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+ +

+ + +Simply Scheme: Introducing Computer Science ch 5: Words and Sentences + + +

Chapter 5

Words and Sentences

+ +

Brian +Harvey
University of California, Berkeley +

Matthew +Wright
University of California, Santa Barbara +

Download PDF version +

Back to Table of Contents +

BACK +chapter thread NEXT +

MIT +Press web page for Simply Scheme +

+ +

+ + +

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. + +

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 +Barry Manilow." If you just type square into Scheme, you +will find out that square is a procedure: + +

> square
+#<PROCEDURE>
+

+ +

(Different versions of Scheme will have different ways of printing +out procedures.) + +

What you need is a way to say that you want to use the word "square" +itself, rather than the value of that word, as an +expression. The way to do this is to use quote: + + +

> (quote square)
+SQUARE
+
+> (quote (tomorrow never knows))
+(TOMORROW NEVER KNOWS)
+
+> (quote (things we said today))
+(THINGS WE SAID TODAY)
+

+ +

Quote is a special form, since its argument isn't evaluated. +Instead, it just returns the argument as is. + +

Scheme programmers use quote a lot, so there is an abbreviation for it: + +

> 'square
+SQUARE
+
+> '(old brown shoe)
+(old brown shoe)
+

+ +

(Since Scheme uses the apostrophe as an abbreviation for +quote, you can't use one as an ordinary punctuation mark in a sentence. +That's why we've been avoiding titles like (can't buy me love). +To Scheme this would mean (can (quote t) buy me +love)!)[1] + +

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. + +

(It's no accident that kids who make jokes like + +

Matt: "Say your name." + +
Brian: "Your name." + +

+ +

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.) + +

Selectors

+ +

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.[2] We'll start with +the take-apart tools; the technical term for them is selectors. + +

> (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
+

+ +

Notice that the first of a sentence is a word, while the +first 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 +butfirst of a sentence is a sentence, and the butfirst of a +word is a word. The corresponding rules hold for last and +butlast. + +

The names butfirst and butlast aren't meant to describe ways to +sled; they abbreviate "all but the first" and "all but +the last." + +

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: + +

(define (second thing)
+  (first (butfirst thing)))
+
+> (second '(like dreamers do))
+DREAMERS
+
+> (second 'michelle)
+I
+

+ +

There is, however, a primitive selector item that takes +two arguments, a number n and a word or sentence, and returns the nth +element of the second argument. + +

> (item 4 '(being for the benefit of mister kite!))
+BENEFIT
+
+> (item 4 'benefit)
+E
+

+ + +

Don't forget that a sentence containing exactly one word is different from +the word itself, and selectors operate on the two differently: + +

> (first 'because)
+B
+
+> (first '(because))
+BECAUSE
+

+ +

> (butfirst 'because)
+ECAUSE
+
+> (butfirst '(because))
+()
+

+ +

The value of that last expression is the empty sentence. You can tell it's a sentence because of the +parentheses, and you can tell it's empty because there's nothing between +them. + +

> (butfirst 'a)
+""
+
+> (butfirst 1024)
+"024"
+

+ +

As these examples show, sometimes butfirst returns a word +that has to have double-quote marks around it. The first example +shows the empty word, 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.) + +

> 024
+24
+
+> "024"
+"024"
+

+ +

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.) + + + + +

Since butfirst and butlast are so hard to type, there are +abbreviations bf and bl. You can figure out which is which. + +

Constructors

+ +

Functions for putting things together are called constructors. +For now, we just have two of them: word and sentence. Word takes any number of words as arguments and joins them all together +into one humongous word: + +

> (word 'ses 'qui 'pe 'da 'lian 'ism)
+SESQUIPEDALIANISM
+
+> (word 'now 'here)
+NOWHERE
+
+> (word 35 893)
+35893
+

+ +

Sentence is similar, but slightly different, since it can take both +words and sentences as arguments: + +

>  (sentence 'carry 'that 'weight)
+(CARRY THAT WEIGHT)
+
+> (sentence '(john paul) '(george ringo))
+(JOHN PAUL GEORGE RINGO)
+

+ +

Sentence is also too hard to type, so there's the +abbreviation se. + +

> (se '(one plus one) 'makes 2)
+(ONE PLUS ONE MAKES 2)
+

+ +

By the way, why did we have to quote makes in the last example, but +not 2? It's because numbers are self-evaluating, as we said + + +in Chapter 3. We have to quote makes because otherwise Scheme +would look for something named makes instead of using the word +itself. But numbers can't be the names of things; they represent +themselves. (In fact, you could quote the 2 and it wouldn't make any +difference—do you see why?) + +

First-Class Words and Sentences

+ +

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 characters 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. + +

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 machine +language 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. + +

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. + +

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. + +

A programming language should let you express your ideas in terms that match +your way of thinking, not the computer's way. Technically, we say +that words and sentences should be first-class data 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 variables) and Chapter 17 (on lists). + +

Pitfalls

+ +

We've been avoiding apostrophes in our words and sentences because +they're abbreviations for the quote 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. + +

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 word and sentence. 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. + +

(define (plural word)                        ;; wrong!
+  (word word 's))
+
+> (plural 'george)
+ERROR: GEORGE isn't a procedure
+

+ +

The result of substitution was not, as you might think, + +

(word 'george 's)
+

+ +

but rather + +

('george 'george 's)
+

+ +

We've been using wd and sent as formal parameters +instead of word and sentence, and we recommend that practice. + +

There's a difference between a word and a single-word sentence. For +example, people often fall into the trap of thinking that the butfirst +of a two-word sentence such as (sexy sadie) is the second word, but it's +not. It's a one-word-long sentence. For example, its count is one, +not five.[3] + +

> (bf '(sexy sadie))
+(SADIE)
+
+> (first (bf '(sexy sadie)))
+SADIE
+

+ +

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 +"6-of-hearts" instead of just 6-of-hearts. + +

Quote doesn't mean "print." Some people look at interactions +like this: + +

> '(good night)
+(GOOD NIGHT)
+

+ +

and think that the quotation mark was an instruction telling +Scheme to print what comes after it. Actually, Scheme always 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 (good night). That value wouldn't +be printed if the quotation were part of some larger expression: + +

> (bf '(good night))
+(NIGHT)
+

+ +

If you see an error message like + +

> (+ 3 (bf 1075))
+ERROR: INVALID ARGUMENT TO +: "075"
+

+ +

try entering the expression + +

> (strings-are-numbers #t)
+OKAY
+

+ +

and try again. (The extension to Scheme that allows arithmetic +operations to work on nonstandard numbers like "075" makes ordinary +arithmetic slower than usual. So we've provided a way to turn the extension +on and off. Invoking strings-are-numbers with the argument #f +turns off the extension.)[4] + +

Boring Exercises

+ +

5.1 What values are printed when you type these expressions to Scheme? (Figure +it out in your head before you try it on the computer.) + +

(sentence 'I '(me mine))
+
+(sentence '() '(is empty))
+
+(word '23 '45)
+
+(se '23 '45)
+
+(bf 'a)
+
+(bf '(aye))
+
+(count (first '(maggie mae)))
+
+(se "" '() "" '())
+
+(count (se "" '() "" '()))
+

+ +

5.2 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. + +

> (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
+

+ +

+5.3 Explain the difference in meaning between (first 'mezzanine) and +(first '(mezzanine)). + + +

+5.4 Explain the difference between the two expressions (first (square 7)) and +(first '(square 7)). + + +

+5.5 Explain the difference between (word 'a 'b 'c) and (se 'a 'b 'c). + + +

+5.6 Explain the difference between (bf 'zabadak) and (butfirst +'zabadak). + +

+5.7 Explain the difference between (bf 'x) and (butfirst '(x)). + + +

+5.8 Which of the following are legal Scheme sentences? + +

(here, there and everywhere)
+(help!)
+(all i've got to do)
+(you know my name (look up the number))
+

+ + +

+ +5.9 Figure out what values each of the following will return before +you try them on the computer: + +

(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)))))
+

+ +

5.10 What kinds of argument can you give butfirst so that it +returns a word? A sentence? + + +

+5.11 What kinds of argument can you give last so that it returns a word? A +sentence? + + +

+5.12 Which of the functions first, last, butfirst, and +butlast can return an empty word? For what arguments? What about +returning an empty sentence? + + +

+ + +

Real Exercises

+ +

5.13 What does ' 'banana stand for? + +

What is (first ' 'banana) and why? + + +

+5.14 Write a procedure third that selects the third letter of a word +(or the third word of a sentence). + + +

+5.15 +Write a procedure first-two that takes a word as its argument, +returning a two-letter word containing the first two letters of the argument. + +

> (first-two 'ambulatory)
+AM
+

+ + +

+ +5.16 Write a procedure two-first that takes two words as arguments, +returning a two-letter word containing the first letters of the two +arguments. + +

> (two-first 'brian 'epstein)
+BE
+

+ +

Now write a procedure two-first-sent that takes a two-word +sentence as argument, returning a two-letter word containing the first +letters of the two words. + +

> (two-first-sent '(brian epstein))
+BE
+

+ + +

+ +5.17 Write a procedure knight that takes a person's name as its +argument and returns the name with "Sir" in front of it. + +

+ +

> (knight '(david wessel))
+(SIR DAVID WESSEL)
+

+ + +

+ +5.18 Try the following and explain the result: + +

(define (ends word)
+  (word (first word) (last word)))
+
+> (ends 'john)
+

+ +

+5.19 Write a procedure insert-and that takes a sentence of items and +returns a new sentence with an "and" in the right place: + +

> (insert-and '(john bill wayne fred joey))
+(JOHN BILL WAYNE FRED AND JOEY)
+

+ +

+ +5.20 Define a procedure to find somebody's middle names: + +

+> (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)
+

+ +

+5.21 Write a procedure query that turns a statement into a question +by swapping the first two words and adding a question mark to the last word: + +

> (query '(you are experienced))
+(ARE YOU EXPERIENCED?)
+
+> (query '(i should have known better))
+(SHOULD I HAVE KNOWN BETTER?)
+

+ +

+[1] Actually, it is possible to put punctuation +inside words as long as the entire word is enclosed in double-quote +marks, like this: + + +

> '("can't" buy me love)
+("can't" BUY ME LOVE)
+

+ +

Words like that are called strings. 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 symbols.)

+[2] + 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.

+[3] You met count 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.

+[4] See Appendix A for a fuller explanation.

(back to Table of Contents)

+BACK +chapter thread NEXT + +

+Brian Harvey, +bh@cs.berkeley.edu +

+ + -- cgit 1.4.1-2-gfad0