What is the case where you would prefer “del x[0]” over “cdr”?
Isn’t this just the difference between mutable and persistent data structures?
Isn’t this just the difference between mutable and persistent data structures?
x = [42, 99]
y = x
del x[0]
y[0] ; 99
Most of the time I try to get rid of variable names so it seems somewhat strange to invent more than one pointing to the same data structure in RAM, but I'm a simple person that gets confused easily.
(defun vector-delete! (vector index)
"Delete the element at INDEX from the given VECTOR.
Destructively modifies the vector.
The vector must have a fill-pointer.
This is not thread-safe, use at your own risk.
Examples:
(let* ((x (make-array 4 :fill-pointer 4 :initial-contents '(1 2 3 4)))
(y x))
(vector-delete! x 0)
y)
;; => #(2 3 4)"
(declare (type (Integer 0 *) index)
(type Vector vector))
(assert (array-has-fill-pointer-p vector))
(let ((l (length vector)))
(assert (< index l))
(let ((removed-element (aref vector index)))
(loop :for i :from (1+ index) :below l
:do
(setf (aref vector (1- i))
(aref vector i)))
(decf (fill-pointer vector))
removed-element)))
I think there is less confusion about this kind of thing with Clojure, where a preference for immutable persistent data structures is strongly stated in the language design. So the persistent nature of lists and other data structures is fore grounded, and there is no confusion over why the equivalent of "del x[0]" is not supported.