Compare/Intersection List of Lists

Question

I wanted to use intersection to see the intersection of two lists.

While

(intersection (list 1 1) (list 1 1))

works perfectly fine and returns (1 1),

 (intersection (list (list 1 2) (list 1 4)) (list (list 1 2) (list 1 5)))

doesnt return me (1 2) but NIL.

Where is my mistake?

Please tag your question with the language that you are using — Spok, May 31 '22 at 18:55
Don't worry about it. The tag looks fine to me. It's possible Nicolas overlooked that tag and/or didn't know "Common Lisp" was the name of a particular language. — Silvio Mayolo, May 31 '22 at 21:08

score 1 · Accepted Answer · answered May 31 '22 at 19:02

1

intersection takes an optional :test keyword argument to specify what definition of "equality" to use. The default is eql, which compares lists for pointer equality. Since your two lists are actually distinct lists in memory, they're not eql. Use equal to compare lists element-wise.

  (intersection (list (list 1 2) (list 1 4)) (list (list 1 2) (list 1 5)) :test #'equal)

answered May 31 '22 at 19:02

Silvio Mayolo

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Short and simple. Thank you for the explanation! – sergeantSalty May 31 '22 at 19:07

Compare/Intersection List of Lists

1 Answers1