Lisp isn't reversing my lists

Question

I'm doing some homework in Lisp, using clisp to test, and I'm loading this code and running in in clisp

(defun myreverse (thelist)

(reverse thelist)
(print thelist)

(if (equal thelist nil)
    nil
    (if (consp (first thelist))
            (cons (myreverse (reverse (first thelist))) 
                (myreverse (reverse (rest thelist))))
            (cons (first thelist) (myreverse (rest thelist))))))

I'm kind of new to Lisp, but this code isn't reversing thelist at all, my output is:

[18]> (myreverse '(a (b c) d))

(A (B C) D)
((B C) D)
(C B)
(B)
NIL
(D)
NIL
(A (C B) D)

The first line of my code says (reverse thelist), why isn't it reversing for the first print statement? Am I missing something?

(null thelist) is more idiomatic than (equal thelist nil) in Common Lisp. — Iceland_jack
– Iceland_jack, Commented Jan 18, 2012 at 17:42

P Varga · Accepted Answer · 2019-08-22 21:50:02Z

4

I believe (reverse) is without side-effects, thus it doesn't reverse the original list, but returns a new, reversed one. This is not so natural in Common Lisp, but expected in Scheme. Nevertheless, here is the doc http://www.lispworks.com/documentation/HyperSpec/Body/f_revers.htm#reverse

What I think you want is (nreverse).

edited Aug 22, 2019 at 21:50

answered Jan 11, 2012 at 5:19

P Varga

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6 Comments

ToastyMallows Over a year ago

Ah ok, so it reverses (B C) because when it returns list it actually goes somewhere else in the code (i.e. - reverse-all)?

P Varga Over a year ago

Yeah it just prints thelist, which happens to be the result of (reverse (first thelist)) in that case

ToastyMallows Over a year ago

Awesome, thanks so much I was really confused there for a second.

P Varga Over a year ago

So anyway, what you want is, in the beginning instead of (reverse thelist) something like (nreverse thelist)

Hugh Over a year ago

I think it's worth noting that even for functions like (nreverse thelist) that modify the list in place you have to consume the return value (e.g. (let ((x (list 1 2 3))) (setf x (nreverse x)))).

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