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row-major-aref

row-major-aref Accessor

Syntax:

row-major-aref array index → element

(setf (row-major-aref array index**)** new-element**)**

Arguments and Values:

array—an array.

index—a valid array row-major index for the array.

element, new-element—an object.

Description:

Considers array as a vector by viewing its elements in row-major order, and returns the element of that vector which is referred to by the given index.

row-major-aref is valid for use with setf.

See Also:

aref, array-row-major-index

Notes:

(row-major-aref array index) ≡

(aref (make-array (array-total-size array)

:displaced-to array

:element-type (array-element-type array))

index)

(aref array i1 i2 ...) ≡

(row-major-aref array (array-row-major-index array i1 i2))

Expanded Reference: row-major-aref​

Basic Usage​

row-major-aref accesses an array element using a single row-major index, treating the array as if it were flattened into one dimension.

(let ((m (make-array '(2 3) :initial-contents '((a b c) (d e f)))))
  (list (row-major-aref m 0)    ; element (0,0) = a
        (row-major-aref m 2)    ; element (0,2) = c
        (row-major-aref m 3)    ; element (1,0) = d
        (row-major-aref m 5)))  ; element (1,2) = f
=> (A C D F)

Setting Elements with SETF​

row-major-aref works with setf.

(let ((m (make-array '(2 2) :initial-element 0)))
  (setf (row-major-aref m 1) 10)   ; element (0,1)
  (setf (row-major-aref m 2) 20)   ; element (1,0)
  m)
=> #2A((0 10) (20 0))

Iterating Over All Elements Generically​

row-major-aref combined with array-total-size lets you iterate over any array regardless of rank.

;; Sum all elements of a 3D array
(let ((a (make-array '(2 2 2) :initial-contents
                     '(((1 2) (3 4)) ((5 6) (7 8)))))
      (total 0))
  (dotimes (i (array-total-size a) total)
    (incf total (row-major-aref a i))))
=> 36

Collecting All Elements Into a List​

(defun array-to-list (array)
  "Flatten any array into a list of its elements in row-major order."
  (loop for i below (array-total-size array)
        collect (row-major-aref array i)))

(array-to-list (make-array '(2 3) :initial-contents '((a b c) (d e f))))
=> (A B C D E F)

Equivalence to aref​

(row-major-aref array idx) accesses the same element as (aref array i0 i1 ...) where idx is (array-row-major-index array i0 i1 ...).

(let ((m (make-array '(3 4) :initial-contents
                     '((0 1 2 3) (4 5 6 7) (8 9 10 11)))))
  (= (aref m 2 1)
     (row-major-aref m (array-row-major-index m 2 1))))
=> T