Counting all occurences of an atom inside a list? - LISP

Question

I need to write a function that counts all occurrences of a given atom inside of a list. This is what I have:

(defun my-count (a L)
    (cond
    ((null L) nil)
    ((equal a (car L))(+ 1 (my-count a (cdr L))))
    (t (my-count a(cdr L)))
    )
)

By my understanding this should work. However when I run a trace I find that when (equals a car L) evaluates to true it makes the recursive call but doesn't initialize the + 1. There is obviously something I am missing.

score 3 · Accepted Answer · answered Sep 21 '13 at 21:54

3

The problem is your null condition. You are in a recursive addition, and when you're done you should add 0, not nil:

(defun my-count (a L)
  (cond
   ((null L) 0)
   ((equal a (car L)) (+ 1 (my-count a (cdr L))))
   (t (my-count a (cdr L)))))

such as

? (my-count 'x '(a x b))
1
? (my-count 'x '(a x  x b))
2
? (my-count 'x '(a b))
0

answered Sep 21 '13 at 21:54

uselpa

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@MaksimKniazev What is `freq`? If you refer to `my-count`, it counts atoms in a flat list, not in a nested list (a.k.a. tree). – uselpa Oct 28 '17 at 09:20
How do I do in tested? Can you help? – Yaroslav Dukal Oct 28 '17 at 09:48
@MaksimKniazev That's even easier. It `L` is an atom, test if it matches. If not, recurse both on the `car` and the `cdr` of `L`. – uselpa Oct 28 '17 at 10:09
Tried here https://stackoverflow.com/questions/46985799/occurrence-of-symbol-a-found-anywhere-in-l-lisp and no luck.. ( – Yaroslav Dukal Oct 28 '17 at 10:31
@MaksimKniazev I just answered there. – uselpa Oct 28 '17 at 10:34

Counting all occurences of an atom inside a list? - LISP

1 Answers1