letter occurence in a text

[nrichard]

hello my problem is to count occurence of letter in a text and come out with
an assoc-list like
'((a.16) (b.10) ... (z.5))
i started a fee function but they don't work so please help me

;; an alist
(define lettre '((""."") ) )
(define lettre2 '(("a". 1) ("b". 1) ("c". 1) ) )

;; this function take a key and an alist
;; add the key to the alist if it is not a space,a newline,and does not
already exist
;; update the value of the given key
(define (lettre-test x alist)
        (cond ((and 
                ( and (not (char=?  x #\space))(not (char=?  x #\newline)))   
                                (eq? (assoc (make-string 1 x) alist) #f))  
                (set! alist
                        (assoc-set! alist (make-string 1 x) 0 )))
                ((char=?  x #\space) '())
                ((char=?  x #\newline) '())              
                (else
                (set! alist   
                        (assoc-set! alist (make-string 1 x) (+ (assoc-ref alist
(make-string 1 x)) 1))))))
        
;; this function open a file
;; read the caracter and count the occurence of each character  
(define (compte-char source alist)      
(let ((p (open-port source)))
  (let f ((x (read-char p)))
    (cond ((eof-object? x)
        (begin
          (close-input-port p)
          '()))        
         (else
        (cons (lettre-test x alist) (f (read-char p))))))))
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