TweetFollow Us on Twitter

Cons Cells
Volume Number:1
Issue Number:9
Column Tag:Lisp Listener

"Cons Cells and Quickdraw!"

By Andy Cohen, Human Factors Engineer, MacTutor Contributing Editor

The items within a list have pointers to values and to other items. These list inhabitants are refered to as cons cells. A list can be illustrated in terms of it's cons cells. For example, ( A B C D) can be illustrated as follows:

(A B (C D) E) looks like this:

Each pointer for each cell is a stored address of the location in memory of either the value of the cons cell or the next cons cell. It follows then that the cons cell has, two halves. The first half, which points to the value, contains the contents of the address register. While the other half which points to the next cons cell contains the contents of the decrement register. CAR and CDR, remember?. If you are wondering about the similarity between these acronyms and the functions discussed in this column three months ago, you are right, there is a relationship. However, this relationship is not necessarily significant, actually they are analogies. CAR returns the first part of a list or in the context of the cons cell, the value. CDR returns the rest ( or the next cons cell) minus the first. The procedures could just as easily have been called something more meaningful and they can be if one assigned one of these procedures to a preferred name using DEFUN. I am not bringing this subject up as an aid in understanding these lower level functions. My point is to mention that the number of cons cells a machine can handle is a good estimate the capabilities of the machine. With a large "AI" workstation such as the Symbolics 3600 (upon which much of Experlisp was developed) one can expect to have enough memory to utilize in various ways approximately 30,000 cons cells. Using a 512K Macintosh with Experlisp one can obviously expect much less. Added into the latest version of Experlisp (v1.02) there is an undocumented procedure called FREECONS. Contact Expertelligence for an update. Entering FREECONS into the Listener Window returns the number of available cons cells. On a 512K Mac with ExperLisp booted up and with only an untouched ªlispinit file compiled, FREECONS returns 5912. With two megabytes in an XL or a suped up Mac 512 one should expect to get at least 23648 cons cells. Not bad considering the Symbolics 3600 costs in excess of $100,000.

More Predicates

In our discussion on predicates from last month there were a few more predefined predicates provided in Experlisp that were not mentioned. Instead of going through each of them the following table contains most of the predicates listed in the Experlisp reference manual:

Experlisp Predicates:

AND
ARRAYP
ATOM
BOUNDP
BUILTINP
CHARACTERP
CONSP
COPY-LIST
EQUAL
FUNCTIONP
LAMBDAP
LISTP
LOCATIVEP
MACROP
MEMBER
NAND
NEQ
NOT
NULL
NUMBERP
OR
SPECIALP
SYMBOLP
TAILP
TYPE-OF
VARIABLEP
=
>
<

As you may recall a predicate is simply a procedure which tests the value or characteristics of an argument. It always returns what could be thought of as either true or false. Some of the more interesting predicates in the above list include ARRAYP which tests to see if an array was set up and assigned an address, BUILTINP which tests whether or not a function is a valid primitive and returns it's address if it is, and TAILP which tests whether or not a smaller list is at the tail end of a larger list. While these predicates are already defined when Experlisp is opened, there is certainly no reason why one would never want to create their own predicates via DEFUN. For example:

;(DEFUN NAMEP (a)
 (EQUAL "Andy" a))

;(NAMEP "Fred")
;nil
;(NAMEP "Andy")
;t

When defining a predicate the "P" at the tail end of the function name is a typical way to identify it as a predicate. However, predicates are not necessarily always written with this syntax. For example, EQUAL tests for equality between groups of alphanumeric characters (the "=" is used fro numerals).

Con-ditionals

As in most other computer languages there must be a way of performing boolean logic. The terms "If-Then" are typical since they illustrate the logical form. This form is consistent within Experlisp although the syntax is a little different. The COND procedure performs this "If-Then" logic. The typical syntax is as follows:

(COND (("IF" clause) "THEN" values, lists or procedures)

In BASIC one will find many different varieties of "If-Then" commands such as "If-Then-Else", "If-or-then-else" or "If-and- then-else", etc. COND provides us with any variety of conditional statement required. Any appropriate predicate, including one defined by the user, may be used within the clause of the COND. ExperLisp looks through each of these tests or clauses within the COND procedure until one returns anything other then nil. If all the clauses return nil, nil is returned. The following is an example:

(Defun Which (x)
   (cond ((= 1 x) '(the number is one))
          ((= 2 x) '(the number is two))
          ((= 3 x) '(the number is three)))

;Which
;(Which 2)
;the number is two
;(Which 4)
; nil

The above is invoked when the function name and the appropriate form of parameter are evaluated. In the first list inside the COND procedure "(= 1 x)", x is tested for equality with the number one. Since the number two was sent this predicate returns nil. The next list containing "the number is one" is skipped, that is it is not returned and the next line's clause is then evaluated. The parameter, x is then tested for equality with the number two. Since the number two was sent, this list returns "t" and the following list containing " the number is two" is returned in the Listener Window. The third line containing the clause "(= 3 x)" is never evaluated. Once Experlisp gets a nonil from an "IF" clause, the "Then" atoms, lists, or procedures are returned and the COND procedure is completed.

Last month the predicate "MEMBER" was discussed. This predicate, as you may recall, tests to see whether or not an argument is a member of a list. The following sample of COND contains this predicate:

(Setq vegies '(carrots, peas, eggplants, broccoli))
(DEFUN foodfun (x)
   (COND  ((member x vegies) print "it is a member")
           (not (member x vegies) print "It is not a member")))

; (carrots, peas, eggplants, Broccoli)
;foodfun
;(foodfun apple)
;It is not a member
;(foodfun peas)
;it is a member

In the above, the list "(carrots, peas, eggplants, Broccoli)" is assigned as a value to the symbol "vegies". Whenever the term "vegies" is then used (without the single quote, since the single quote will prevent ExperLisp from evaluating the symbol. Symbols have to be evaluated in order to get to their values. Lists or atoms are not evaluated, hence the single quote) it will refer to this list of vegetables. When the defined procedure foodfun is evaluated, along with an appropriate parameter, it passes control to the COND procedure. In this procedure the passed parameter's value is evaluated to see if it is contained within the list represented by the symbol vegies. In the first call to foodfun, the parameter "apple" is not a member of vegie. MEMBER returns nil and control is passed to the next "IF" clause in the COND procedure. This next line is a little more complicated. The parameter is again tested for it's membership within vegies, however there is also an evaluation for the return of a nil or a nonnil. NOT returns "t" for nil and "nil" for a nonnil. Since "apple" is still not a member of vegies the nil is returned. NOT then returns a "t" and then the following atoms, lists or procedures are returned. In this case "It is not a member" is printed in the Listener Window. In the second call to foodfun the parameter value is "peas". This time the first list evaluates it's membership in vegies and returns "t". The arguments, lists or procedures following this evaluation are then invoked. In this second call to "foodfun", "it is a member" is printed.

Note that in the two above samples two different forms of output are used. In "Which" the contents of the list are returned. In "foodfun" the PRINT procedure prints the following text inside the Listener Window. One should also note that the second clause in "foodfun" could be performed just as well with the following:

(t ( print "It is not a member"))

The clause is "t" which forces COND to return "it is not a member". In the above one makes it impossible for COND to return "nil".

ExperLisp provides another simpler form of conditional which can be used instead of COND for situations which don't require multiple clauses. IF acts in the same way as COND except it only has one If-Then-Else sequence. The following demonstrates the IF syntax:

(DEFUN Iftry (x)
 (IF (= 5 x)  '(the number is five) '(the number isn‘t five)))

;(Iftry 2)
;(the number isn‘t five)
;(Iftry 5)
;(the number is five)

IF contains three parts; the "If" condition, the "Then" or true return and the "Else" or false return. Note, that the apostrophe (option "]") was used in the word "isin't". If the single quote was used as an apostrophe by accident the following would result:

;(Iftry 2)
;(the number isn (quote t) five)

ExperLisp reads the single quote as the special symbol used as described above.

Quickdraw!

A language for the Mac without access to the Quickdraw routines in ROM cannot be taken seriously. Obviously, ExperLisp can utilize these routines. They are used with the same syntax as that described in Inside Macintosh. For example the following draws a rectangular figure:

(Paintrect '(top left bottom right))

To draw a circular object or oval try the following:

(Paintoval '(top left bottom right))

Top left and bottom right refers to the coordinates within the specified or default graphics window. These coordinates are as described in last month's column. Top corresponds to the vertical coordinate, left corresponds to the horizontal coordinate. The coordinate order for the bottom points are consistent.

PENPAT is an ExperLisp function which sets the graphic pen output to one of five patterns. The following demonstrates the above Quickdraw routines and the patterns available via PENPAT:

(DEFUN QukDrw ()
   (paintrect '(-52 -56 -32 -32))
   (penpat dkgray)
   (paintoval '(-32 -32 -8 -12))
   (penpat gray)
   (paintrect '(-8 -8 12 16))
   (penpat ltgray)
   (paintoval '(12 16 32 40))
   (penpat black)
   (paintrect '(36 44 52 64))
   (penpat white)
   (paintrect '(38 46 50 62)))

(QukDrw)

The first rectangle uses the default pattern, black. After each object is drawn the pen pattern is changed. The bottom rectangle is actually two. The first is drawn in black, the second is slightly smaller and drawn in white. If drawn by itself the last rectangle would not be visible.

DRAWSTRING puts text into the graphics window. TEXTFONT and TEXTFACE allow the manipulation of the font and style of the text produced from DRAWSTRING, respectively. TEXTSIZE changes the size of the text produced by DRAWSTRING. The following illustrates the syntax of these text control routines in ExperLisp:

(penup) (moveto -82 -35) (pendown)
(textsize 12) (textface (+ 1 8))
(drawstring "Mactutor Magazine")
(penup) (moveto -89 -20) (pendown)
(textfont 1) (textsize 10) (textface (+ 0 4))
(Drawstring "The ONLY Mag On Mac Programming")

Penup and Pendown control when pen movement will and will not produce graphic output. Moveto moves the pen to the given coordinates. The above lists produce the following when compiled:

The text styles are controlled in TEXTFACE using numbers which represent the different styles. The styles and their respective numbers are as follows:

Bold (1) Shadow (16)

Italic (2) Underline (4)

Outline (3) Normal (0)

Text fonts are also represented by numerals. The system font (Chicago) is represented by the number zero. Geneva is 1 and Monaco is 4. Other fonts are supposedly available, but it is not clear how the numbers are assigned. It is likely that the numbers representing the fonts are related to the font's resource ID. This is assigned by the DA/Font Mover just released by Apple.

Next month the Lisp Listener will continue with a discussion on iteration, more on Quickdraw routines and how Mouse input is achieved.

Dr. Tom's Reference Decks

Tom Programs of Washington DC has announced a set of reference cards keyed toInside Macintosh for developers and programmers. Each card lists information about a single toolbox trap call, giving the trap name, address, parameters and notes on how to make use of the trap routine. It provides a handy quick reference during programming that can eliminate the need to flip through the un-referenced Inside Macintosh book. Three decks are being offered, printed on card stock and color coded by manager. Each of the three sets sell for $21.95, a very reasonable price. Alternately, you can buy the entire set as a Microsoft File data base for $59.95, but we think the index card format is much more handy, since your computer is hardly free to run File while your writing code! Contact Tom Programs, Suite 34T, 1500 Massachusetts Ave., NW in Washington, DC. 20005. Or call (223-6813).

One Flew Over The QuickDraw's Nest

Valuable Information Press announces a new technical Macintosh Programming book by Gregg Lewis of Montreal, Canada, titled "One Flew Over the QuickDraw's Nest" (a fan of Jack Nicholson's no doubt!) This book is designed for people who want to program their Macintosh! (Where have I heard that before?) Should be great stuff for MacTutor fans. The book sells for $24.95 and is being distributed by Jim Fitzsimmons at Mac America, the same distributor who handles MacTutor, Macazine and the Macintosh Buyer's Guide. Contact Jim directly to reserve an advance copy. The final manuscript is being prepared for printing now and is expected to be available from Jim at the Mac Expo in Boston at the MacTutor booth. Contact Mac America, (714) 779-2922.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Apple Remote Desktop 3.8 - Remotely cont...
Apple Remote Desktop 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, and... Read more
NeoOffice 2014.7 - Mac-tailored, OpenOff...
NeoOffice is a complete office suite for OS X. With NeoOffice, users can view, edit, and save OpenOffice documents, PDF files, and most Microsoft Word, Excel, and PowerPoint documents. NeoOffice 3.x... Read more
DesktopLyrics 2.6.6 - Displays current i...
DesktopLyrics is an application that displays the lyrics of the song currently playing in "iTunes" right on your desktop. The lyrics for the song have to be set in iTunes; DesktopLyrics does nothing... Read more
Ember 1.8.3 - Versatile digital scrapboo...
Ember (formerly LittleSnapper) is your digital scrapbook of things that inspire you: websites, photos, apps or other things. Just drag in images that you want to keep, organize them into relevant... Read more
Apple iTunes 12.1 - Manage your music, m...
Apple iTunes lets you organize and play digital music and video on your computer. It can automatically download new music, app, and book purchases across all your devices and computers. And it's a... Read more
LibreOffice 4.4.3 - Free, open-source of...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more
FoldersSynchronizer 4.2.1 - Synchronize...
FoldersSynchronizer is a popular and useful utility that synchronizes and backs-up files, folders, disks and boot disks. On each session you can apply special options like Timers, Multiple Folders,... Read more
Simon 4.0.2 - Monitor changes and crashe...
Simon monitors websites and alerts you of crashes and changes. Select pages to monitor, choose your alert options, and customize your settings. Simon does the rest. Keep a watchful eye on your... Read more
Cocktail 8.1.2 - General maintenance and...
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
Cyberduck 4.6.4 - FTP and SFTP browser....
Cyberduck is a robust FTP/FTP-TLS/SFTP browser for the Mac whose lack of visual clutter and cleverly intuitive features make it easy to use. Support for external editors and system technologies such... Read more

Playworld Superheroes Review
Playworld Superheroes Review By Tre Lawrence on January 30th, 2015 Our Rating: :: HERO CRAFTINGUniversal App - Designed for iPhone and iPad It’s all about the imagination, fighting bad creatures — and looking good while doing so.   | Read more »
Join the SpongeBob Bubble Party in this...
Join the SpongeBob Bubble Party in this New Match 3 Bubble Poppin’ Frenzy Posted by Jessica Fisher on January 30th, 2015 [ permalink ] | Read more »
Handpick Review
Handpick Review By Jennifer Allen on January 30th, 2015 Our Rating: :: TANTALIZING SUGGESTIONSiPhone App - Designed for the iPhone, compatible with the iPad Handpick will make you hungry, as well as inspire you to cook something... | Read more »
Storm the Halls of Echo Base in First St...
Storm the Halls of Echo Base in First Star Wars: Galactic Defense Event Posted by Jessica Fisher on January 30th, 2015 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
Contradiction Review
Contradiction Review By Tre Lawrence on January 30th, 2015 Our Rating: :: SPOT THE LIEiPad Only App - Designed for the iPad Contradiction is a live action point and click adventure that’s pretty engaging.   Developer: Tim Follin... | Read more »
Unlock Sunshine Girl in Ironkill with th...
Unlock Sunshine Girl in Ironkill with this special 148Apps code Posted by Rob Rich on January 29th, 2015 [ permalink ] Robo-fighter Ironkill has been out on iOS a | Read more »
Crossroad Zombies Review
Crossroad Zombies Review By Jordan Minor on January 29th, 2015 Our Rating: :: CROSSWALKING DEADiPad Only App - Designed for the iPad Crossroad Zombies is a rough draft of a cool genre mash-up.   | Read more »
Blood Brothers 2 – Tips, Cheats, and Str...
War is hell: Is it the kind of hell you want to check out? Read our Blood Brothers 2 review to find out! Blood Brothers 2, DeNA’s follow-up to the original Blood Brothers, is an intriguing card collecting / role-playing / strategy hybrid. There’s... | Read more »
Blood Brothers 2 Review
Blood Brothers 2 Review By Nadia Oxford on January 29th, 2015 Our Rating: :: AN AGGRAVATING RELATIVEUniversal App - Designed for iPhone and iPad Blood Brothers 2 is built on a simple, solid foundation, but its free-to-play system... | Read more »
I AM BREAD, the Toast of the Town, is Ro...
Have you ever dreamt of being deliciously gluten-y? Do you feel passionate about Rye and Wheat? The guys at Bossa Studios do and that is why they are bringing I AM BREAD to iOS soon. The loafy app will feature all the new content that is being... | Read more »

Price Scanner via MacPrices.net

Amazon offers 15-inch 2.2GHz Retina MacBook P...
 Amazon.com has the 15″ 2.2GHz Retina MacBook Pro on sale for $1879 including free shipping. Their price is $120 off MSRP, and it’s the lowest price available for this model from any reseller (except... Read more
Intel Aims to Transform Workplace With 5th-Ge...
Intel Corporation today announced the availability of its 5th generation Intel Core vPro processor family that provides cutting-edge features to enable a new and rapidly shifting workplace. To meet... Read more
iOS App Sharalike Introduces New Instant Smar...
Sharalike slideshow and photo management software for iOS, is making it easier than ever to create shareable meaningful moments with its new instant SmartShow technology. Staying organized is a goal... Read more
Apple Becomes World’s Largest Smartphone Vend...
According to the latest research data from Strategy Analytics, as global smartphone shipments grew 31 percent annually to reach a record 380 million units in the fourth quarter of 2014. Apple became... Read more
Cut the Cord: OtterBox Resurgence Power Case...
Dead batteries and broken phones are two of the biggest issues for smartphone users today. Otterbox addresses both with the new Resurgence Power Case for Apple iPhone 6, promising to make those panic... Read more
13-inch Retina MacBook Pros on sale for up to...
B&H Photo has 13″ Retina MacBook Pros on sale for $200 off MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 2.6GHz/128GB Retina MacBook Pro: $1199.99 save $100 - 13″ 2.6GHz/... Read more
15-inch 2.5GHz Retina MacBook Pro on sale for...
 B&H Photo has the 15″ 2.5GHz Retina MacBook Pro on sale for $2319.99 including free shipping plus NY sales tax only. Their price is $180 off MSRP, and it’s the lowest price available for this... Read more
Back in stock: Refurbished iPod nanos for $99...
The Apple Store has Apple Certified Refurbished 16GB iPod nanos available for $99 including free shipping and Apple’s standard one-year warranty. That’s $50 off the cost of new nanos. Most colors are... Read more
Apple lowers price on refurbished 256GB MacBo...
The Apple Store has lowered prices on Apple Certified Refurbished 2014 MacBook Airs with 256GB SSDs, now available for up to $200 off the cost of new models. An Apple one-year warranty is included... Read more
New Good Management Suite Simplifies Enterpri...
Good Technology has announced the availability of the Good Management Suite, a comprehensive cross-platform solution for organizations getting started with mobile business initiatives. Built on the... Read more

Jobs Board

At-Home Chat Specialist- *Apple* Online Stor...
**Job Summary** At Apple , we believe in hard work, a fun environment, and the kind of creativity and innovation that only comes about when talented people from diverse Read more
Sr. Mac Expert- *Apple* Online Store - Apple...
**Job Summary** The World Wide Apple Online Store (AOS) Sales and Service team is looking for motivated, outgoing, and tech savvy individuals who want to offer Apple Read more
*Apple* Solutions Consultant- Retail Sales (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Event Director, *Apple* Retail Marketing -...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global engagement strategy and team. Delivering an overarching brand Read more
At-Home Chat Specialist- *Apple* Online Stor...
**Job Summary** At Apple , we believe in hard work, a fun environment, and the kind of creativity and innovation that only comes about when talented people from diverse Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.