I'd like to make copies of my 2D array, which feels like the nice, functional, nondestructive way of handling arrays. What is the lispy way of doing this?
I use the following, which I believe is better than the alexandria version:
(defun copy-array (array &key (element-type (array-element-type array)) (fill-pointer (and (array-has-fill-pointer-p array) (fill-pointer array))) (adjustable (adjustable-array-p array))) "Returns an undisplaced copy of ARRAY, with same fill-pointer and adjustability (if any) as the original, unless overridden by the keyword arguments." (let* ((dimensions (array-dimensions array)) (new-array (make-array dimensions :element-type element-type :adjustable adjustable :fill-pointer fill-pointer))) (dotimes (i (array-total-size array)) (setf (row-major-aref new-array i) (row-major-aref array i))) new-array))
The problem with the alexandria version is that the
adjust-array hack causes the result (at least on SBCL) to never be a
simple-array, which some other libraries (e.g. opticl) expect. The above version also was faster for me.
Someone else has published a very similar version in a different library, but I forgot the names of both person and library.
(defun copy-array (array &key (element-type (array-element-type array)) (fill-pointer (and (array-has-fill-pointer-p array) (fill-pointer array))) (adjustable (adjustable-array-p array))) "Returns an undisplaced copy of ARRAY, with same fill-pointer and adjustability (if any) as the original, unless overridden by the keyword arguments. Performance depends on efficiency of general ADJUST-ARRAY in the host lisp -- for most cases a special purpose copying function is likely to perform better." (let ((dims (array-dimensions array))) ;; Dictionary entry for ADJUST-ARRAY requires adjusting a ;; displaced array to a non-displaced one to make a copy. (adjust-array (make-array dims :element-type element-type :fill-pointer fill-pointer :adjustable adjustable :displaced-to array) dims)))
It depends how your 2D array is represented, and what flavor of Lisp are you using.
If you are using Common Lisp, then copy-seq could be useful.
If you want to do things
the nice, functional, nondestructive way, then why do you even need to copy it?
if you're copying it in order to update it -- then you're not doing it the functional way.
如果你要复制它以便更新它 - 那么你就不是以功能的方式来做它。
if you're doing it the functional way -- then you don't need a copy. You can just pass it anywhere and everywhere.
如果你正在以功能的方式做 - 那么你不需要副本。你可以随时随地传递它。
Maybe you want to transform it. In that case, you could use one of Lisp's many pure functions, such as