Question

In python, you might do something like

i = (0, 3, 2)
x = [x+1 for x in range(0,5)]
operator.itemgetter(*i)(x)

to get (1, 4, 3).
In (emacs) lisp, I wrote this function called extract which does something similar,

(defun extract (elems seq)
  (mapcar (lambda (x) (nth x seq)) elems))

(extract '(0 3 2) (number-sequence 1 5))

but I feel like there should be something built in? All I know is first, last, rest, nth, car, cdr… What’s the way to go? ~ Thanks in advance ~

Answer 1

If your problem is the speed then use (vector 1 2 3 4 5) instead of a list, and (aref vec index) to get the element.

(defun extract (elems seq)
  (let ((av (vconcat seq)))
    (mapcar (lambda (x) (aref av x)) elems)))

If you’re going to extract from the same sequence many times of course it make sense to store the sequence in a vector just once.
Python lists are indeed one-dimensional arrays, the equivalent in LISP are vectors.