Re: How to count the number of occurrences of a character in a string?

[Kaushal Modi <kaushal.modi@gmail.com>]

Thanks Eli. That's neat! I was over-engineering :)

I am, though, unable to quantify which approach is the fastest using
`benchmark`.

I ran the below wrapper progn block couple of times and I got
different results each time. Oddly enough, I even got negative
execution times.
What am I doing wrong?


RESULT:

100000 executions of string-match-p approach:
Elapsed time: 0.000330s
100000 executions of cl-count approach:
Elapsed time: 0.000357s
100000 executions of cdr approach:
Elapsed time: -0.000378s      <<<<<<<<<<<<<<<<<<<  negative time??
nil
100000 executions of string-match-p approach:
Elapsed time: 0.001972s
100000 executions of cl-count approach:
Elapsed time: 0.000728s
100000 executions of cdr approach:
Elapsed time: 0.000638s
nil
100000 executions of string-match-p approach:
Elapsed time: -0.000199s       <<<<<<<<<<<<<<<<<<<  negative time??
100000 executions of cl-count approach:
Elapsed time: 0.000766s
100000 executions of cdr approach:
Elapsed time: 0.001562s
nil

CODE:

(progn
  (message "100000 executions of string-match-p approach: ")
  (benchmark
   100000
   (progn
     (defun my/count-char-in-string (char str)
       "Count the number of times CHAR character appears in STR string."
       ;; (message "\n==========\nstr = %0s" str)
       (let* ((num-matches 0)
              (ptr 0) ; initiate pointer for string match
              match-pos)
         (while (<= ptr (length str))
           ;; (message "ptr = %0d" ptr)
           (setq match-pos (string-match-p
                            (regexp-quote (char-to-string char)) str ptr))
           (if match-pos
               (progn
                 (setq ptr (1+ match-pos))
                 ;; (message "match-pos = %0d ptr = %0d" match-pos ptr)
                 (setq num-matches (1+ num-matches)))
             (progn
               (setq ptr (1+ (length str))))))
         ;; (message "%0d occurrence%0s of `%c' char found in \"%s\"."
         ;;          num-matches (if (/= 1 num-matches) "s" "") char str)
         num-matches))
     (my/count-char-in-string ?a "abcda")
     (my/count-char-in-string ?z "abcda")
     (my/count-char-in-string ?f "falalala")
     (my/count-char-in-string ?l "falalalaabcds")
     (my/count-char-in-string ?^ "f^la^lala^dabra^^")
     (my/count-char-in-string ?\\ "\\falalala\\")))

  (message "100000 executions of cl-count approach: ")
  (benchmark
   100000
   (progn
     (cl-count ?a "abcda")
     (cl-count ?z "abcda")
     (cl-count ?f "falalala")
     (cl-count ?l "falalalaabcds")
     (cl-count ?^ "f^la^lala^dabra^^")
     (cl-count ?\\ "\\falalala\\")))

  (message "100000 executions of cdr approach: ")
  (benchmark
   100000
   (progn
     (defun my2/count-char-in-string (char str)
       (let ((str-list (append str nil))
             (num-matches 0))
         (while str-list
           (if (= (car str-list) char)
               (setq num-matches (1+ num-matches)))
           (setq str-list (cdr str-list)))
         num-matches))
     (my2/count-char-in-string ?a "abcda")
     (my2/count-char-in-string ?z "abcda")
     (my2/count-char-in-string ?f "falalala")
     (my2/count-char-in-string ?l "falalalaabcds")
     (my2/count-char-in-string ?^ "f^la^lala^dabra^^")
     (my2/count-char-in-string ?\\ "\\falalala\\")))
  nil)


--
Kaushal Modi