CODEWITHSUNDEEP
sortinglispcommon-lisp
Pedro Costa
Pedro Costa

Reputation: 437

How to sort a list by a certain order in Common Lisp?

So i know how to sort a list like (1 2 3) by ascended order for example in Common Lisp.

But i need to sort this list: ( (1 2 3) nil 1) in descending order of the 3rd element.

Is there a way to do that?

Upvotes: 1

Views: 4446

Answers (1)

Jay
Jay

Reputation: 9656

The sort function accepts a predicate that will be used to tell when an element precedes another (see the hyperspec).

So, define a function compare-3rd:

(defun compare-3rd (a b)
  (< (nth 2 a) (nth 2 b)))

And use it as the predicate:

(sort '(( (1 2 3) nil 4)
    ( (1 2 3) nil 2)
    ( (1 2 3) nil 3))
      'compare-3rd)

==> (((1 2 3) NIL 2) ((1 2 3) NIL 3) ((1 2 3) NIL 4))

Of course, if you want descending order you may use > instead of < in compare-3rd.

Upvotes: 3

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