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Path: ...!feed.opticnetworks.net!eternal-september.org!feeder3.eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail From: "B. Pym" <No_spamming@noWhere_7073.org> Newsgroups: comp.lang.lisp Subject: Re: Another code review perhaps? Date: Thu, 27 Jun 2024 17:27:25 -0000 (UTC) Organization: A noiseless patient Spider Lines: 125 Message-ID: <v5k7dr$2rb8r$1@dont-email.me> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Injection-Date: Thu, 27 Jun 2024 19:27:26 +0200 (CEST) Injection-Info: dont-email.me; posting-host="722d2135e1f4d991409dbc16d21c8b9f"; logging-data="2993435"; mail-complaints-to="abuse@eternal-september.org"; posting-account="U2FsdGVkX18dvavRAlbZVf6foTB6Lw+n" User-Agent: XanaNews/1.18.1.6 Cancel-Lock: sha1:UJZe2XtQYxU4CKOnCEJ1a6RRFxY= Bytes: 5964 > > This is my solution to Ex. 5 on p. 97 of Paul Graham's "ANSI Common > > Lisp" > > > > <QUOTE> > > Define iterative and recursive versions of a function that takes an > > object x and a vector v, and returns a list of all the objects that > > immediately precede x in v. > > > > > (precedes #\a "abracadabra") > > (#\c #\d #\r) > > </QUOTE> > > > > (I'll just ask about the iterative solution I developed.) > > > > ;;;;Ex. 5 > > (defun precedes (object vector) > > (let ((maximum-vector-index (- (length vector) 1)) > > (return-list nil)) > > (dotimes (vector-index maximum-vector-index return-list) > > (let ((test-vector-element (aref vector (+ vector-index 1))) > > (preceding-vector-element (aref vector vector-index))) > > (if (and (eql object test-vector-element) > > (not (member preceding-vector-element return-list))) > > (push preceding-vector-element return-list)))))) > > > > Do you think that the use of DOTIMES is better than DO in this case? > DOTIMES is fine. My main commentt is, while it's good to use > descriptive names, you don't want to go overboard. And you don't > really need to pull the (- (length vector) 1) expression out since > it's only used once--I think it's actually more clear to use it > directly in place; seeing it in place in the DOTIMES I know what it's > for immediately. (Also, that's one of the benefits of using DOTIMES > compared to DO, is that it only evaluates the count-form once). > Finally, you can use PUSHNEW to do the membership test for you. > Anyway, here's how I'd modify your original to make it (IMO) a bit > more clear but otherwise about the same. Note how I haven't > abbreviated any names--they're all full words. But I don't think > anything is lost by, for example, using a single word, `index' instead > of `vector-index': > > (defun precedes (object vector) > (let ((results nil)) > (dotimes (index (1- (length vector)) results) > (let ((current (aref vector (1+ index))) > (previous (aref vector index))) > (when (eql object current) > (pushnew previous results)))))) > > Now that I can sort of see what's going on, I see that `current' and > `previous' are also only used once each so I think it'll further > clarify things to inline the expressions. I'd also abbreviate the > index variable--not because it's less typing but because I can tell at > a glance that it's just a regular index variable. Matter of taste: > > (defun precedes (object vector) > (let ((results nil)) > (dotimes (idx (1- (length vector)) results) > (when (eql object (aref vector (1+ idx))) > (pushnew (aref vector idx) results))))) > > That we've got things boiled down a bit we can try writing the > equivalent using DO. Because we can control the starting value of the > index with a DO loop I switch to starting at 1 and looping up to the > end of the vector. I always try to have my index variable actually > looping over the indices I want to use--I always screw it up if the > end test is anything more complicated than (= idx (length vector)). > > (defun precedes (object vector) > (do ((length (length vector)) > (results nil) > (idx 1 (1+ idx))) > ((= idx length) results) > (when (eql object (aref vector idx)) > (pushnew (aref vector (1- idx)) results)))) > > I don't think that's really any better. Maybe LOOP: > > (defun precedes (object vector) > (loop with results = nil > for idx from 1 below (length vector) > when (eql object (aref vector idx)) > do (pushnew (aref vector (1- idx)) results) > finally (return results))) > > About the same as the DOTIMES version. However I might opt for > expressing the removal of duplicates more explicitly, by using > DELETE-DUPLICATES (which also lets me use LOOP's collect mechanism): > > (defun precedes (object vector) > (delete-duplicates > (loop for idx from 1 below (length vector) > when (eql object (aref vector idx)) > collect (aref vector (1- idx))) > > Or for a fairly different way of looking at it, there's already a > function to find the position of a given object in a sequence. Maybe > we can use it: > > (defun precedes (object vector) > (delete-duplicates > (loop for start = 1 then (1+ pos) > for pos = (position object vector :start start) > while pos collect (aref vector (1- pos))))) > > I don't know if this last one is really better in any way but it's > worth considering that there are a bunch of built in functions for > doing good stuff with sequences. Gauche Scheme (use gauche.sequence) ;; subseq (define (precedes object vec) (fold (lambda (a b accum) (if (and (equal? object b) (not (member a accum))) (cons a accum) accum)) '() vec (subseq vec 1))) (precedes 'b #(b a b c d b e f b f b)) ===> (f d a)