[TOC=2]
## 12.1 简介
我会在本章讲解在Lisp/Scheme程序设计语言中具有字符性质的数据类型——符号。
## 12.2 有关符号的基本函数
下列都是有关符号的基本函数。
`(symbol? x)`
如果`x`是一个符号则返回`#t`。
`(string->symbol str)`
将`str`转换为符号。`str`应该都是小写的，否则地址系统可能无法正常工作。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)`
将sym转换为字符。
## 12.3 统计文本中的单词
下面的代码是一段统计文本中单词个数的程序，这也是被经常用作演示符号的例子。这个程序使用了**哈希表（Hash table）**和**关联表（Association list）**，这些都将在下一章中讲解。
~~~
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)
..........)
~~~
说明：
行号 函数 说明 09 `(list->symbo ls0)` 将一个由字符构成的列表（`ls0`）转换为一个符号 12 `(char-in c . ls)` 检查
## 12.4 小结
符号是Lisp/Scheme中用于解析分析文段（例如词数统计，解析等）的一种字符式数据类型，有一些速度很快的函数适用于符号。
