TweetFollow Us on Twitter

Iteration
Volume Number:1
Issue Number:10
Column Tag:Lisp Listener

"Iteration Techniques in Lisp"

By Andy Cohen, Human Factors Engineer, Hughes Aircraft, MacTutor Contributing Editor

Last month I showed how ExperLisp uses some of the simpler Quickdraw routines. For the curious, the following table contains all the Quickdraw routines discussed in the ExperLisp Reference Guide.

Quickdraw Routines

CLOSEPOLY CLOSERGN COPYRGN DIFFRGN DISPOSERGN DRAWCHAR DRAWSTRING EMPTYRGN EQUALRGN ERASEARC ERASEOVAL ERASEPOLY ERASERECT ERASERGN ERASEROUNDRECT FILLARC FILLOVAL FILLRECT FILLRGN FILLROUNDRECT FRAMEARC FRAMEOVAL FRAMEPOLY FRAMERECT FRAMERGN FRAMEROUNDRECT GETFONTINFO HIDECURSOR HIDEPEN INSETRGN INVERTARC INVERTOVAL INVERTPOLY INVERTRECT INVERTRGN INVERTROUNDRECT KILLPOLY LINE LINETO MOVE MOVETO NEWRGN OBSCURECURSOR OFFSETPOLY OFFSETRGN PAINTARC PAINTOVAL PAINTPOLY PAINTRECT PAINTRGN PAINTROUNDRECT PENMODE PENNORMAL PENPAT PENPOS PENSIZE SECTRGN SETEMPTYRGN SETRECTRGN SHOWCURSOR SHOWPEN SPACEEXTRA TEXTFACE T EXTFONT TEXTMODE TEXTSIZE UNIONRGN XORRGN

Quickdraw commands will be discussed in more detail as each command is used within the ExperLisp examples.

Iteration

Iteration is a process which provides a method of performing repetitive actions. In BASIC, iteration is accomplished with the FOR...NEXT or GOTO statements. Fortran uses the DO loop and Pascal REPEAT.....UNTIL. Experlisp has a number of different kinds of iteration, some of which look and act like those of the above.

The first kind of iteration requires use of the PROG special form. PROG stands for program and has a list of variables associated with it that are equal to nil when the PROG is evaluated. These variables are local and are bound to the PROG. The body of the PROG follows the variable list. The body contains forms or lists which are evaluated and consist of symbols, values or procedures. PROG by itself returns nil and it does not provide iteration. The special form GO within PROG is used with an arbitrary tag for iteration. The tag is placed where the loop begins within the PROG. GO tagname, placed further along within the PROG, puts the next evaluation to where the tag is. For example;

(defun iter () ;1
   (prog ((y 1)) ;2
     loop ;3
       (setq y (add1 y))  ;4
       (print y) ;5
       (if (not(= 6 y)) (go loop)))) ;6

(iter)  

When the above is compiled...

;iter
;2
;3
;4
;5
;6
;nil

The semicolon on each line of the sample is the method one uses for placing remarks within ExperLisp source code. Everything after the semicolon is ignored. The above PROG is within a defined procedure (iter). This makes it possible for the code to be compiled and then called as a procedure. Multiple PROGs are allowed within a DEFUN. However, the variables are local to each of the PROGs. It is wise to keep the number of PROGS within a defined procedure down to a minimum. The smaller the procedures, the easier to debug. Line 2 starts the PROG, declares the variable "y" and gives "y" the value of 1. ExperLisp syntax requires the variable or variables and their associated values be contained within an overall variable list. That is why the list "(y 1)" is contained within parentheses. Line 3 is the GO tag. Note, that it is not within it's own list. However, it is within the overall PROG list. In line 4 the value of y has the number 1 added to it. The value of this sum is then assigned to the symbol "y" using SETQ. Line 5 prints the value of "y" for each loop. Line 6 tests to see if the value of "y" equals ten. If it doesnt then the evaluation is sent to the GO tag, "loop". The entire process is then repeated until "(not(= 10 y))" returns "nil". If it does then the next evaluation takes place after line 6. Since Line 6 is actually the end of the PROG, nil is returned and the iteration is stopped. In order to get a value returned from a PROG one must use RETURN. RETURN not only returns a value, it also terminates the iteration. For example;

(defun iter (x)
 (prog ((y 1))
     loop
  (setq y (add1 y))
 (if(= x y) (RETURN "Y IS EQUAL TO X")
  (print y))
      (go loop)))

(iter 5)

In the above "x" is compared to "y". When found to be equal ExperLisp stops the iteration and returns the string as follows;

;Iter
;2
;3
;4
;Y IS EQUAL TO X

Early versions of Lisp use PROG and GO for iteration. Later versions, which strive to conform to the Common Lisp standard, are using a more efficient form of iteration form called DO. DO not only specifies the variable label, it can indicate an initial value, increment the value and contain a conditional for halting the iteration. The syntax for the DO special form is as follows:

(DO ((Variable   initial value 
        increment ) (Conditional))
       (form)
       (form)
       ....)

(Defun Iter2 (x)
   (DO ((y 1 (add1 y)))
       ((= y x) "Y IS EQUAL TO X")
      (Print y)))

The above is a DO version of the PROG sample, "Iter". It performs the same task however, it's output is slightly different.

; (iter2 5)
;1
;2
;3
;4
;"Y IS EQUAL TO X"

In the PROG version the addition of the number one to "y" is performed prior to the print command. Hence, the first number printed is two. In the DO version the ADD1 procedure is not performed until the second loop. The print "y" statement is performed in the first loop so that the number one is printed first. Note that when the conditional "(= y x)" is true the iteration is halted and what follows the conditional is returned. For another example, check out following:

(defun iterfun (x) ;1
   (do ((a 0 (add1 a))) ;2
       ((= a 5) 'done)  ;3
       (print (* a x))))  ;4

(iterfun 3)

;iterfun
;0 
;3 
;6 
;9 
;12 
;done

Line one of the above specifies the defined procedure and creates the symbol "x". In line two the variable "a" is initialized and assigned the value zero. On that same line the method of incrementation of the variable is also specified. Line three is the conditional and tests the variable "a" for equivalence with the number five. Line four prints the product of the value of "a" for each iteration and the value assigned to the symbol, "x". One of the features of DO is that one may use GOs and RETURNs just as in the PROG.

There is another form of DO for faster and simpler iteration. DOLIST assigns the elements of a list to a symbol. It then performs the functions within it's body upon each element of the list one at a time. It's syntax is as follows:

DOLIST (Symbol '(list)) (functions......)

(DOLIST (FUN '(Buy Mactutor Mag))
   (print fun)


;Buy
;Mactutor
;Mag
;nil

The print command is performed upon each member of the list assigned to "FUN". Note that it always returns nil when the iteration is complete. This is because the list assigned to "FUN" is exhausted. The list then contains nil.

One of the easiest to use forms of iteration (especially to someone who knows BASIC) is DOTIMES. Using DOTIMES one specifies only a variable which represents each loop and the maximum number of loops one wants performed. For example:

(Dotimes (turn 150)
   (forward (/ turn 2))
   (back (/ turn 2))
   (right turn))

When compiled or typed into the Listener Window the above list produces the following:

The symbol "turn" is initially zero. The body of DOTIMES is iterated 150 times. I used 150 so that enough lines would be drawn to make it interesting. Remember FORWARD? Well, BACK moves the pen backward. I divided "TURN" in half when the pen is moved so that the lines don't go off the column's margins. I'll continue with another form of iteration after the next couple of paragraphs.

Using the Mouse

Using the mouse as an input or interactive device with ExperLisp is quite simple. GETMOUSE reports the X-Y coordinates of the mouse's's pointer on the active window. Type the following in the Listener Window:

(GETMOUSE)
;(146  107 )

BUTTON is a predicate which returns "t" when it is evaluated and the mouse's button is held down. It returns nil when the button is not held down. The following is an example of GETMOUSE and BUTTON in iterative functions.

(defun Watch ()
   (prog ()
     look
     (if (button) (Mousey) 
     (go look))))


(defun Mousey ()
  (prog ()
   another
   (print (getmouse))
   (if (not(button)) 
    (halt) (go another))))



(defun halt ()
   (print"that's all folks!"))

(Watch)

The first of the three above procedures, Watch, waits for the mouse's button to be pressed. If it is not then the loop continues. When the button is pressed then Watch calls Mousey. Mousey will print the X-Y position of the mouse as long as the button is held down (remember, "(IF (then) (else))" ?). When the button is released Mousey calls Halt. Halt simply prints a message with no iteration.

One more form of iteration is the function, WHILE. WHILE includes a conditional. If the conditional returns true then the body of the WHILE is performed. What makes WHILE an iterative function and one that is different from those above, is that the conditional repeats the test as long as it returns true or nonnil.For example:

(While (Not(Button)) (Print(Getmouse)))

The above performs alot like "Mousey" above. However it all takes place on one line and operates from the mouse differently. Instead of showing the mouse's position with the button down, the above shows until the button is pressed down.

One aspect of ExperLisp which might not be apparent to a novice is the fact that once a procedure is defined via DEFUN, it is available until one quits ExperLisp. ExperLisp is not just a language, it is an environment. If one was to compile all of the samples in this month's issue, then each sample can be run by simply typing the procedure's name within a list in the Listener Window. The value of this fact will be more apparent after the discussion on menus in ExperLisp.

User Warning

In using ExperLisp one must be aware from the very beginning of a bug in Experlisp's design of the user interface. When one tells the Mac to save from the menu selection under "File", ExperLisp saves the active window into the active file. If one has the Listener Window as the active window and tells ExperLisp to save, the entire file will be replaced by the contents of the Listener Window. BE CAREFUL. Check which window is active before you save. Eventually, later versions of ExperLisp will have a "snapshot" feature. This feature will save the contents of the environment. One will be able to get ExperLisp back to exactly where it was when the snapshot was taken without recompiling files. This might be performed by saving with the Listener Window active. If one has a couple of files compiled and is using them in conjunction, this feature will save lots of time when saving and restarting ExperLisp.

Last month I described how one can measure a Lisp machine's capacity by seeing how many cons cells it can handle. I also described the FREECONS procedure available in the latest versions of ExperLisp (v1.02). I'd like to report that after upgrading my 512K Mac to 2 megabytes (from Levco in San Diego) I found that ExperLisp can have as many as 30,440 cons cells. However, that many cons cells is far from necessary for the simple examples contained in the Lisp Listener column. A 512K mac will do fine. In writing this article though, I put Experlisp, Macwrite and Macpaint all into the Switcher utility (v3.5). Most of which I load from a RAMdisk! No more waiting to boot up the applications, verify code, or copy and paste. I have switcher allocate 500K to ExperLisp and I can still access 5000 cons cells. When using the full two megabytes one has as many cons cells as alot of the very expensive Lisp workstations. Note that the upgrades of 1 or 2 megabytes available by most companies do not necessarily access the extra memory in the same way. I can assure all those interested that the 2 megabyte (or Monster Mac) upgrade from Levco can access the extra RAM using ExperLisp or most of the other applications which were designed in accordance with the Macintosh guidelines.

Next month I will show how to use the Mouse in interacting with the Quickdraw routines. I will also discuss recursion.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Microsoft Office 2016 15.31 - Popular pr...
Microsoft Office 2016 - Unmistakably Office, designed for Mac. The new versions of Word, Excel, PowerPoint, Outlook and OneNote provide the best of both worlds for Mac users - the familiar Office... Read more
Spotify 1.0.49.125. - Stream music, crea...
Spotify is a streaming music service that gives you on-demand access to millions of songs. Whether you like driving rock, silky R&B, or grandiose classical music, Spotify's massive catalogue puts... Read more
QuickBooks 16.1.12.1564 R13 - Financial...
QuickBooks helps you manage your business easily and efficiently. Organize your finances all in one place, track money going in and out of your business, and spot areas where you can save. Built for... Read more
Dash 4.0.1 - Instant search and offline...
Dash is an API documentation browser and code snippet manager. Dash helps you store snippets of code, as well as instantly search and browse documentation for almost any API you might use (for a full... Read more
Tinderbox 7.0.0 - Store and organize you...
Tinderbox is a personal content management assistant. It stores your notes, ideas, and plans. It can help you organize and understand them. And Tinderbox helps you share ideas through Web journals... Read more
Apple Remote Desktop Client 3.9 - Client...
Apple Remote Desktop Client is the best way to manage the Mac computers on your network. Distribute software, provide real-time online help to end users, create detailed software and hardware reports... Read more
Sparkle 2.1.1 - $79.99
Sparkle will change your mind if you thought building websites wasn't for you. Sparkle is the intuitive site builder that lets you create sites for your online portfolio, team or band pages, or... Read more
1Password 6.5.5 - Powerful password mana...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
WhatRoute 2.0.15 - Geographically trace...
WhatRoute is designed to find the names of all the routers an IP packet passes through on its way from your Mac to a destination host. It also measures the round-trip time from your Mac to the router... Read more
Art Text 3.2.2 - $49.99
Art Text is graphic design software specifically tuned for lettering, typography, text mockups and various artistic text effects. Supplied with a great variety of ready to use styles and materials,... Read more

Tiny Striker: World Football Guide - How...
| Read more »
Good news everyone! Futurama: Worlds of...
Futurama is finding a new home on mobile in TinyCo and Fox Interactive's new game, Futurama: Worlds of Tomorrow. They're really doing it up, bringing on board Futurama creator Matt Groening along with the original cast and writers. TinyCo wants... | Read more »
MUL.MASH.TAB.BA.GAL.GAL (Games)
MUL.MASH.TAB.BA.GAL.GAL 1.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0 (iTunes) Description: ENDLESS UPGRADES. CONSTANT DANGER. ANCIENT WISDOM. BOUNCY BALLS. Launch Sale, 40% OFF for a very limited time!!! MUL.... | Read more »
Dungeon Rushers (Games)
Dungeon Rushers 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: Dungeon Rushers is a 2D tactical RPG combining dungeon crawler’s gameplay and turn based fights. Manage your team, loot dusty... | Read more »
Blasty Bubs is a colorful Pinball and Br...
QuickByte Games has another arcade treat in the works -- this time it's a mishmash of brick breaking and Pinball mechanics. It's called Blasty Bubs, and it's a top down brickbreaker that has you slinging balls around a board. [Read more] | Read more »
Corsola and Heracross are the new region...
Generation 2 finally launched in Pokémon GO, unleashing a brand new batch of Pokémon into the wild. Even before the update went live people were speculating on how to catch elusive Pokémon like the legendary "dogs", Unknown, and whether or not... | Read more »
The Warlock of Firetop Mountain (Games)
The Warlock of Firetop Mountain 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: An epic adventure through a mysterious mountain filled with monsters, magic and mayhem! “...it looks downright... | Read more »
Fantasy MMORPG MU Origin’s receives a hu...
Developer Webzen are looking to take their highly popular fantasy battler MU Origin to the next level this month, with its most ambitious overhaul yet. The latest update introduces the long sought after Server Arena, new treasure dungeons, and much... | Read more »
RPG Djinn Caster (Games)
RPG Djinn Caster 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: SPECIAL PRICE 38% OFF(USD 7.99 -> USD 4.99)!!!A Fantasy Action RPG of far foreign lands! Summon the Djinns and rise to... | Read more »
Alto's Odyssey gets its first trail...
There's finally video evidence of Alto's Odyssey, the follow up to the 2015 App Store hit, Alto's Adventure. It looks just as soothing and atmospheric as Alto's last outing, but this time players will be journeying to the desert. Whereas Alto's... | Read more »

Price Scanner via MacPrices.net

Apple’s New iPad Ads Don’t Address Pro Users’...
Apple launched a new tranche of iPad Pro TV ads last week addressing actual queries and challenges from the Twitterverse, albeit using actors for the visuals. That’s great. As an iPad fan and heavy... Read more
Free Verbum Catholic Bible Study App For iOS
The Verbum mobile app runs on Logos’ powerful Bible software and is an advanced resource for mobile Catholic study. The Verbum app surrounds the Bible with the Tradition. Verbum comes with 15 free... Read more
27-inch Apple iMacs on sale for up to $200 of...
B&H Photo has 27″ Apple iMacs on sale for up to $200 off MSRP, each including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099.99 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $... Read more
15-inch 2.2GHz Retina MacBook Pro on sale for...
Amazon has 2015 15″ 2.2GHz Retina MacBook Pros (MJLQ2LL/A) available for $1849.99 including free shipping. Apple charges $1999 for this model, so Amazon’s price is represents a $150 savings. Read more
Apple refurbished iPad Air 2s available start...
Apple has Certified Refurbished iPad Air 2 WiFis available for starting at $319 including free shipping. A standard Apple one-year warranty is included: - 16GB iPad Air 2 WiFi: $319 $60 off original... Read more
Apple refurbished iPad Pros available for up...
Apple has Certified Refurbished 9″ and 12″ Apple iPad Pros available for up to $160 off the cost of new iPads. An Apple one-year warranty is included with each model, and shipping is free: - 32GB 9″... Read more
Apple restocks refurbished 2015 and 2016 13-i...
Apple has Certified Refurbished 2015 and 2016 13″ MacBook Airs available starting at $759. An Apple one-year warranty is included with each MacBook, and shipping is free: - 2016 13″ 1.6GHz/8GB/128GB... Read more
13-inch 2.5GHz MacBook Pro (Apple refurbished...
Apple has Certified Refurbished 13″ 2.5GHz MacBook Pros (MD101LL/A) available for $829, or $270 off original MSRP. Apple’s one-year warranty is standard, and shipping is free: - 13″ 2.5GHz MacBook... Read more
QuickerTek Announces 5TB Apple AC AirPort Tim...
QuickerTek Inc. has announced their new 5TB hard drive upgrade for Apple’s AC AirPort Time Capsule. By customer request, this upgrade also features six external antennas and offers the highest... Read more
Apple Certified Refurbished iMacs available f...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more

Jobs Board

*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
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and 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
Manager *Apple* Systems Administration - Pu...
Req ID 3315BR Position Title Manager, Apple Systems Administration Job Description The Manager of Apple Systems Administration oversees the administration and Read more
*Apple* Technician - nfrastructure (United S...
Let’s Work Together Apple Technician This position is based in Portland, ME Life at nfrastructure At nfrastructure, we understand that our success results from our Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.