[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。