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| author | elioat <elioat@tilde.institute> | 2023-08-23 07:52:19 -0400 |
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| committer | elioat <elioat@tilde.institute> | 2023-08-23 07:52:19 -0400 |
| commit | 562a9a52d599d9a05f871404050968a5fd282640 (patch) | |
| tree | 7d3305c1252c043bfe246ccc7deff0056aa6b5ab /js/games/nluqo.github.io/~bh/ssch0 | |
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| download | tour-562a9a52d599d9a05f871404050968a5fd282640.tar.gz | |
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diff --git a/js/games/nluqo.github.io/~bh/ssch0/ack.html b/js/games/nluqo.github.io/~bh/ssch0/ack.html new file mode 100644 index 0000000..7c99711 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/ssch0/ack.html @@ -0,0 +1,122 @@ +<P> + +<P> + +<HTML> +<HEAD> +<TITLE>Simply Scheme:Acknowledgments</TITLE> +</HEAD> +<BODY> +<CITE>Simply Scheme</CITE>: +<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT +<H1>Acknowledgments</H1> + +<TABLE width="100%"><TR><TD> +<IMG SRC="../simply.jpg" ALT="cover photo"> +<TD><TABLE> +<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian +Harvey</A><BR>University of California, Berkeley</CITE> +<TR><TD align="right"><CITE><A HREF="http://ccrma.stanford.edu/~matt">Matthew +Wright</A><BR>University of California, Santa Barbara</CITE> +<TR><TD align="right"><BR> +<TR><TD align="right"><A HREF="../pdf/ssch00.pdf">Download PDF version</A> +<TR><TD align="right"><A HREF="../ss-toc2.html">Back to Table of Contents</A> +<TR><TD align="right"><A HREF="instructor.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../ssch1/part1.html"><STRONG>NEXT</STRONG></A> +<TR><TD align="right"><A HREF="http://mitpress.mit.edu/0262082810">MIT +Press web page for <CITE>Simply Scheme</CITE></A> +</TABLE></TABLE> + +<HR> + + +<P>Obviously our greatest debt is to Harold Abelson, +Gerald Jay Sussman, and Julie Sussman. They have +inspired us and taught us, and gave birth to the movement to which we are +minor contributors. Julie carefully read what we thought was the final +draft, made thousands of suggestions, both small and large, improved the +book enormously, and set us back two months. Hal encouraged us, read early +drafts, and also made this a better book than we could have created on our +own. + +<P> +Mike Clancy, Ed Dubinsky, Dan Friedman, +Tessa Harvey, and Yehuda Katz +also read drafts and made detailed and very helpful +suggestions for improvement. Mike contributed many exercises. +(We didn't take their advice about everything, though, so they get none of +the blame for anything you don't like here.) + +<P> +Terry Ehling, Bob Prior, and everyone at the MIT +Press have given this project the benefit of their enthusiasm and their +technical support. We're happy to be working with them. + +<P>The Computer Science Division at the University of California, Berkeley, +allowed us to teach a special section of the CS 3 course using the first +draft of this book. The book now in your hands is much better because of +that experience. We thank Annika Rogers, our teaching assistant +in the course, and also the thirty students who served not merely as guinea +pigs but as collaborators in pinning down the weak points in our +explanations. + +<P>Some of the ideas in this book, especially the different approaches to +recursion, are taken from Brian's earlier Logo-based +textbook.<A NAME="text1" HREF="ack.html#ft1">[1]</A> +Many of our explanatory metaphors, especially the "little people" model, +were invented by members of the Logo community. We also took the word and +sentence data types from Logo. Although this book doesn't use Logo itself, +we tried to write it in the Logo spirit. + +<P>We wrote much of this book during the summer of 1992, while we were on the +faculty of the Institute for Secondary Mathematics and Computer Science +Education, an inservice teacher training program at Kent State University. +Several of our IFSMACSE colleagues contributed to our ideas both about +computer science and about teaching; we are especially indebted to +Ed Dubinsky and Uri Leron. + +<P>We stole the idea of a "pitfalls" section at the end of each chapter from +Dave Patterson and John Hennessy. + + + +<P>We stole some of the ideas for illustrations from Douglas +Hofstadter's wonderful <EM>Godel, Escher, Bach.</EM> + + +<P>David Zabel helped us get software ready for students, + +especially with compiling SCM for the PC. + +<P>We conclude this list with an acknowledgment of each other. Because of the +difference in our ages, it may occur to some readers to suspect that we +contributed unequally to this book—either that Matt did all the work and +Brian just lent his name and status to impress publishers, or that Brian had +all the ideas and Matt did the typing. Neither of these is true. Almost +everything in the book was written with both of us in front of the computer, +arguing out every paragraph. When we did split up to write some sections +separately, each of us read and criticized the other's work. (We're a +little surprised that we still like each other, after all the arguments!) +Luckily we both like the Beatles, +Chinese food, and ice cream, so we had a common ground for +programming examples. But when you see an example about +Bill Frisell, you can be pretty sure it's Matt's writing, and when +the example is about Dave Dee, Dozy, Beaky, Mick, and Tich, it's probably + +Brian's. + +<P> + +<A NAME="ft1" HREF="ack.html#text1">[1]</A> <EM>Computer Science Logo Style, volume 1: +Intermediate Programming,</EM> MIT Press, 1985.<P> +<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P> +<A HREF="instructor.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../ssch1/part1.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> diff --git a/js/games/nluqo.github.io/~bh/ssch0/foreword.html b/js/games/nluqo.github.io/~bh/ssch0/foreword.html new file mode 100644 index 0000000..2a531c3 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/ssch0/foreword.html @@ -0,0 +1,142 @@ +<P> + +<P> + +<HTML> +<HEAD> +<TITLE>Simply Scheme:Foreword</TITLE> +</HEAD> +<BODY> +<CITE>Simply Scheme</CITE>: +<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT +<H1>Foreword</H1> + +<TABLE width="100%"><TR><TD> +<IMG SRC="../simply.jpg" ALT="cover photo"> +<TD><TABLE> +<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian +Harvey</A><BR>University of California, Berkeley</CITE> +<TR><TD align="right"><CITE><A HREF="http://ccrma.stanford.edu/~matt">Matthew +Wright</A><BR>University of California, Santa Barbara</CITE> +<TR><TD align="right"><BR> +<TR><TD align="right"><A HREF="../pdf/ssch00.pdf">Download PDF version</A> +<TR><TD align="right"><A HREF="../ss-toc2.html">Back to Table of Contents</A> +<TR><TD align="right">[no back] +chapter thread <A HREF="preface.html"><STRONG>NEXT</STRONG></A> +<TR><TD align="right"><A HREF="http://mitpress.mit.edu/0262082810">MIT +Press web page for <CITE>Simply Scheme</CITE></A> +</TABLE></TABLE> + +<HR> + + +<P> + +<P>One of the best ways to stifle the growth of an idea is to enshrine it +in an educational curriculum. The textbook publishers, certification +panels, professional organizations, the folks who write the college +entrance exams—once they've settled on an approach, they +become frozen in a straitjacket of interlocking constraints that +thwarts the ability to evolve. So it is common that students learn +the "modern" geography of countries that no longer exist and +practice using logarithm tables when calculators have made tables +obsolete. And in computer science, beginning courses are trapped in +an approach that was already ten years out of date by the time it was +canonized in the mid-1980s, when the College Entrance Examination Board +adopted an advanced placement exam based on Pascal.<A NAME="text1" HREF="foreword.html#ft1">[1]</A> + +<P>This book points the way out of the trap. It emphasizes programming +as a way to express ideas, rather than just a way to get computers to +perform tasks. + +<P>Julie and Gerry Sussman and I are flattered that Harvey and Wright +characterize their revolutionary introduction to computer science as a +"prequel" to our text <EM>Structure and Interpretation of Computer +Programs.</EM> When we were writing <EM>SICP,</EM> we often drew upon +the words of the great American computer scientist Alan Perlis +(1922-1990). Perlis was one of the designers of the Algol +programming language, which, beginning in 1958, established the +tradition of formalism and precision that Pascal embodies. Here's +what Perlis had to say about this tradition in 1975, +nine years <EM>before</EM> the start of the AP exam: + +<P><BLOCKQUOTE> + +Algol is a blight. You can't have fun with Algol. Algol is a code +that now belongs in a plumber's union. It helps you design correct +structures that don't collapse, but it doesn't have any fun in it. +There are no pleasures in writing Algol programs. It's a labor of +necessity, a preoccupation with the details of tedium. + +</BLOCKQUOTE> + +<P>Harvey and Wright's introduction to computing emerges from a different +intellectual heritage, one rooted in research in artificial +intelligence and the programming language Lisp. In approaching +computing through this book, you'll focus on two essential techniques. + +<P>First is the notion of <EM>symbolic programming.</EM> This means that +you deal not only with numbers and letters, but with structured +collections of data—a word is a list of characters, a sentence is a +list of words, a paragraph is a list of sentences, a story is a list +of paragraphs, and so on. You assemble things in terms of natural +parts, rather than always viewing data in terms of its tiniest pieces. +It's the difference between saying "find the fifth character of the +third word in the sentence" and "scan the sentence until you pass +two spaces, then scan past four more characters, and return the next +character." + +<P>The second technique is to work with <EM>higher-order functions.</EM> +That means that you don't only write programs, but rather you <EM> +write programs that write programs,</EM> so you can bootstrap your methods +into more powerful methods. + +<P>These two techniques belong at center stage in any beginning +programming course, which is exactly where Harvey and Wright put them. +The underlying principle in both cases is that you work with general +parts that you extend and combine in flexible ways, rather than tiny +fragments that you fit together into rigid structures. + +<P>You should come to this introduction to computing ready to think about +ideas rather than details of syntax, ready to design your own +languages rather than to memorize the rules of languages other people +have designed. This kind of activity changes your outlook not only on +programming, but on any area where design plays an important role, +because you learn to appreciate the relations among parts rather than +always fixating on the individual pieces. To quote Alan Perlis again, + +<P><BLOCKQUOTE> + +You begin to think in terms of patterns and idioms and phrases, and no +longer pick up a trowel and some cement and lay things down brick by +brick. The Great Wall, standing for centuries, is a monument. But +building it must have been a bore. + +</BLOCKQUOTE> + +<P><P> + +<P> + +<P>Hal Abelson + +<P>Cambridge, MA + +<P> + +<P> + +<A NAME="ft1" HREF="foreword.html#text1">[1]</A> Since Hal +wrote this Foreword, they've switched the AP exam to use Java, but +the principle is the same.<P> +<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P> +[no back] +chapter thread <A HREF="preface.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> diff --git a/js/games/nluqo.github.io/~bh/ssch0/instructor.html b/js/games/nluqo.github.io/~bh/ssch0/instructor.html new file mode 100644 index 0000000..0dafa73 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/ssch0/instructor.html @@ -0,0 +1,232 @@ +<P> + +<P> +<HTML> +<HEAD> +<TITLE>Simply Scheme:To the Instructor</TITLE> +</HEAD> +<BODY> +<CITE>Simply Scheme</CITE>: +<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT +<H1>To the Instructor</H1> + +<TABLE width="100%"><TR><TD> +<IMG SRC="../simply.jpg" ALT="cover photo"> +<TD><TABLE> +<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian +Harvey</A><BR>University of California, Berkeley</CITE> +<TR><TD align="right"><CITE><A HREF="http://ccrma.stanford.edu/~matt">Matthew +Wright</A><BR>University of California, Santa Barbara</CITE> +<TR><TD align="right"><BR> +<TR><TD align="right"><A HREF="../pdf/ssch00.pdf">Download PDF version</A> +<TR><TD align="right"><A HREF="../ss-toc2.html">Back to Table of Contents</A> +<TR><TD align="right"><A HREF="preface.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="ack.html"><STRONG>NEXT</STRONG></A> +<TR><TD align="right"><A HREF="http://mitpress.mit.edu/0262082810">MIT +Press web page for <CITE>Simply Scheme</CITE></A> +</TABLE></TABLE> + +<HR> + + +<P>The language that we use in this book isn't exactly standard Scheme. We've +provided several extensions that may seem unusual to an experienced Scheme +programmer. This may make the book feel weird at first, but there's a +pedagogic reason for each extension. + +<P>Along with our slightly strange version of Scheme, our book has a slightly +unusual order of topics. Several ideas that are introduced very early in +the typical Scheme-based text are delayed in ours, most notably recursion. +Quite a few people have looked at our table of contents, noted some +particular big idea of computer science, and remarked, "I can't believe +you wait so long before getting to <EM>such and such</EM>!" + +<P>In this preface for instructors, we describe and explain the unusual +elements of our approach. Other teaching issues, including the timing and +ordering of topics, are discussed in the Instructor's Manual. + +<P><H2>Lists and Sentences</H2> + +<P>The chapter named "Lists" in this book is Chapter 17, about halfway +through the book. But really we use lists much earlier than that, almost +from the beginning. + +<P>Teachers of Lisp have always had trouble deciding when and how to introduce +lists. The advantage of an early introduction is that students can then +write interesting symbolic programs instead of boring numeric ones. The +disadvantage is that students must struggle with the complexity of the +implementation, such as the asymmetry between the two ends of a list, while +still also struggling with the idea of composition of functions and Lisp's +prefix notation. + +<P>We prefer to have it both ways. We want to spare beginning students the +risk of accidentally constructing ill-formed lists such as + +<P><PRE>((((() . D) . C) . B) . A) +</PRE> + +<P>but we also want to write natural-language programs from the +beginning of the book. Our solution is to borrow from Logo the idea of a +<EM>sentence</EM> abstract data type.<A NAME="text1" HREF="instructor.html#ft1">[1]</A> Sentences are +guaranteed to be flat, proper lists, and they appear to be symmetrical to +the user of the abstraction. (That is, it's as easy to ask for the last +word of a sentence as to ask for the first word.) The <CODE>sentence</CODE> +constructor accepts either a word or a sentence in any argument position. + +<P>We defer <EM>structured</EM> lists until we have higher-order functions and +recursion, the tools we need to be able to use the structure +effectively.<A NAME="text2" HREF="instructor.html#ft2">[2]</A> A +structured list can be understood as a tree, and Lisp programmers generally +use that understanding implicitly. After introducing <CODE>car</CODE>-<CODE>cdr</CODE> +recursion, we present an explicit abstract data type for trees, without +reference to its implementation. Then we make the connection between these +formal trees and the name "tree recursion" used for structured lists +generally. But Chapter 18 can be omitted, if the instructor finds +the tree ADT unnecessary, and the reader of Chapter 17 will still +be able to use structured lists. + +<P><H2>Sentences and Words</H2> + +<P>We haven't said what a <EM>word</EM> is. Scheme includes separate data types +for characters, symbols, strings, and numbers. We want to be able to +dissect words into letters, just as we can dissect sentences into words, so +that we can write programs like <CODE>plural</CODE> and <CODE>pig-latin</CODE>. Orthodox +Scheme style would use strings for such purposes, but we want a sentence to +look <CODE>(like this)</CODE> and not <CODE>("like" "this")</CODE>. We've arranged that +in most contexts symbols, strings, and numbers can be used interchangeably; +our readers never see Scheme characters at all.<A NAME="text3" HREF="instructor.html#ft3">[3]</A> +Although a word made of letters is represented internally as a symbol, while +a word made of digits is represented as a number, above the abstraction line +they're both words. (A word that standard Scheme won't accept as a symbol +nor as a number is represented as a string.) + +<P>There is an efficiency cost to treating both words and sentences as abstract +aggregates, since it's slow to disassemble a sentence from right to left and +slow to disassemble a word in either direction. Many simple procedures that +seem linear actually behave quadratically. Luckily, words aren't usually +very long, and the applications we undertake in the early chapters don't use +large amounts of data in any form. We write our large projects as +efficiently as we can without making the programs unreadable, but we +generally don't make a fuss about it. Near the end of the book we discuss +explicitly the efficient use of data structures. + +<P><H2>Overloading in the Text Abstraction</H2> + +<P>Even though computers represent numbers internally in many different ways +(fixed point, bignum, floating point, exact rational, complex), when people +visit mathland, they expect to meet numbers there, and they expect that all +the numbers will understand how to add, subtract, multiply, and divide with +each other. (The exception is dividing by zero, but that's because of the +inherent rules of mathematics, not because of the separation of numbers into +categories by representation format.) + +<P>We feel the same way about visiting textland. We expect to meet English +text there. It takes the form of words and sentences. The operations that +text understands include <CODE>first</CODE>, <CODE>last</CODE>, <CODE>butfirst</CODE>, and <CODE> +butlast</CODE> to divide the text into its component parts. You can't divide an +empty word or sentence into parts, but it's just as natural to divide a word +into letters as to divide a sentence into words. (The ideas of mathland and +textland, as well as the details of the word and sentence procedures, come +from Logo.) + +<P>Some people who are accustomed to Scheme's view of data types consider <CODE> +first</CODE> to be badly "overloaded"; they feel that a procedure that selects an +element from a list shouldn't also extract a letter from a symbol. Some of +them would prefer that we use <CODE>car</CODE> for lists, use <CODE>substring</CODE> for +strings, and not disassemble symbols at all. Others want us to define <CODE> +word-first</CODE> and <CODE>sentence-first</CODE>. + +<P>To us, <CODE>word-first</CODE> and <CODE>sentence-first</CODE> sound no less awkward than +<CODE>fixnum-+</CODE> and <CODE>bignum-+</CODE>. Everyone agrees that it's reasonable to +overload the name <CODE>+</CODE> because the purposes are so similar. Our students +find it just as reasonable that <CODE>first</CODE> works for words as well as for +sentences; they don't get confused by this. + +<P>As for the inviolability of symbols—the wall between names and data—we +are following an older Lisp tradition, in which it was commonplace to <CODE> +explode</CODE> symbols and to construct new names within a program. Practically +speaking, all that prevents us from representing words as strings is that +Scheme requires quotation marks around them. But in any case, the +abstraction we're presenting is that the data we're dissecting are neither +strings nor symbols, but words. + +<P><H2>Higher-Order Procedures, Lambda, and Recursion</H2> + +<P>Scheme relies on procedure invocation as virtually its only control +mechanism. In order to write interesting programs, a Scheme user must +understand at least one of two hard ideas: recursion or procedure as object +(in order to use higher-order procedures). We believe that higher-order +procedures are easier to learn, especially because we begin in Chapter +8 by applying them only to named procedures. Using a named procedure +as an argument to another procedure is the way to use procedures as objects +that's least upsetting to a beginner. After the reader is comfortable with +higher-order procedures, we introduce <CODE>lambda</CODE>; after that we introduce +recursion. We do the tic-tac-toe example with higher-order procedures and +<CODE>lambda</CODE>, but not recursion. + +<P>In this edition, however, we have made the necessary minor revisions so that +an instructor who prefers to begin with recursion can assign Part IV before +Part III. + +<P>When we get to recursion, we begin with an example of embedded recursion. +Many books begin with the simplest possible recursive procedure, which turns +out to be a simple sequential recursion, or even a tail recursion. We feel +that starting with such examples allows students to invent the "go back" +model of recursion as looping. + +<P><H2>Mutators and Environments</H2> + +<P>One of the most unusual characteristics of this book is that there is no +assignment to variables in it. The reason we avoid <CODE>set!</CODE> is that the +environment model of evaluation is very hard for most students. We use a +pure substitution model throughout most of the book. (With the background +they get from this book, students should be ready for the environment model +when they see a rigorous presentation, as they will, for example, in Chapter +3 of <EM>SICP.</EM>) + +<P>As the last topic in the book, we do introduce a form of mutation, namely +<CODE>vector-set!</CODE>. Mutation of vectors is less problematic than mutation of +lists, because lists naturally share storage. You really have to go out of +your way to get two pointers to the same vector.<A NAME="text4" HREF="instructor.html#ft4">[4]</A> Mutation of data +structures is less problematic than assignment to variables because it +separates the issue of mutation from the issues of binding and scope. Using +vectors raises no new questions about the evaluation process, so we present +mutation without reference to any formal model of evaluation. We +acknowledge that we're on thin ice here, but it seems to work for our +students. + +<P>In effect, our model of mutation is the "shoebox" model that you'd find in +a mainstream programming language text. Before we get to mutation, we use +input/output programming to introduce the ideas of effect and sequence; +assigning a value to a vector element introduces the important idea of +state. We use the sequential model to write two more or less practical +programs, a spreadsheet and a database system. A more traditional approach +to assignment in Scheme would be to build an object-oriented language +extension, but the use of local state variables would definitely force us to +pay attention to environments. + +<P> + +<A NAME="ft1" HREF="instructor.html#text1">[1]</A> Speaking of abstraction, even +though that's the name of Part V, we do make an occasion in each of the +earlier parts to talk about abstraction as examples come up.<P> +<A NAME="ft2" HREF="instructor.html#text2">[2]</A> Even then, we take lists as a primitive data type. We +don't teach about pairs or improper lists, except as a potential pitfall.<P> +<A NAME="ft3" HREF="instructor.html#text3">[3]</A> Scheme's primitive +I/O facility gives you the choice of expressions or characters. Instead of +using <CODE>read-char</CODE>, we invent <CODE>read-line</CODE>, which reads a line as a +sentence, and <CODE>read-string</CODE>, which returns the line as one long word.<P> +<A NAME="ft4" HREF="instructor.html#text4">[4]</A> We don't talk about +<CODE>eq?</CODE> at all. We're careful to write our programs in such a way that +the issue of identity doesn't arise for the reader.<P> +<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P> +<A HREF="preface.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="ack.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> diff --git a/js/games/nluqo.github.io/~bh/ssch0/preface.html b/js/games/nluqo.github.io/~bh/ssch0/preface.html new file mode 100644 index 0000000..7c367d1 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/ssch0/preface.html @@ -0,0 +1,413 @@ +<P> + +<P> +<HTML> +<HEAD> +<TITLE>Simply Scheme:Preface</TITLE> +</HEAD> +<BODY> +<CITE>Simply Scheme</CITE>: +<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT +<H1>Preface</H1> + +<TABLE width="100%"><TR><TD> +<IMG SRC="../simply.jpg" ALT="cover photo"> +<TD><TABLE> +<TR><TD align="right"><CITE><A HREF="http://www.cs.berkeley.edu/~bh/">Brian +Harvey</A><BR>University of California, Berkeley</CITE> +<TR><TD align="right"><CITE><A HREF="http://ccrma.stanford.edu/~matt">Matthew +Wright</A><BR>University of California, Santa Barbara</CITE> +<TR><TD align="right"><BR> +<TR><TD align="right"><A HREF="../pdf/ssch00.pdf">Download PDF version</A> +<TR><TD align="right"><A HREF="../ss-toc2.html">Back to Table of Contents</A> +<TR><TD align="right"><A HREF="foreword.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="instructor.html"><STRONG>NEXT</STRONG></A> +<TR><TD align="right"><A HREF="http://mitpress.mit.edu/0262082810">MIT +Press web page for <CITE>Simply Scheme</CITE></A> +</TABLE></TABLE> + +<HR> + + +<P>There are two schools of thought about teaching computer science. We might +caricature the two views this way: + +<P><P> +<TABLE><TR><TH align="right" valign="top">•<TD> <TD valign="top"><STRONG>The conservative view:</STRONG> Computer programs have become too large and +complex to encompass in a human mind. Therefore, the job of computer +science education is to teach people how to discipline their work in such a +way that 500 mediocre programmers can join together and produce a program +that correctly meets its specification. + +</TABLE><TABLE><TR><TH align="right" valign="top">•<TD> <TD valign="top"><STRONG>The radical view:</STRONG> Computer programs have become too large and +complex to encompass in a human mind. Therefore, the job of computer +science education is to teach people how to expand their minds so that the +programs <EM>can</EM> fit, by learning to think in a vocabulary of larger, +more powerful, more flexible ideas than the obvious ones. Each unit of +programming thought must have a big payoff in the capabilities of the +program. + +</TABLE><P> + +<P>Of course nobody would admit to endorsing the first approach as we've +described it. Yet many introductory programming courses seem to spend half +their time on obscure rules of the programming language (semicolons go +<EM>between</EM> the instructions in Pascal, but <EM>after</EM> each +instruction in C) and the other half on stylistic commandments (thou shalt +comment each procedure with its preconditions and postconditions; thou shalt +not use <CODE>goto</CODE>). In an article that was <EM>not</EM> intended as a +caricature, the noted computer scientist Edsger Dijkstra argues that +beginning computer science students <EM>should not be allowed to use +computers,</EM> lest they learn to debug their programs interactively instead of +writing programs that can be proven correct by formal methods before +testing.<A NAME="text1" HREF="preface.html#ft1">[1]</A> + +<P>If you are about to be a student in an introductory computer science course, +you may already be an experienced programmer of your home computer, or +instead you may have only a vague idea of what you're getting into. Perhaps +you suspect that programming a computer is like programming a VCR: +entering endless obscure numeric codes. Even if you're already a computer +programmer, you may not yet have a clear idea of what computer <EM> +science</EM> means. In either case, what we want to do in this book is put +our best foot forward—introduce you to some new ideas, get you excited, +rather than mold you into a disciplined soldier of the programming army. + +<P>In order to understand the big ideas, though, we'll also have to expend some +effort on technical details; studying computer science without writing +computer programs is like trying to study German grammar without learning +any of the words in the language. But we'll try to keep the ideas in view +while struggling with the details, and we hope you'll remember them too. + +<P><H2>One Big Idea: Symbolic Programming</H2> + +<P>We said that our approach to teaching computer science emphasizes big +ideas. Our explanation of symbolic programming in the following + +paragraphs is in part just an illustration of that approach. But we chose +this particular example for another reason also. Scheme, the programming +language used in this book, is an unusual choice for an introductory +computer science course. You may wonder why we didn't use a more +traditional language, such as Pascal, Modula-2, or C. Our discussion of +symbolic programming is the beginning of an answer to that question. + +<P>Originally computers were about numbers. Scientists used them to solve +equations; businesses used them to compute the payroll and the inventory. +We were rescued from this boring state of affairs mainly by researchers in +<EM>artificial intelligence—</EM>people who wanted to get +computers to think more nearly the way people do, about ideas in general +rather than just numbers. + +<P>What does it mean to represent <EM>ideas</EM> in a computer? Here's a simple +example: We want to teach the computer to answer the question, "Was +so-and-so a Beatle?" We can't quite ask the question in English; in this +book we interact with the computer using Scheme. Our interactions will look +like this: + +<P><P> +You type: <CODE>(beatle? 'paul)</CODE> +<P>Computer replies: <CODE>#t</CODE> (computerese for "true") +<P> +<P>You type: <CODE>(beatle? 'elvis)</CODE> +<P>Computer replies: <CODE>#f</CODE> ("false") +<P> +<P> + +<P>Here's the program that does the job: + +<P><PRE>(define (beatle? person) + (member? person '(john paul george ringo))) +</PRE> + +<P>If you examine this program with a (metaphoric) magnifying glass, +you'll find that it's really still full of numbers. In fact, each letter or +punctuation character is represented in the computer by its own unique +number.<A NAME="text2" HREF="preface.html#ft2">[2]</A>But the point +of the example is that you don't have to know that! When you see + +<P><PRE>(john paul george ringo) +</PRE> + +<P>you don't have to worry about the numbers that represent the +letters inside the computer; all you have to know is that you're seeing a +<EM>sentence</EM> made up of four <EM>words.</EM> Our programming +language hides the underlying mechanism and lets us think in terms more +appropriate to the problem we're trying to solve. That hiding of details is +called <EM>abstraction,</EM> one of the big ideas in this book. + +<P> + +<P>Programming with words and sentences is an example of +symbolic programming. In 1960 John McCarthy invented the +Lisp programming language to handle symbolic computations like this +one. Our programming language, Scheme, is a modern dialect of Lisp. + +<P><H2>Lisp and Radical Computer Science</H2> + +<P>Symbolic programming is one aspect of the reason why we like to teach +computer science using Scheme instead of a more traditional language. More +generally, Lisp (and therefore Scheme) was designed to support what we've +called the radical view of computer science. In this view, computer science +is about tools for expressing ideas. Symbolic programming allows <EM>the +computer</EM> to express ideas; other aspects of Lisp's design help <EM>the +programmer</EM> express ideas conveniently. Sometimes that goal +comes in conflict with the conservative computer scientist's goal of +protection against errors. + +<P>Here's an example. We want to tell our computer, "To square a number, +multiply it by itself." In Scheme we can say + +<P><PRE>(define (square num) + (* num num)) +</PRE> + +<P>The asterisk represents multiplication, and is followed by the +two operands—in this case, both the same number. This short program works +for any number, of course, as we can see in the following dialogue. (The +lines with <CODE>></CODE> in front are the ones you type.) + +<P><PRE>> (square 4) +16 +> (square 3.14) +9.8596 +> (square -0.3) +0.09 +</PRE> + +<P>But the proponents of the 500-mediocre-programmer +school<A NAME="text3" HREF="preface.html#ft3">[3]</A> think this straightforward approach +is sinful. "What!" they cry. "You haven't said whether <CODE>num</CODE> +is a whole number or a number with a decimal fraction!" They're +afraid that you might write the <CODE>square</CODE> program with whole +numbers in mind, and then apply it to a decimal fraction <EM>by +mistake.</EM> If you're on a team with 499 other programmers, it's easy +to have failures of communication so that one programmer uses +another's program in unintended ways. + +<P>To avoid that danger, they want you to write these two separate programs: + +<P><PRE>function SquareOfWholeNumber(num: integer): integer; + begin + SquareOfWholeNumber := num * num + end; + +function SquareOfDecimalNumber(num: real): real; + begin + SquareOfDecimalNumber := num * num + end; +</PRE> + +<P>Isn't this silly? Why do they pick this particular distinction +(whole numbers and decimals) to worry about? Why not positive and negative +numbers, for example? Why not odd and even numbers? + +<P>That two-separate-program example is written in the Pascal language. +Pascal was designed by Niklaus Wirth, one of the leaders of the +structured programming school, specifically to <EM>force</EM> programming +students to write programs that fit conservative ideas about programming +style and technique; you can't write a program in Pascal at all unless you +write it in the approved style. Naturally, this language has been very +popular with school teachers.<A NAME="text4" HREF="preface.html#ft4">[4]</A> That's why, as +we write this in 1993, the overwhelming majority of introductory computer +science classes are taught using Pascal, even though no professional +programmer would be caught dead using it.<A NAME="text5" HREF="preface.html#ft5">[5]</A> + +<P> + +<P>For fourteen years after the introduction of Pascal in 1970, its hegemony in +computer science education was essentially unchallenged. But in 1984, two +professors at the Massachusetts Institute of Technology and a programmer +at Bolt, Beranek and Newman (a commercial research lab) published +the Scheme-based <EM>Structure and Interpretation of Computer Programs</EM> +(Harold Abelson and Gerald Jay Sussman with +Julie Sussman, MIT Press/McGraw-Hill). That ground-breaking text +brought the artificial intelligence approach to a wide audience for the +first time. We (Brian and Matt) have been teaching their course together +for several years. Each time, we learn something new. + +<P>The only trouble with <EM>SICP</EM> is that it was written for MIT students, +all of whom love science and are quite comfortable with formal mathematics. +Also, most of the students who use <EM>SICP</EM> at MIT have already learned +to program computers before they begin. As a result, many other schools +have found the book too challenging for a beginning course. We believe that +everyone who is seriously interested in computer science must read <EM> +SICP</EM> eventually. Our book is a <EM>prequel;</EM> it's meant to teach you +what you need to know in order to read that book +successfully.<A NAME="text6" HREF="preface.html#ft6">[6]</A> Generally +speaking, our primary goal in Parts I-V has been preparation for <EM> +SICP,</EM> while the focus of Part VI is to connect the course with the kinds +of programming used in "real world" application programs like spreadsheets +and databases. (These are the last example and the last project in the +book.) + +<P><H2>Who Should Read This Book</H2> + +<P>This book is intended as an introduction to computer programming and to +computer science for two kinds of students. + +<P>For those whose main interest is in some other field, we provide a +self-contained, one-semester experience with computer programming in a +language with a minimum of complicated notation, so that students can +quickly come in contact with high-level ideas about algorithms, functions, +and recursion. The book ends with the implementation of a spreadsheet +program and a database program, so it complements a computer application +course in which the commercial versions of such programs are used. + +<P>For those who intend to continue the study of computer science but who have +no prior programming experience, we offer a preparatory course, less intense +than a traditional CS 1 but not limited to programming technique; we give +the flavor of computer science ideas that will be studied in more depth later +in the curriculum. We also include an extensive discussion of recursion, +which is a stumbling block for many beginning students. + +<P>The course at Berkeley for which we wrote this book includes both categories +of students. About 90% of the first-year students who intend to major in +computer science have already had a programming course in high school, and +most of them begin with <EM>SICP.</EM> The other 10% are advised to take +this course first. But many of the students in this course aren't computer +science majors. A few other departments (business administration and +architecture are the main ones) have a specific computer course requirement, +and all students must meet a broader "quantitative reasoning" requirement; +our course satisfies these requirements. Finally, some students come just +out of curiosity about computers. + +<P>We assume that you have never programmed a computer. On the other hand, we +do assume that you can <EM>use</EM> a computer; we don't talk about how to +turn it on, how to edit text, and so on, because those details are too +different from one computer model to another. If you've never used a +computer before, you may wish to spend a few days with a book written +specifically for your machine that will introduce you to its operation. +It won't take more than a few days, because you don't have to be an expert +before you read our book. As long as you can start up the Scheme interpreter +and correct your typing mistakes, you're ready. + +<P>We assume that you're not a mathematics lover. (If you are, you might be +ready to read <EM>SICP</EM> right away.) The earlier example about squaring +a number is about as advanced as we get. And of course you don't have to do +any arithmetic at all; computers are good at that. You'll learn how to <EM> +tell</EM> the computer to do arithmetic, but that's no harder than using a +pocket calculator. Most of our programming examples are concerned with +words and sentences rather than with numbers. A typical example is to get +Scheme to figure out the plural form of a noun. Usually that means putting +an "s" on the end, but not quite always. (What's the plural of "French +fry"?) + +<P><H2>How to Read This Book</H2> + +<P>Do the exercises! Whenever we teach programming, we always get students who +say, "When I read the book it all makes sense, but on the exams, when you +ask me to write a program, I never know where to start." Computer science +is two things: a bunch of big ideas, as we've been saying, and also a +skill. You can't learn the skill by watching. + +<P>Do the exercises on a computer! It's not good enough to solve the exercises +on paper, even if you feel sure your solution is correct. Maybe it's 99% +correct but there's some little detail you've overlooked. When you run such +a program, you won't get 99% of the answer you wanted. By trying the +exercise on the computer, you get unambiguous feedback. If your program is +correct, you get the response you expected. If not, not. + +<P>Don't feel bad if you don't get things right the first time. Even the most +experienced programmers have to <EM>debug</EM> their programs—that is, fix +the parts that don't work. In fact, an important part of what you'll learn +from the exercises is the <EM>process</EM> of debugging your +solutions. It would be too bad if all of your programs in this course +worked the first time, because that would let you avoid the practice in +debugging that you'll certainly need when you write more complicated +programs later. Also, don't be afraid or ashamed to ask for help if you get +stuck. That, too, is part of the working style of professional programmers. + +<P>In some of the chapters, we've divided the exercises into two categories, +"boring" and "real." The boring exercises ask you to work through +examples mechanically, to make sure you understand the rules. The +real exercises ask you to <EM>invent</EM> something, usually a small +computer program, but sometimes an explanation of some situation that we +present. (In some chapters, the exercises are just labeled "exercises," +which means that they're all considered "real.") We don't intend that the +boring exercises be handed in; the idea is for you to do as many of them as +you need to make sure you understand the mechanics of whatever topic you're +learning. + +<P>Occasionally we introduce some idea with a simplified explanation, saving +the whole truth for later. We warn you when we do this. Also, we sometimes +write preliminary, partial, or incorrect example programs, and we always +flag these with a comment like + +<P><PRE>(define (something foo baz) ;; first version + ) +</PRE> + +<P>When we introduce technical terms, we sometimes mention the +origin of the word, if it's not obvious, to help prevent the terminology +from seeming arbitrary. + +<P>This book starts easy but gets harder, in two different ways. One is that we +spend some time teaching you the basics of Scheme before we get to two +hard big ideas, namely, function as object and recursion. The +earlier chapters are short and simple. You may get the idea that the whole +book will be trivial. You'll change your mind in Parts III and IV. + +<P>The other kind of difficulty in the book is that it includes long +programming examples and projects. ("Examples" are programs we write and +describe; "projects" are programs we ask you to write.) Writing a long +program is quite different from writing a short one. Each small piece may +be easy, but fitting them together and remembering all of them at once is a +challenge. The examples and projects get longer as the book progresses, but +even the first example, tic-tac-toe, is much longer and more complex than +anything that comes before it. + +<P>As the text explains more fully later, in this book we use some extensions + + +to the standard Scheme language—features that we implemented ourselves, as +Scheme programs. If you are using this book in a course, your instructor +will provide our programs for you, and you don't have to worry about it. +But if you're reading the book on your own, you'll need to follow the +instructions in Appendix A. + +<P>There are several reference documents at the end of the book. If you +don't understand a technical term in the text, try the Glossary for a +short definition, or the General Index to find the more complete +explanation in the text. If you've forgotten how to use a particular +Scheme primitive procedure, look in the Alphabetical Table +of Scheme Primitives, or in the General Index. If you've +forgotten the name of the relevant primitive, refer to the inside +back cover, where all the primitive procedures are listed by +category. Some of our example programs make reference to procedures +that were defined earlier, either in another example or in an +exercise. If you're reading an example program and it refers to some +procedure that's defined elsewhere, you can find that other procedure +in the Index of Defined Procedures. + +<P> + +<A NAME="ft1" HREF="preface.html#text1">[1]</A> "On the Cruelty of Really Teaching Computer Science," +<EM>Communications of the ACM,</EM> vol. 32, no. 12, December, 1989.<P> +<A NAME="ft2" HREF="preface.html#text2">[2]</A> The left parenthesis is 40, for example, and the letter +<CODE>d</CODE> is 100. If it were a capital <CODE>D</CODE> it would be 68.<P> +<A NAME="ft3" HREF="preface.html#text3">[3]</A> Their own names for their approach are <EM> +structured programming</EM> and <EM> +software engineering.</EM><P> +<A NAME="ft4" HREF="preface.html#text4">[4]</A> Of course, <EM>your</EM> teacher isn't +an uptight authoritarian, or you wouldn't be using our book!<P> +<A NAME="ft5" HREF="preface.html#text5">[5]</A> Okay, we're exaggerating. +But even Professor Wirth himself has found Pascal so restrictive that he had +to design more flexible languages—although not flexible enough—called +Modula and Oberon.<P> +<A NAME="ft6" HREF="preface.html#text6">[6]</A> As the ideas pioneered by <EM>SICP</EM> have +spread, we are starting to see other intellectually respectable +introductions to computer science that are meant as alternatives to <EM> +SICP.</EM> In particular, we should acknowledge <EM>Scheme and the Art of +Programming</EM> (George Springer and +Daniel P. Friedman, MIT Press/McGraw-Hill, 1989) as a recognized +classic. We believe our book will serve as preparation for theirs, too.<P> +<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P> +<A HREF="foreword.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="instructor.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> |