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vector-pop

vector-pop Function

Syntax:

vector-pop vector → element

Arguments and Values:

vector—a vector with a fill pointer .

element—an object.

Description:

Decreases the fill pointer of vector by one, and retrieves the element of vector that is designated by the new fill pointer .

Examples:

(vector-push (setq fable (list ’fable)) 
	     (setq fa (make-array 8 
				  :fill-pointer 2 
				  :initial-element ’sisyphus))) → 2 
(fill-pointer fa) → 3 
(eq (vector-pop fa) fable) → true 
(vector-pop fa) → SISYPHUS 
(fill-pointer fa) → 1

Side Effects:

The fill pointer is decreased by one.

Affected By:

The value of the fill pointer .

Exceptional Situations:

An error of type type-error is signaled if vector does not have a fill pointer .

If the fill pointer is zero, vector-pop signals an error of type error.

See Also:

vector-push, vector-push-extend, fill-pointer

Expanded Reference: vector-pop​

Basic Usage​

vector-pop decreases the fill pointer by one and returns the element that was at that position. It is the inverse of vector-push.

(let ((v (make-array 5 :fill-pointer 3
                     :initial-contents '(a b c nil nil))))
  (list (vector-pop v)      ; removes and returns C
        (vector-pop v)      ; removes and returns B
        (fill-pointer v)
        v))
=> (C B 1 #(A))

Stack-Like Behavior​

vector-push and vector-pop together implement a stack (LIFO) on a vector.

(let ((stack (make-array 10 :fill-pointer 0)))
  (vector-push 'first stack)
  (vector-push 'second stack)
  (vector-push 'third stack)
  (list (vector-pop stack)    ; LIFO: third comes out first
        (vector-pop stack)
        (vector-pop stack)))
=> (THIRD SECOND FIRST)

Error on Empty Vector​

vector-pop signals an error if the fill pointer is zero (the vector is logically empty).

;; This would signal an error:
;; (vector-pop (make-array 5 :fill-pointer 0))
;; --> ERROR: fill pointer is zero

;; Safe usage -- check before popping
(let ((v (make-array 5 :fill-pointer 0)))
  (if (> (fill-pointer v) 0)
      (vector-pop v)
      :empty))
=> :EMPTY

Push/Pop Cycle with vector-push-extend​

Combining vector-push-extend and vector-pop for a dynamically-sized stack.

(let ((stack (make-array 2 :fill-pointer 0 :adjustable t)))
  (dotimes (i 5)
    (vector-push-extend (* i 10) stack))
  (let ((results '()))
    (dotimes (i 3)
      (push (vector-pop stack) results))
    (list :popped (nreverse results)
          :remaining (copy-seq stack))))
=> (:POPPED (40 30 20) :REMAINING #(0 10))