Scheme 练习 - 统计字符串中每个字符出现的次数

#lang racket

(define (count-characters astr)
(begin
(define ht (make-hash))
(map (lambda (c)
(hash-set! ht c
(+ 1 (hash-ref ht c 0))))
(string->list astr)))
ht)

(display (count-characters "asdfjghasdfjaskdjhasdfkjhasdf"))

#hash((k . 2) (j . 4) (h . 3) (g . 1) (f . 4) (d . 5) (s . 5) (a . 5))