Flatten a non-linear list in lisp

Asked Jan 13 '18 at 16:08

Active Feb 21 '18 at 03:37

Viewed 120 times

I want to flatten a list in Lisp, for example:

(1 2 (3 (4 5) 6)) --> (1 2 3 4 5 6)

Here is the code I wrote:

(defun flatten (l)
    (cond
        ((endp l) nil)
        ((listp (car l)) (cons (flatten (car l)) (flatten (cdr l))))
        ((atom (car l)) (cons (car l) (flatten (cdr l))))))

I don't know what is wrong here. I even found on a forum the exact same code and the guy said that it is working but this code is not changing my list at all. Some help?

edited Feb 20 '18 at 10:20

Will Ness

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asked Jan 13 '18 at 16:08

Xyntell

2

If `flatten` makes a list then `(cons (flatten..)...) ` makes a nested list. – Sylwester Jan 13 '18 at 16:23
In the case of (listp (car l)), you should use append, not cons. See more at the linked duplicate. – Will Ness Feb 21 '18 at 03:50

Flatten a non-linear list in lisp

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