APPEND
and NCONC
for an example.
Well, you can have your cake and eat it too, by using optional (or
keyword) parameters. Here's an example: Let's assume you're writing a
function COMPLEX-MATRIX-STUFF which takes two matrices
M1 and M2 as its arguments and computes and
returns a resulting matrix the size of which depends on
M1 and M2, i.e. for a fresh result you'll
need an empty matrix which'll be created by, say,
(MAKE-APPROPRIATE-RESULT-MATRIX-FOR M1 M2).
The classical textbook way to implement this function will more or less look like this:
(defun complex-matrix-stuff (m1 m2)
(let ((result (make-appropriate-result-matrix-for m1 m2)))
;; ... compute storing the results in RESULT
result))
And you'll use it like this:
(setq some-matrix (complex-matrix-stuff A B))But why not write it like so:
(defun complex-matrix-stuff (m1 m2
&optional
(result
(make-appropriate-result-matrix-for m1 m2)))
;; ... compute storing the results in RESULT
result)
Now you have it both ways. You can still "make up results"
on the fly as in:
(setq some-matrix (complex-matrix-stuff A B))But you can also (destructively) re-use previously allocated matrices:
(complex-matrix-stuff A B some-appropriate-matrix-I-built-before)Or use your function like this:
(setq some-other-matrix
(complex-matrix-stuff A B some-appropriate-matrix-I-built-before))
in which case you'll end up with:
* (eq some-other-matrix some-appropriate-matrix-I-built-before) T
ADJUST-ARRAY instead of consing up new sequences with SUBSEQSTART and END keywords so you can make them
operate on a sub-sequence without actually creating it, i.e. instead of
(count #\a (subseq long-string from to))you should of course use
(count #\a long-string :start from :end to)which'll yield the same result but not create an unnecessary intermediate sub-sequence.
However, sometimes it looks like you can't avoid creating new data. Consider a hash table the keys of which are strings. If the key you're looking for is a sub-string of another string you'll most likely end up with
(gethash (subseq original-string from to)
has-table)
But you don't have to. You can create one displaced string and reuse
it multiple times with ADJUST-ARRAY:
(let ((substring (make-array 0
:element-type 'character
:displaced-to ""
:displaced-index-offset 0)))
;; more code
(gethash
(adjust-array substring (- to from)
:displaced-to original-string
:displaced-index-offset from)
hash-table)
;; even more code
)