CODEWITHSUNDEEP
lispcommon-lisp
Anna
Anna

Reputation: 11

Explanation of a lisp code

I start studying Lisp and I find a code on the book as example but I do not understand what is it for. Are you able to help me understanding that? I don't know if it is the right place to do it. Thanks everyone

(defun compress (l1)               
  (cond ((null (cdr l1)) '())
        (t (accumula (car l1) 1 (cdr l1)))))


(defun accumula (val acc lst)
  (cond ((null lst) (cons (comp-list val acc) nil))
        ((eq val (car lst)) (accumula val (1+ acc) (cdr lst)))
        (t (cons (comp-list val acc) (accumula (car lst) 1 (cdr lst))))))

(defun comp-list (val acc)
  (if (> acc 1) (list acc val) val))

Upvotes: 0

Views: 538

Answers (2)

Sylwester
Sylwester

Reputation: 48745

This is an answer to problem 13 in The 99 Lisp problems (L99). It has a bug:

(compress '(a))
; ==> nil

The correct result would have been (a).

Upvotes: 1

Robert Harvey
Robert Harvey

Reputation: 180788

It's a compression function, of the Run Length Encoding variety.

(compress '(3 3 4 3 3 2 1 1 1 1 0))

will yield 

((2 3) 4 (2 3) 2 (4 1) 0)

where the first number in each sublist is the number of times the second number repeats in the original sequence.

It doesn't look like much from the example, but for long sequences where numbers repeat a lot, you can get significant savings in storage costs.

Upvotes: 1

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