Re: Q: on hashes and counting

Erik Naggum wrote:

| Suppose i have a text file and i want to count strings in it.
| Specifically, the first 5 characters of each line in this file are a
| vendor code.  Then i want to ouptut all vendor codes and how many
| times they show up in the input.
 
  Here's my cut at that task:
 
(let ((hash (make-hash-table :test #'equal)))
  (with-open-file (stream ...file-args...)
    (handler-case
        (loop
          (let ((vendor (read-sequence stream (make-string 5))))
            ;; skip to end of line and consume
            (peek-char #\newline stream)
            (read-char stream)
            ;; real body of loop
            (incf (gethash vendor hash 0))))
      ;; use conditions to simplify error handling
      (end-of-file ())))
  (dolist (key (sort (hash-table-keys hash) #'string<))
    (format t "~A,~A~%" key (gethash key hash))))

Gauche Scheme

(use srfi-13)  ;; string functions

Using a collector that collects into an association list.

(define table (malistbag))

(with-input-from-file "input.dat"
  (lambda ()
    (do. ((line (read-line) <>))  ;; <> means repeat preceding expr.
      ((eof-object? line))
      (when (> (string-length line) 4)
        (table (string-take line 5) 1 +)))))

(dolist (e (sort (table) > cdr))
  (print (car e) " " (cdr e)))

  ===>
V0003 3
V0002 2
V0001 1

Given:

;; Non-destructive.
(define (update-alist alist k v :optional (func #f) (default 0))
  (define (alter-entry e)
    (if func
      (let ((new-v (func v (if e (cdr e) default))))
        (cons k new-v))
      (cons k v)))
  (let go ((the-list alist) (seen '()))
    (cond ((null? the-list) (cons (alter-entry #f) seen))
          ((equal? k (caar the-list))
           (append (cons (alter-entry (car the-list)) seen)
                   (cdr the-list)))
          (#t (go (cdr the-list) (cons (car the-list) seen))))))

(define (malistbag)
  (let ((bag '()))
    (case-lambda
      [()  bag]
      [(k)  (let ((e (assoc k bag))) (and e (cdr e)))]
      [(k val) (set! bag (update-alist bag k val))]
      [(k val func) (set! bag (update-alist bag k val func))]
      [(k val func def) (set! bag (update-alist bag k val func def))])))

(define-syntax do.-aux
  (syntax-rules  ( <> )
    [ (do.-aux ((v init <>) more ...) seen stuff ...)
      (do.-aux ((v init init) more ...) seen stuff ...) ]
    [ (do.-aux (what more ...) (seen ...) stuff ...)
      (do.-aux (more ...) (seen ... what) stuff ...) ]
    [ (do.-aux () seen stuff ...)
      (do seen stuff ...) ] ))
(define-syntax do.
  (syntax-rules ()
    [ (do. things more ...)
      (do.-aux things () more ...) ] ))