第十二章 符號

12.1?簡介

我會在本章講解在Lisp/Scheme程序設(shè)計語言中具有字符性質(zhì)的數(shù)據(jù)類型——符號。

12.2?有關(guān)符號的基本函數(shù)

下列都是有關(guān)符號的基本函數(shù)。

(symbol? x)

如果x是一個符號則返回#t。

(string->symbol str)

將str轉(zhuǎn)換為符號。str應(yīng)該都是小寫的，否則地址系統(tǒng)可能無法正常工作。在MIT-Scheme中，(string->symbol "Hello")和'Hello是不同的。 scheme (eq? (string->symbol "Hello") 'Hello) ;Value: () (eq? (string->symbol "Hello") (string->symbol "Hello")) ;Value: #t (symbol->string (string->symbol "Hello")) ;Value 15: "Hello"

(symbol->string sym)

將sym轉(zhuǎn)換為字符。

12.3?統(tǒng)計文本中的單詞

下面的代碼是一段統(tǒng)計文本中單詞個數(shù)的程序，這也是被經(jīng)常用作演示符號的例子。這個程序使用了哈希表（Hash table）和關(guān)聯(lián)表（Association list），這些都將在下一章中講解。

01:     ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
02:     ;;;   wc.scm
03:     ;;;   a scheme word-count program
04:     ;;;
05:     ;;;    by T.Shido
06:     ;;;    on August 19, 2005
07:     ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
08:     
09:     (define (list->symbol ls0)
10:       (string->symbol (list->string (reverse! ls0))))
11:     
12:     (define (char-in c . ls)
13:       (let loop((ls0 ls))
14:         (if (null? ls0)
15:             #f
16:           (or (char=? c (car ls0))
17:               (loop (cdr ls0))))))
18:     
19:     (define (read-words fname)
20:       (with-input-from-file fname
21:         (lambda ()
22:           (let loop((w '()) (wls '()))
23:             (let ((c (read-char)))
24:           (cond
25:            ((eof-object? c)
26:                 (reverse! (if (pair? w)
27:                               (cons (list->symbol w) wls)
28:                             wls)))
29:            ((char-in c #\Space #\Linefeed #\Tab #\, #\.  #\ #\( #\) #\= #\? #\! #\; #\:)
30:                 (loop '() (if (pair? w)
31:                               (cons (list->symbol w) wls)
32:                             wls)))
33:            (else
34:             (loop (cons (char-downcase c) w) wls))))))))
35:     
36:     (define (sort-by-frequency al)
37:       (sort al (lambda (x y) (> (cdr x) (cdr y)))))
38:     
39:     (define (wc fname)
40:       (let ((wh (make-eq-hash-table)))
41:         (let loop((ls (read-words fname)))
42:           (if (null? ls)
43:               (sort-by-frequency (hash-table->alist wh))
44:             (begin
45:              (hash-table/put! wh (car ls) (1+ (hash-table/get wh (car ls) 0)))
46:              (loop (cdr ls)))))))
(wc "opensource.txt")
?
((the . 208) (to . 142) (a . 104) (of . 103) (and . 83) (that . 75) (is . 73) (in . 65) (i . 64)
(you . 55) (it . 54) (they . 48) (for . 46) (what . 38) (work . 37) (but . 35) (have . 32) (on . 32)
(people . 32) (are . 30) (be . 29) (do . 29) (from . 27) (so . 26) (like . 25) (as . 25) (by . 24)
(source . 24) (not . 23) (open . 23) (can . 23) (we . 22) (was . 22) (one . 22) (it's . 22) (an . 21)
(this . 20) (about . 20) (business . 18) (working . 18) (most . 17) (there . 17) (at . 17) (with . 16)
(don't . 16) (just . 16) (their . 16) (something . 15) (than . 15) (has . 15) (if . 15) (when . 14)
(because . 14) (more . 14) (were . 13) (office . 13) (own . 13) (or . 12) (online . 12) (now . 12)
(blogging . 12) (how . 12) (employees . 11) (them . 11) (think . 11) (time . 11) (company . 11)
(lot . 11) (want . 11) (companies . 10) (could . 10) (know . 10) (get . 10) (learn . 10) (better . 10)
(some . 10) (who . 10) (even . 9) (thing . 9) (much . 9) (no . 9) (make . 9) (up . 9) (being . 9)
(money . 9) (relationship . 9) (that's . 9) (us . 9) (anyone . 8) (average . 8) (bad . 8) (same . 8)
..........)

說明：

行號 函數(shù) 說明 09?(list->symbo ls0)?將一個由字符構(gòu)成的列表（ls0）轉(zhuǎn)換為一個符號 12?(char-in c . ls)?檢查

12.4?小結(jié)

符號是Lisp/Scheme中用于解析分析文段（例如詞數(shù)統(tǒng)計，解析等）的一種字符式數(shù)據(jù)類型，有一些速度很快的函數(shù)適用于符號。