shipping. That’s $50 off MSRP and among the lowest prices available for these iPads from any Apple reseller. Read more

Jobs Board

QA Automation Engineer, *Apple* Pay - Apple...
# QA Automation Engineer, Apple Pay Job Number: 113202642 Santa Clara Valley, California, United States Posted: 11-Dec-2017 Weekly Hours: 40.00 **Job Summary** At Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions - Apple,...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Information Security - Security Data...
# Apple Information Security - Security Data Analyst Job Number: 113119545 Austin, Texas, United States Posted: 10-Nov-2017 Weekly Hours: 40.00 **Job Summary** This Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.