(take n l) is that n is a nonnegative integer
and l is a true list.
Major Section: PROGRAMMING
For any natural number n not exceeding the length of l,
(take n l) collects the first n elements of the list l.
The following is a theorem (though it takes some effort, including lemmas, to get ACL2 to prove it):
(equal (length (take n l)) (nfix n))If
n is is an integer greater than the length of l, then
take pads the list with the appropriate number of nil
elements. Thus, the following is also a theorem.
(implies (and (integerp n)
(true-listp l)
(<= (length l) n))
(equal (take n l)
(append l (make-list (- n (length l))))))
For related functions, see nthcdr and see butlast.
The guard for (take n l) is that n is a nonnegative integer
and l is a true list.