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diff --git a/js/games/nluqo.github.io/~bh/ssch5/words b/js/games/nluqo.github.io/~bh/ssch5/words new file mode 100644 index 0000000..86ec8e1 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/ssch5/words @@ -0,0 +1,661 @@ +<P> + +<P><A NAME="larson"></A> +<P><CENTER><IMG SRC="../ss-pics/farside.jpg" ALT="figure: farside"></CENTER> +<HTML> +<HEAD> +<TITLE>Simply Scheme: Introducing Computer Science ch 5: Words and Sentences</TITLE> +</HEAD> +<BODY> +<HR> +<CITE>Simply Scheme:</CITE> +<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT +<H2>Chapter 5</H2> +<H1>Words and Sentences</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/ssch05.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="../ssch4/defining.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../ssch6/true.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>We started out, in Part I, with examples about acronyms and so on, but since +then we've been working with numbery old numbers. That's because the +discussions about evaluation and procedure definition were complicated +enough without introducing extra ideas at the same time. But now we're +ready to get back to symbolic programming. + +<P>As we mentioned in Chapter 3, everything that you type into Scheme is +evaluated and the resulting value is printed out. Let's say you want to use +"square" as a word in your program. For example, you want your program to +solve the problem, "Give me an adjective that describes +<A NAME="g1"></A>Barry Manilow." If you just type <CODE>square</CODE> into Scheme, you +will find out that <CODE>square</CODE> is a procedure: + +<P><PRE>> square +#<PROCEDURE> +</PRE> + +<P>(Different versions of Scheme will have different ways of printing +out procedures.) + +<P>What you need is a way to say that you want to use the word "<CODE>square</CODE>" +<EM>itself,</EM> rather than the <EM>value</EM> of that word, as an +expression. The way to do this is to use <A NAME="g2"></A><CODE>quote</CODE>: +<A NAME="spquote"></A> + +<P><PRE>> (quote square) +SQUARE + +> (quote (tomorrow never knows)) +(TOMORROW NEVER KNOWS) + +> (quote (things we said today)) +(THINGS WE SAID TODAY) +</PRE> + +<P><CODE>Quote</CODE> is a <A NAME="g3"></A><A NAME="g4"></A>special form, since its argument isn't evaluated. +Instead, it just returns the argument as is. + +<P>Scheme programmers use <CODE>quote</CODE> a lot, so there is an abbreviation for it: + +<P><PRE>> 'square +SQUARE + +> '(old brown shoe) +(old brown shoe) +</PRE> + +<P>(Since Scheme uses the apostrophe as an abbreviation for <CODE><A NAME="g5"></A> +quote</CODE>, you can't use one as an ordinary punctuation mark in a sentence. +That's why we've been avoiding titles like <CODE>(can't buy me love)</CODE>. +To Scheme this would mean <CODE>(can (quote t) buy me +love)</CODE>!)<A NAME="text1" HREF="words#ft1">[1]</A> + +<P>This idea of quoting, although it may seem arbitrary in the context of +computer programming, is actually quite familiar from ordinary English. +What is a book? It's a bunch of pieces of paper, with printing on them, +bound together. What is "a book"? It's a noun phrase, made up of an article +and a noun. See? Similarly, what's 2+3? It's five. What's "2+3"? +It's an arithmetic formula. When you see words inside quotation marks, you +understand that you're supposed to think about the words themselves; you +don't evaluate what they mean. Scheme is the same way. + +<P>(It's no accident that kids who make jokes like + +<P><P><BLOCKQUOTE>Matt: "Say your name." + +<P>Brian: "Your name." + +<P></BLOCKQUOTE> + +<P><P>grow up to be computer programmers. The difference between a +thing and its name is one of the important ideas that programmers need to +understand.) + +<P><H2>Selectors</H2> + +<P>So far all we've done with words and sentences is quote them. To do more +interesting work, we need tools for two kinds of operations: We have to be +able to take them apart, and we have to be able to put them +together.<A NAME="text2" HREF="words#ft2">[2]</A> We'll start with +the take-apart tools; the technical term for them is <EM>selectors.</EM> + +<P><PRE>> (first 'something) +S + +> (first '(eight days a week)) +EIGHT + +> (first 910) +9 + +> (last 'something) +G + +> (last '(eight days a week)) +WEEK + +> (last 910) +0 + +> (butfirst 'something) +OMETHING + +> (butfirst '(eight days a week)) +(DAYS A WEEK) + +> (butfirst 910) +10 + +> (butlast 'something) +SOMETHIN + +> (butlast '(eight days a week)) +(EIGHT DAYS A) + +> (butlast 910) +91 +</PRE> + +<P>Notice that the <A NAME="g9"></A><CODE>first</CODE> of a sentence is a word, while the +<CODE>first</CODE> of a word is a letter. (But there's no separate data type +called "letter"; a letter is the same as a one-letter word.) The +<A NAME="g10"></A><CODE>butfirst</CODE> of a sentence is a sentence, and the <CODE>butfirst</CODE> of a +word is a word. The corresponding rules hold for <A NAME="g11"></A><CODE>last</CODE> and +<A NAME="g12"></A><CODE>butlast</CODE>. + +<P>The names <CODE>butfirst</CODE> and <CODE>butlast</CODE> aren't meant to describe ways to +sled; they abbreviate "all <CODE>but</CODE> the <CODE>first</CODE>" and "all <CODE>but</CODE> +the <CODE>last</CODE>." + +<P>You may be wondering why we're given ways to find the first and last +elements but not the 42nd element. It turns out that the ones we have are +enough, since we can use these primitive selectors to define others: + +<P> +<PRE>(define (<A NAME="g13"></A>second thing) + (first (butfirst thing))) + +> (second '(like dreamers do)) +DREAMERS + +> (second 'michelle) +I +</PRE> + +<P> + +<P>There is, however, a primitive selector <CODE>item</CODE> that takes +two arguments, a number <EM>n</EM> and a word or sentence, and returns the <EM>n</EM>th +element of the second argument. + +<P> +<PRE>> (item 4 '(being for the benefit of mister kite!)) +BENEFIT + +> (item 4 'benefit) +E +</PRE> + + +<P>Don't forget that a sentence containing exactly one word is different from +the word itself, and selectors operate on the two differently: + +<P> +<PRE>> (first 'because) +B + +> (first '(because)) +BECAUSE +</PRE> + +<P> +<PRE>> (butfirst 'because) +ECAUSE + +<A NAME="g14"></A><A NAME="g15"></A>> (butfirst '(because)) +() +</PRE> + +<P>The value of that last expression is the <EM>empty sentence.</EM> You can tell it's a sentence because of the +parentheses, and you can tell it's empty because there's nothing between +them. + +<P><PRE>> (butfirst 'a) +"" + +> (butfirst 1024) +"024" +</PRE> + +<P>As these examples show, sometimes <CODE>butfirst</CODE> returns a word +that has to have double-quote marks around it. The first example +shows the <EM><A NAME="g16"></A>empty word,</EM> while the second shows a number +that's not in its ordinary form. (Its numeric value is 24, but you don't +usually see a zero in front.) + +<P><PRE>> 024 +24 + +> "024" +"024" +</PRE> + +<P>We're going to try to avoid printing these funny words. But don't +be surprised if you see one as the return value from one of the selectors +for words. (Notice that you don't have to put a single quote in front of +the double quotes. Strings are self-evaluating, just as numbers are.) +<A NAME="g17"></A> +<A NAME="g18"></A> +<A NAME="g19"></A> + +<P>Since <CODE>butfirst</CODE> and <CODE>butlast</CODE> are so hard to type, there are +abbreviations <A NAME="g20"></A><CODE>bf</CODE> and <A NAME="g21"></A><CODE>bl</CODE>. You can figure out which is which. + +<P><H2>Constructors</H2> + +<P>Functions for putting things together are called <EM>constructors.</EM> +For now, we just have two of them: <A NAME="g22"></A><CODE>word</CODE> and <A NAME="g23"></A><CODE>sentence</CODE>. <CODE>Word</CODE> takes any number of words as arguments and joins them all together +into one humongous word: + +<P><PRE>> (word 'ses 'qui 'pe 'da 'lian 'ism) +SESQUIPEDALIANISM + +> (word 'now 'here) +NOWHERE + +> (word 35 893) +35893 +</PRE> + +<P><CODE>Sentence</CODE> is similar, but slightly different, since it can take both +words and sentences as arguments: + +<P><PRE>> (sentence 'carry 'that 'weight) +(CARRY THAT WEIGHT) + +> (sentence '(john paul) '(george ringo)) +(JOHN PAUL GEORGE RINGO) +</PRE> + +<P><CODE>Sentence</CODE> is also too hard to type, so there's the +abbreviation <A NAME="g24"></A><CODE>se</CODE>. + +<P><PRE>> (se '(one plus one) 'makes 2) +(ONE PLUS ONE MAKES 2) +</PRE> + +<P>By the way, why did we have to quote <CODE>makes</CODE> in the last example, but +not <CODE>2</CODE>? It's because numbers are self-evaluating, as we said +<A NAME="g25"></A> +<A NAME="g26"></A> +in Chapter 3. We have to quote <CODE>makes</CODE> because otherwise Scheme +would look for something named <CODE>makes</CODE> instead of using the word +itself. But numbers can't be the names of things; they represent +themselves. (In fact, you could quote the <CODE>2</CODE> and it wouldn't make any +difference—do you see why?) + +<P><H2>First-Class Words and Sentences</H2> + +<P>If Scheme isn't your first programming language, you're probably accustomed +to dealing with English text on a computer quite differently. Many other +languages treat a sentence, for example, as simply a collection (a +"string") of <EM>characters</EM> such as letters, spaces, and punctuation. +Those languages don't help you maintain the two-level nature of English +text, in which a sentence is composed of words, and a word is composed of +letters. + +<P>Historically, computers just dealt with numbers. You could add two numbers, +move a number from one place in the computer's memory to another place, and +so on. Since each instruction in the computer's native <EM>machine +language</EM> couldn't process anything larger than a number, programmers +developed the attitude that a single number is a "real thing" while +anything more complicated has to be considered as a collection of things, +rather than as a single thing in itself. + +<P>The computer represents a text character as a single number. In many +programming languages, therefore, a character is a "real thing," but +a word or sentence is understood only as a collection of these +character-code numbers. + +<P>But this isn't the way in which human beings normally think about their own +language. To you, a word isn't primarily a string of characters (although +it may temporarily seem like one if you're competing in a spelling bee). +It's more like a single unit of meaning. Similarly, a sentence is a +linguistic structure whose parts are words, not letters and spaces. + +<P>A programming language should let you express your ideas in terms that match +<EM>your</EM> way of thinking, not the computer's way. Technically, we say +that words and sentences should be <EM>first-class data</EM> in our +language. This means that a sentence, for example, can be an argument to a +procedure; it can be the value returned by a procedure; we can give it a name; +and we can build aggregates whose elements are sentences. So far we've seen +how to do the first two of these. We'll finish the job in Chapter +7 (on <EM>variables</EM>) and Chapter 17 (on <EM>lists</EM>). + +<P><H2>Pitfalls</H2> + +<P>We've been avoiding apostrophes in our words and sentences because +they're abbreviations for the <CODE>quote</CODE> special form. You must also avoid +periods, commas, semicolons, quotation marks, vertical bars, and, of course, +parentheses, since all of these have special meanings in Scheme. You may, +however, use question marks and exclamation points. + +<P>Although we've already mentioned the need to avoid names of primitives +when choosing formal parameters, we want to remind you specifically about the +names <CODE>word</CODE> and <CODE>sentence</CODE>. These are often very tempting formal +parameters, because many procedures have words or sentences as their +domains. Unfortunately, if you choose these names for parameters, you won't +be able to use the corresponding procedures within your definition. + +<P><PRE>(define (plural word) ;; wrong! + (word word 's)) + +> (plural 'george) +ERROR: GEORGE isn't a procedure +</PRE> + +<P>The result of substitution was not, as you might think, + +<P><PRE>(word 'george 's) +</PRE> + +<P>but rather + +<P><PRE>('george 'george 's) +</PRE> + +<P>We've been using <CODE>wd</CODE> and <CODE>sent</CODE> as formal parameters +instead of <CODE>word</CODE> and <CODE>sentence</CODE>, and we recommend that practice. + +<P>There's a difference between a word and a single-word sentence. For +example, people often fall into the trap of thinking that the <CODE>butfirst</CODE> +of a two-word sentence such as <CODE>(sexy sadie)</CODE> is the second word, but it's +not. It's a one-word-long sentence. For example, its <CODE>count</CODE> is one, +not five.<A NAME="text3" HREF="words#ft3">[3]</A> + +<P><PRE>> (bf '(sexy sadie)) +(SADIE) + +> (first (bf '(sexy sadie))) +SADIE +</PRE> + +<P>We mentioned earlier that sometimes Scheme has to put double-quote marks +around words. Just ignore them; don't get upset if your procedure returns +<CODE>"6-of-hearts"</CODE> instead of just <CODE>6-of-hearts</CODE>. + +<P><CODE>Quote</CODE> doesn't mean "print." Some people look at interactions +like this: + +<P><PRE>> '(good night) +(GOOD NIGHT) +</PRE> + +<P>and think that the quotation mark was an instruction telling +Scheme to print what comes after it. Actually, Scheme <EM>always</EM> prints +the value of each expression you type, as part of the read-eval-print loop. +In this case, the value of the entire expression is the subexpression that's +being quoted, namely, the sentence <CODE>(good night)</CODE>. That value wouldn't +be printed if the quotation were part of some larger expression: + +<P><PRE>> (bf '(good night)) +(NIGHT) +</PRE> + +<P>If you see an error message like + +<P><PRE>> (+ 3 (bf 1075)) +ERROR: INVALID ARGUMENT TO +: "075" +</PRE> + +<P>try entering the expression + +<P><PRE>> (strings-are-numbers #t) +OKAY +</PRE> + +<P>and try again. (The extension to Scheme that allows arithmetic +operations to work on nonstandard numbers like <CODE>"075"</CODE> makes ordinary +arithmetic slower than usual. So we've provided a way to turn the extension +on and off. Invoking <CODE>strings-are-numbers</CODE> with the argument <CODE>#f</CODE> +turns off the extension.)<A NAME="text4" HREF="words#ft4">[4]</A> + +<P><H2>Boring Exercises</H2> + +<P><B>5.1</B> What values are printed when you type these expressions to Scheme? (Figure +it out in your head before you try it on the computer.) + +<P><PRE>(sentence 'I '(me mine)) + +(sentence '() '(is empty)) + +(word '23 '45) + +(se '23 '45) + +(bf 'a) + +(bf '(aye)) + +(count (first '(maggie mae))) + +(se "" '() "" '()) + +(count (se "" '() "" '())) +</PRE> + +<P><B>5.2</B> For each of the following examples, write a procedure of two arguments +that, when applied to the sample arguments, returns the sample result. +Your procedures may not include any quoted data. + +<P><PRE>> (f1 '(a b c) '(d e f)) +(B C D E) + +> (f2 '(a b c) '(d e f)) +(B C D E AF) + +> (f3 '(a b c) '(d e f)) +(A B C A B C) + +> (f4 '(a b c) '(d e f)) +BE +</PRE> + +<P> +<B>5.3</B> Explain the difference in meaning between <CODE>(first 'mezzanine)</CODE> and +<CODE>(first '(mezzanine))</CODE>. + + +<P> +<B>5.4</B> Explain the difference between the two expressions <CODE>(first (square 7))</CODE> and +<CODE>(first '(square 7))</CODE>. + + +<P> +<B>5.5</B> Explain the difference between <CODE>(word 'a 'b 'c)</CODE> and <CODE>(se 'a 'b 'c)</CODE>. + + +<P> +<B>5.6</B> Explain the difference between <CODE>(bf 'zabadak)</CODE> and <CODE>(butfirst +'zabadak)</CODE>. + +<P> +<B>5.7</B> Explain the difference between <CODE>(bf 'x)</CODE> and <CODE>(butfirst '(x))</CODE>. + + +<P> +<B>5.8</B> Which of the following are legal Scheme sentences? + +<P><PRE>(here, there and everywhere) +(help!) +(all i've got to do) +(you know my name (look up the number)) +</PRE> + + +<P> + +<B>5.9</B> Figure out what values each of the following will return <EM>before</EM> +you try them on the computer: + +<P><PRE>(se (word (bl (bl (first '(make a)))) + (bf (bf (last '(baseball mitt))))) + (word (first 'with) (bl (bl (bl (bl 'rigidly)))) + (first 'held) (first (bf 'stitches)))) + +(se (word (bl (bl 'bring)) 'a (last 'clean)) + (word (bl (last '(baseball hat))) (last 'for) (bl (bl 'very)) + (last (first '(sunny days))))) +</PRE> + +<P><B>5.10</B> What kinds of argument can you give <CODE>butfirst</CODE> so that it +returns a word? A sentence? + + +<P> +<B>5.11</B> What kinds of argument can you give <CODE>last</CODE> so that it returns a word? A +sentence? + + +<P> +<B>5.12</B> Which of the functions <CODE>first</CODE>, <CODE>last</CODE>, <CODE>butfirst</CODE>, and +<CODE>butlast</CODE> can return an empty word? For what arguments? What about +returning an empty sentence? + + +<P> + + +<H2>Real Exercises</H2> + +<P><B>5.13</B> What does <CODE>'</CODE> <CODE>'banana</CODE> stand for? + +<P>What is <CODE>(first '</CODE> <CODE>'banana)</CODE> and why? + + +<P> +<B>5.14</B> Write a procedure <CODE><A NAME="g27"></A>third</CODE> that selects the third letter of a word +(or the third word of a sentence). + + +<P> +<B>5.15</B> <A NAME="firsttwo"></A> +Write a procedure <CODE><A NAME="g28"></A>first-two</CODE> that takes a word as its argument, +returning a two-letter word containing the first two letters of the argument. + +<P><PRE>> (first-two 'ambulatory) +AM +</PRE> + + +<P> + +<B>5.16</B> Write a procedure <CODE><A NAME="g29"></A>two-first</CODE> that takes two words as arguments, +returning a two-letter word containing the first letters of the two +arguments. + +<P><PRE>> (two-first 'brian 'epstein) +BE +</PRE> + +<P>Now write a procedure <CODE><A NAME="g30"></A>two-first-sent</CODE> that takes a two-word +sentence as argument, returning a two-letter word containing the first +letters of the two words. + +<P><PRE>> (two-first-sent '(brian epstein)) +BE +</PRE> + + +<P> + +<B>5.17</B> Write a procedure <CODE><A NAME="g31"></A>knight</CODE> that takes a person's name as its +argument and returns the name with "Sir" in front of it. + +<P> + +<P><PRE>> (knight '(david wessel)) +(SIR DAVID WESSEL) +</PRE> + + +<P> + +<B>5.18</B> Try the following and explain the result: + +<P><PRE>(define (<A NAME="g32"></A>ends word) + (word (first word) (last word))) + +> (ends 'john) +</PRE> + +<P> +<B>5.19</B> Write a procedure <CODE><A NAME="g33"></A>insert-and</CODE> that takes a sentence of items and +returns a new sentence with an "and" in the right place: + +<P><PRE>> (insert-and '(john bill wayne fred joey)) +(JOHN BILL WAYNE FRED AND JOEY) +</PRE> + +<P> + +<B>5.20</B> Define a procedure to find somebody's middle names: + +<P><PRE> +> (<A NAME="g34"></A>middle-names '(james paul mccartney)) +(PAUL) + +> (middle-names '(john ronald raoul tolkien)) +(RONALD RAOUL) + +> (middle-names '(bugs bunny)) +() + +> (middle-names '(peter blair denis bernard noone)) +(BLAIR DENIS BERNARD) +</PRE> + +<P> +<B>5.21</B> Write a procedure <CODE><A NAME="g35"></A>query</CODE> that turns a statement into a question +by swapping the first two words and adding a question mark to the last word: + +<P><PRE>> (query '(you are experienced)) +(ARE YOU EXPERIENCED?) + +> (query '(i should have known better)) +(SHOULD I HAVE KNOWN BETTER?) +</PRE> + +<P> + +<HR> +<A NAME="ft1" HREF="words#text1">[1]</A> Actually, it <EM>is</EM> possible to put punctuation +inside words as long as the entire word is enclosed in double-quote +marks, like this: +<A NAME="g6"></A> + +<P><PRE>> '("can't" buy me love) +("can't" BUY ME LOVE) +</PRE> + +<P>Words like that are called <EM>strings.</EM> We're not going +to use them in any examples until almost the end of the book. Stay away from +punctuation and you won't get in trouble. However, question marks and +exclamation points are okay. (Ordinary words, the ones that are neither +strings nor numbers, are officially called <EM>symbols.</EM>)<P> +<A NAME="ft2" HREF="words#text2">[2]</A><A NAME="g7"></A> +<A NAME="g8"></A> The procedures we're about to show you are not part of +standard, official Scheme. Scheme does provide ways to do these things, but +the regular ways are somewhat more complicated and error-prone for +beginners. We've provided a simpler way to do symbolic computing, using +ideas developed as part of the Logo programming language.<P> +<A NAME="ft3" HREF="words#text3">[3]</A> You met <CODE>count</CODE> in Chapter 2. It takes a word +or sentence as its argument, returning either the number of letters in the +word or the number of words in the sentence.<P> +<A NAME="ft4" HREF="words#text4">[4]</A> See Appendix A for a fuller explanation.<P> +<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P> +<A HREF="../ssch4/defining.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../ssch6/true.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/ssch5/words.html b/js/games/nluqo.github.io/~bh/ssch5/words.html new file mode 100644 index 0000000..a6e1536 --- /dev/null +++ b/js/games/nluqo.github.io/~bh/ssch5/words.html @@ -0,0 +1,661 @@ +<P> + +<P><A NAME="larson"></A> +<P><CENTER><IMG SRC="../ss-pics/farside.jpg" ALT="figure: farside"></CENTER> +<HTML> +<HEAD> +<TITLE>Simply Scheme: Introducing Computer Science ch 5: Words and Sentences</TITLE> +</HEAD> +<BODY> +<HR> +<CITE>Simply Scheme:</CITE> +<CITE>Introducing Computer Science</CITE> 2/e Copyright (C) 1999 MIT +<H2>Chapter 5</H2> +<H1>Words and Sentences</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/ssch05.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="../ssch4/defining.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../ssch6/true.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>We started out, in Part I, with examples about acronyms and so on, but since +then we've been working with numbery old numbers. That's because the +discussions about evaluation and procedure definition were complicated +enough without introducing extra ideas at the same time. But now we're +ready to get back to symbolic programming. + +<P>As we mentioned in Chapter 3, everything that you type into Scheme is +evaluated and the resulting value is printed out. Let's say you want to use +"square" as a word in your program. For example, you want your program to +solve the problem, "Give me an adjective that describes +<A NAME="g1"></A>Barry Manilow." If you just type <CODE>square</CODE> into Scheme, you +will find out that <CODE>square</CODE> is a procedure: + +<P><PRE>> square +#<PROCEDURE> +</PRE> + +<P>(Different versions of Scheme will have different ways of printing +out procedures.) + +<P>What you need is a way to say that you want to use the word "<CODE>square</CODE>" +<EM>itself,</EM> rather than the <EM>value</EM> of that word, as an +expression. The way to do this is to use <A NAME="g2"></A><CODE>quote</CODE>: +<A NAME="spquote"></A> + +<P><PRE>> (quote square) +SQUARE + +> (quote (tomorrow never knows)) +(TOMORROW NEVER KNOWS) + +> (quote (things we said today)) +(THINGS WE SAID TODAY) +</PRE> + +<P><CODE>Quote</CODE> is a <A NAME="g3"></A><A NAME="g4"></A>special form, since its argument isn't evaluated. +Instead, it just returns the argument as is. + +<P>Scheme programmers use <CODE>quote</CODE> a lot, so there is an abbreviation for it: + +<P><PRE>> 'square +SQUARE + +> '(old brown shoe) +(old brown shoe) +</PRE> + +<P>(Since Scheme uses the apostrophe as an abbreviation for <CODE><A NAME="g5"></A> +quote</CODE>, you can't use one as an ordinary punctuation mark in a sentence. +That's why we've been avoiding titles like <CODE>(can't buy me love)</CODE>. +To Scheme this would mean <CODE>(can (quote t) buy me +love)</CODE>!)<A NAME="text1" HREF="words.html#ft1">[1]</A> + +<P>This idea of quoting, although it may seem arbitrary in the context of +computer programming, is actually quite familiar from ordinary English. +What is a book? It's a bunch of pieces of paper, with printing on them, +bound together. What is "a book"? It's a noun phrase, made up of an article +and a noun. See? Similarly, what's 2+3? It's five. What's "2+3"? +It's an arithmetic formula. When you see words inside quotation marks, you +understand that you're supposed to think about the words themselves; you +don't evaluate what they mean. Scheme is the same way. + +<P>(It's no accident that kids who make jokes like + +<P><P><BLOCKQUOTE>Matt: "Say your name." + +<P>Brian: "Your name." + +<P></BLOCKQUOTE> + +<P><P>grow up to be computer programmers. The difference between a +thing and its name is one of the important ideas that programmers need to +understand.) + +<P><H2>Selectors</H2> + +<P>So far all we've done with words and sentences is quote them. To do more +interesting work, we need tools for two kinds of operations: We have to be +able to take them apart, and we have to be able to put them +together.<A NAME="text2" HREF="words.html#ft2">[2]</A> We'll start with +the take-apart tools; the technical term for them is <EM>selectors.</EM> + +<P><PRE>> (first 'something) +S + +> (first '(eight days a week)) +EIGHT + +> (first 910) +9 + +> (last 'something) +G + +> (last '(eight days a week)) +WEEK + +> (last 910) +0 + +> (butfirst 'something) +OMETHING + +> (butfirst '(eight days a week)) +(DAYS A WEEK) + +> (butfirst 910) +10 + +> (butlast 'something) +SOMETHIN + +> (butlast '(eight days a week)) +(EIGHT DAYS A) + +> (butlast 910) +91 +</PRE> + +<P>Notice that the <A NAME="g9"></A><CODE>first</CODE> of a sentence is a word, while the +<CODE>first</CODE> of a word is a letter. (But there's no separate data type +called "letter"; a letter is the same as a one-letter word.) The +<A NAME="g10"></A><CODE>butfirst</CODE> of a sentence is a sentence, and the <CODE>butfirst</CODE> of a +word is a word. The corresponding rules hold for <A NAME="g11"></A><CODE>last</CODE> and +<A NAME="g12"></A><CODE>butlast</CODE>. + +<P>The names <CODE>butfirst</CODE> and <CODE>butlast</CODE> aren't meant to describe ways to +sled; they abbreviate "all <CODE>but</CODE> the <CODE>first</CODE>" and "all <CODE>but</CODE> +the <CODE>last</CODE>." + +<P>You may be wondering why we're given ways to find the first and last +elements but not the 42nd element. It turns out that the ones we have are +enough, since we can use these primitive selectors to define others: + +<P> +<PRE>(define (<A NAME="g13"></A>second thing) + (first (butfirst thing))) + +> (second '(like dreamers do)) +DREAMERS + +> (second 'michelle) +I +</PRE> + +<P> + +<P>There is, however, a primitive selector <CODE>item</CODE> that takes +two arguments, a number <EM>n</EM> and a word or sentence, and returns the <EM>n</EM>th +element of the second argument. + +<P> +<PRE>> (item 4 '(being for the benefit of mister kite!)) +BENEFIT + +> (item 4 'benefit) +E +</PRE> + + +<P>Don't forget that a sentence containing exactly one word is different from +the word itself, and selectors operate on the two differently: + +<P> +<PRE>> (first 'because) +B + +> (first '(because)) +BECAUSE +</PRE> + +<P> +<PRE>> (butfirst 'because) +ECAUSE + +<A NAME="g14"></A><A NAME="g15"></A>> (butfirst '(because)) +() +</PRE> + +<P>The value of that last expression is the <EM>empty sentence.</EM> You can tell it's a sentence because of the +parentheses, and you can tell it's empty because there's nothing between +them. + +<P><PRE>> (butfirst 'a) +"" + +> (butfirst 1024) +"024" +</PRE> + +<P>As these examples show, sometimes <CODE>butfirst</CODE> returns a word +that has to have double-quote marks around it. The first example +shows the <EM><A NAME="g16"></A>empty word,</EM> while the second shows a number +that's not in its ordinary form. (Its numeric value is 24, but you don't +usually see a zero in front.) + +<P><PRE>> 024 +24 + +> "024" +"024" +</PRE> + +<P>We're going to try to avoid printing these funny words. But don't +be surprised if you see one as the return value from one of the selectors +for words. (Notice that you don't have to put a single quote in front of +the double quotes. Strings are self-evaluating, just as numbers are.) +<A NAME="g17"></A> +<A NAME="g18"></A> +<A NAME="g19"></A> + +<P>Since <CODE>butfirst</CODE> and <CODE>butlast</CODE> are so hard to type, there are +abbreviations <A NAME="g20"></A><CODE>bf</CODE> and <A NAME="g21"></A><CODE>bl</CODE>. You can figure out which is which. + +<P><H2>Constructors</H2> + +<P>Functions for putting things together are called <EM>constructors.</EM> +For now, we just have two of them: <A NAME="g22"></A><CODE>word</CODE> and <A NAME="g23"></A><CODE>sentence</CODE>. <CODE>Word</CODE> takes any number of words as arguments and joins them all together +into one humongous word: + +<P><PRE>> (word 'ses 'qui 'pe 'da 'lian 'ism) +SESQUIPEDALIANISM + +> (word 'now 'here) +NOWHERE + +> (word 35 893) +35893 +</PRE> + +<P><CODE>Sentence</CODE> is similar, but slightly different, since it can take both +words and sentences as arguments: + +<P><PRE>> (sentence 'carry 'that 'weight) +(CARRY THAT WEIGHT) + +> (sentence '(john paul) '(george ringo)) +(JOHN PAUL GEORGE RINGO) +</PRE> + +<P><CODE>Sentence</CODE> is also too hard to type, so there's the +abbreviation <A NAME="g24"></A><CODE>se</CODE>. + +<P><PRE>> (se '(one plus one) 'makes 2) +(ONE PLUS ONE MAKES 2) +</PRE> + +<P>By the way, why did we have to quote <CODE>makes</CODE> in the last example, but +not <CODE>2</CODE>? It's because numbers are self-evaluating, as we said +<A NAME="g25"></A> +<A NAME="g26"></A> +in Chapter 3. We have to quote <CODE>makes</CODE> because otherwise Scheme +would look for something named <CODE>makes</CODE> instead of using the word +itself. But numbers can't be the names of things; they represent +themselves. (In fact, you could quote the <CODE>2</CODE> and it wouldn't make any +difference—do you see why?) + +<P><H2>First-Class Words and Sentences</H2> + +<P>If Scheme isn't your first programming language, you're probably accustomed +to dealing with English text on a computer quite differently. Many other +languages treat a sentence, for example, as simply a collection (a +"string") of <EM>characters</EM> such as letters, spaces, and punctuation. +Those languages don't help you maintain the two-level nature of English +text, in which a sentence is composed of words, and a word is composed of +letters. + +<P>Historically, computers just dealt with numbers. You could add two numbers, +move a number from one place in the computer's memory to another place, and +so on. Since each instruction in the computer's native <EM>machine +language</EM> couldn't process anything larger than a number, programmers +developed the attitude that a single number is a "real thing" while +anything more complicated has to be considered as a collection of things, +rather than as a single thing in itself. + +<P>The computer represents a text character as a single number. In many +programming languages, therefore, a character is a "real thing," but +a word or sentence is understood only as a collection of these +character-code numbers. + +<P>But this isn't the way in which human beings normally think about their own +language. To you, a word isn't primarily a string of characters (although +it may temporarily seem like one if you're competing in a spelling bee). +It's more like a single unit of meaning. Similarly, a sentence is a +linguistic structure whose parts are words, not letters and spaces. + +<P>A programming language should let you express your ideas in terms that match +<EM>your</EM> way of thinking, not the computer's way. Technically, we say +that words and sentences should be <EM>first-class data</EM> in our +language. This means that a sentence, for example, can be an argument to a +procedure; it can be the value returned by a procedure; we can give it a name; +and we can build aggregates whose elements are sentences. So far we've seen +how to do the first two of these. We'll finish the job in Chapter +7 (on <EM>variables</EM>) and Chapter 17 (on <EM>lists</EM>). + +<P><H2>Pitfalls</H2> + +<P>We've been avoiding apostrophes in our words and sentences because +they're abbreviations for the <CODE>quote</CODE> special form. You must also avoid +periods, commas, semicolons, quotation marks, vertical bars, and, of course, +parentheses, since all of these have special meanings in Scheme. You may, +however, use question marks and exclamation points. + +<P>Although we've already mentioned the need to avoid names of primitives +when choosing formal parameters, we want to remind you specifically about the +names <CODE>word</CODE> and <CODE>sentence</CODE>. These are often very tempting formal +parameters, because many procedures have words or sentences as their +domains. Unfortunately, if you choose these names for parameters, you won't +be able to use the corresponding procedures within your definition. + +<P><PRE>(define (plural word) ;; wrong! + (word word 's)) + +> (plural 'george) +ERROR: GEORGE isn't a procedure +</PRE> + +<P>The result of substitution was not, as you might think, + +<P><PRE>(word 'george 's) +</PRE> + +<P>but rather + +<P><PRE>('george 'george 's) +</PRE> + +<P>We've been using <CODE>wd</CODE> and <CODE>sent</CODE> as formal parameters +instead of <CODE>word</CODE> and <CODE>sentence</CODE>, and we recommend that practice. + +<P>There's a difference between a word and a single-word sentence. For +example, people often fall into the trap of thinking that the <CODE>butfirst</CODE> +of a two-word sentence such as <CODE>(sexy sadie)</CODE> is the second word, but it's +not. It's a one-word-long sentence. For example, its <CODE>count</CODE> is one, +not five.<A NAME="text3" HREF="words.html#ft3">[3]</A> + +<P><PRE>> (bf '(sexy sadie)) +(SADIE) + +> (first (bf '(sexy sadie))) +SADIE +</PRE> + +<P>We mentioned earlier that sometimes Scheme has to put double-quote marks +around words. Just ignore them; don't get upset if your procedure returns +<CODE>"6-of-hearts"</CODE> instead of just <CODE>6-of-hearts</CODE>. + +<P><CODE>Quote</CODE> doesn't mean "print." Some people look at interactions +like this: + +<P><PRE>> '(good night) +(GOOD NIGHT) +</PRE> + +<P>and think that the quotation mark was an instruction telling +Scheme to print what comes after it. Actually, Scheme <EM>always</EM> prints +the value of each expression you type, as part of the read-eval-print loop. +In this case, the value of the entire expression is the subexpression that's +being quoted, namely, the sentence <CODE>(good night)</CODE>. That value wouldn't +be printed if the quotation were part of some larger expression: + +<P><PRE>> (bf '(good night)) +(NIGHT) +</PRE> + +<P>If you see an error message like + +<P><PRE>> (+ 3 (bf 1075)) +ERROR: INVALID ARGUMENT TO +: "075" +</PRE> + +<P>try entering the expression + +<P><PRE>> (strings-are-numbers #t) +OKAY +</PRE> + +<P>and try again. (The extension to Scheme that allows arithmetic +operations to work on nonstandard numbers like <CODE>"075"</CODE> makes ordinary +arithmetic slower than usual. So we've provided a way to turn the extension +on and off. Invoking <CODE>strings-are-numbers</CODE> with the argument <CODE>#f</CODE> +turns off the extension.)<A NAME="text4" HREF="words.html#ft4">[4]</A> + +<P><H2>Boring Exercises</H2> + +<P><B>5.1</B> What values are printed when you type these expressions to Scheme? (Figure +it out in your head before you try it on the computer.) + +<P><PRE>(sentence 'I '(me mine)) + +(sentence '() '(is empty)) + +(word '23 '45) + +(se '23 '45) + +(bf 'a) + +(bf '(aye)) + +(count (first '(maggie mae))) + +(se "" '() "" '()) + +(count (se "" '() "" '())) +</PRE> + +<P><B>5.2</B> For each of the following examples, write a procedure of two arguments +that, when applied to the sample arguments, returns the sample result. +Your procedures may not include any quoted data. + +<P><PRE>> (f1 '(a b c) '(d e f)) +(B C D E) + +> (f2 '(a b c) '(d e f)) +(B C D E AF) + +> (f3 '(a b c) '(d e f)) +(A B C A B C) + +> (f4 '(a b c) '(d e f)) +BE +</PRE> + +<P> +<B>5.3</B> Explain the difference in meaning between <CODE>(first 'mezzanine)</CODE> and +<CODE>(first '(mezzanine))</CODE>. + + +<P> +<B>5.4</B> Explain the difference between the two expressions <CODE>(first (square 7))</CODE> and +<CODE>(first '(square 7))</CODE>. + + +<P> +<B>5.5</B> Explain the difference between <CODE>(word 'a 'b 'c)</CODE> and <CODE>(se 'a 'b 'c)</CODE>. + + +<P> +<B>5.6</B> Explain the difference between <CODE>(bf 'zabadak)</CODE> and <CODE>(butfirst +'zabadak)</CODE>. + +<P> +<B>5.7</B> Explain the difference between <CODE>(bf 'x)</CODE> and <CODE>(butfirst '(x))</CODE>. + + +<P> +<B>5.8</B> Which of the following are legal Scheme sentences? + +<P><PRE>(here, there and everywhere) +(help!) +(all i've got to do) +(you know my name (look up the number)) +</PRE> + + +<P> + +<B>5.9</B> Figure out what values each of the following will return <EM>before</EM> +you try them on the computer: + +<P><PRE>(se (word (bl (bl (first '(make a)))) + (bf (bf (last '(baseball mitt))))) + (word (first 'with) (bl (bl (bl (bl 'rigidly)))) + (first 'held) (first (bf 'stitches)))) + +(se (word (bl (bl 'bring)) 'a (last 'clean)) + (word (bl (last '(baseball hat))) (last 'for) (bl (bl 'very)) + (last (first '(sunny days))))) +</PRE> + +<P><B>5.10</B> What kinds of argument can you give <CODE>butfirst</CODE> so that it +returns a word? A sentence? + + +<P> +<B>5.11</B> What kinds of argument can you give <CODE>last</CODE> so that it returns a word? A +sentence? + + +<P> +<B>5.12</B> Which of the functions <CODE>first</CODE>, <CODE>last</CODE>, <CODE>butfirst</CODE>, and +<CODE>butlast</CODE> can return an empty word? For what arguments? What about +returning an empty sentence? + + +<P> + + +<H2>Real Exercises</H2> + +<P><B>5.13</B> What does <CODE>'</CODE> <CODE>'banana</CODE> stand for? + +<P>What is <CODE>(first '</CODE> <CODE>'banana)</CODE> and why? + + +<P> +<B>5.14</B> Write a procedure <CODE><A NAME="g27"></A>third</CODE> that selects the third letter of a word +(or the third word of a sentence). + + +<P> +<B>5.15</B> <A NAME="firsttwo"></A> +Write a procedure <CODE><A NAME="g28"></A>first-two</CODE> that takes a word as its argument, +returning a two-letter word containing the first two letters of the argument. + +<P><PRE>> (first-two 'ambulatory) +AM +</PRE> + + +<P> + +<B>5.16</B> Write a procedure <CODE><A NAME="g29"></A>two-first</CODE> that takes two words as arguments, +returning a two-letter word containing the first letters of the two +arguments. + +<P><PRE>> (two-first 'brian 'epstein) +BE +</PRE> + +<P>Now write a procedure <CODE><A NAME="g30"></A>two-first-sent</CODE> that takes a two-word +sentence as argument, returning a two-letter word containing the first +letters of the two words. + +<P><PRE>> (two-first-sent '(brian epstein)) +BE +</PRE> + + +<P> + +<B>5.17</B> Write a procedure <CODE><A NAME="g31"></A>knight</CODE> that takes a person's name as its +argument and returns the name with "Sir" in front of it. + +<P> + +<P><PRE>> (knight '(david wessel)) +(SIR DAVID WESSEL) +</PRE> + + +<P> + +<B>5.18</B> Try the following and explain the result: + +<P><PRE>(define (<A NAME="g32"></A>ends word) + (word (first word) (last word))) + +> (ends 'john) +</PRE> + +<P> +<B>5.19</B> Write a procedure <CODE><A NAME="g33"></A>insert-and</CODE> that takes a sentence of items and +returns a new sentence with an "and" in the right place: + +<P><PRE>> (insert-and '(john bill wayne fred joey)) +(JOHN BILL WAYNE FRED AND JOEY) +</PRE> + +<P> + +<B>5.20</B> Define a procedure to find somebody's middle names: + +<P><PRE> +> (<A NAME="g34"></A>middle-names '(james paul mccartney)) +(PAUL) + +> (middle-names '(john ronald raoul tolkien)) +(RONALD RAOUL) + +> (middle-names '(bugs bunny)) +() + +> (middle-names '(peter blair denis bernard noone)) +(BLAIR DENIS BERNARD) +</PRE> + +<P> +<B>5.21</B> Write a procedure <CODE><A NAME="g35"></A>query</CODE> that turns a statement into a question +by swapping the first two words and adding a question mark to the last word: + +<P><PRE>> (query '(you are experienced)) +(ARE YOU EXPERIENCED?) + +> (query '(i should have known better)) +(SHOULD I HAVE KNOWN BETTER?) +</PRE> + +<P> + +<HR> +<A NAME="ft1" HREF="words.html#text1">[1]</A> Actually, it <EM>is</EM> possible to put punctuation +inside words as long as the entire word is enclosed in double-quote +marks, like this: +<A NAME="g6"></A> + +<P><PRE>> '("can't" buy me love) +("can't" BUY ME LOVE) +</PRE> + +<P>Words like that are called <EM>strings.</EM> We're not going +to use them in any examples until almost the end of the book. Stay away from +punctuation and you won't get in trouble. However, question marks and +exclamation points are okay. (Ordinary words, the ones that are neither +strings nor numbers, are officially called <EM>symbols.</EM>)<P> +<A NAME="ft2" HREF="words.html#text2">[2]</A><A NAME="g7"></A> +<A NAME="g8"></A> The procedures we're about to show you are not part of +standard, official Scheme. Scheme does provide ways to do these things, but +the regular ways are somewhat more complicated and error-prone for +beginners. We've provided a simpler way to do symbolic computing, using +ideas developed as part of the Logo programming language.<P> +<A NAME="ft3" HREF="words.html#text3">[3]</A> You met <CODE>count</CODE> in Chapter 2. It takes a word +or sentence as its argument, returning either the number of letters in the +word or the number of words in the sentence.<P> +<A NAME="ft4" HREF="words.html#text4">[4]</A> See Appendix A for a fuller explanation.<P> +<P><A HREF="../ss-toc2.html">(back to Table of Contents)</A><P> +<A HREF="../ssch4/defining.html"><STRONG>BACK</STRONG></A> +chapter thread <A HREF="../ssch6/true.html"><STRONG>NEXT</STRONG></A> + +<P> +<ADDRESS> +<A HREF="../index.html">Brian Harvey</A>, +<CODE>bh@cs.berkeley.edu</CODE> +</ADDRESS> +</BODY> +</HTML> |