Explanation of a lisp code

Question

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))

This page provides some sample input and output to demonstrate the function. (compress '(3 3 4 3 3 2 1 1 1 1 0)) yields ((2 3) 4 (2 3) 2 (4 1) 0) — Robert Harvey
– Robert Harvey, Commented Jan 4, 2017 at 15:55
Is there a way to "sum" the common results? For example (2 3) 4 (2 3) will be (4 3) 4 2 (4 1) 0 — Anna
– Anna, Commented Jan 4, 2017 at 16:03
You could, but you would lose the compression information; i.e. you would no longer be able to reconstruct the original list. Compression is supposed to be reversible. — Robert Harvey
– Robert Harvey, Commented Jan 4, 2017 at 16:04

Robert Harvey · Accepted Answer · 2017-01-04 16:12:56Z

1

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.

answered Jan 4, 2017 at 16:12

Robert Harvey

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3 Comments

Anna Over a year ago

Thanks @Robert for making it. I will try to compress again trying to do (4 3) 4 2... Which is basically what I need. I find it quite difficult

Robert Harvey Over a year ago

Just understand that you won't be able to reconstruct the original sequence if you do that, because you will lose the positioning information (where the 3's were located in the original sequence).

Anna Over a year ago

Yes I perfectly understand that I will lose the sort property. It is just a way to do a difficult exercise

Sylwester · Accepted Answer · 2017-01-04 16:24:32Z

1

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).

answered Jan 4, 2017 at 16:24

Sylwester

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