position - Lisp array element swap -


i'm new lisp , i'm trying trade 2 elements in array. know if there function specified position can use rotatef swap them.

i've tried function position doesn't work on arrays since isn't sequence.

what best computational solution in case there isn't built in function array?

i've searched around , can't seem find simple solution. row-major-aref solution?

basically, want find position of element in 2 dimensional array , return position used in rotatef

i think should work - arrays indeed sequences.

(let* ((an-array (make-array 6 :initial-contents '(#\f #\o #\o #\b #\a #\r)))        (f-pos (position #\f an-array))        (r-pos (position #\r an-array)))   (rotatef (elt an-array f-pos)            (elt an-array r-pos))   an-array) ;=> #(#\r #\o #\o #\b #\a #\f)

of course, don't need bind positions names. work too:

(let ((an-array (make-array 6 :initial-contents '(#\f #\o #\o #\b #\a #\r))))   (rotatef (elt an-array (position #\f an-array))            (elt an-array (position #\r an-array)))   an-array) ;=> #(#\r #\o #\o #\b #\a #\f)

if problem position not finding element need, :test argument may helpful. there position-if , position-if-not functions let supply own predicate identifying element. 3 described here in hyperspec.

here's example won't work without :test argument, since default (which eql sequence functions :test arguments - see table 11-2 here nice summary of standard sequence function keyword arguments) doesn't work on lists.

(let ((an-array (make-array 3 :initial-contents '((1 2 3)                                                   (4 5 6)                                                   (7 8 9)))))   (rotatef (elt an-array (position '(1 2 3) an-array :test #'equal))            (elt an-array (position '(7 8 9) an-array :test #'equal)))   an-array) ;=> ((7 8 9) (4 5 6) (1 2 3))

(tested on sbcl 1.0.55.0.debian).

added:

here's brute force way two-dimensional array. find-position assumes array of size (3 3) easy make more general.

i don't advocate best solution, didn't want leave empty handed after misunderstanding question :)

(defvar an-array #2a((1 2 3) (4 5 6) (7 8 9)))  (defun find-position (array item &key (test #'eql))   (loop below 3         (loop j below 3               (when (funcall test (aref array j) item)                 (return-from find-position (list j))))))  (defun swap-4-and-7 (array)   ;; example use   (destructuring-bind (i1 j1) (find-position array 4)     (destructuring-bind (i2 j2) (find-position array 7)       (rotatef (aref array i1 j1)                (aref array i2 j2))))   array)  (swap-4-and-7 an-array) ;=> #2a((1 2 3) (7 5 6) (4 8 9))

Comments

Post a Comment

Popular posts from this blog

android - Get AccessToken using signpost OAuth without opening a browser (Two legged Oauth) -

i using signpost oauth access data magento server. have read various tutorials on same , reach point open browser user can enter credentials. however, per requirement have automate part. hence, user should not browser page. have set-up done on server side ( magento ), hit url , returned call page. same through program in android. i have tried below, consumer = new commonshttpoauthconsumer(key, secret); provider = new commonshttpoauthprovider(oauth_init_url,access_token_url, authorize_url); try { provider.retrieverequesttoken(consumer, oauth.out_of_band); log.d("tokens" , consumer.gettoken() + " -- " + consumer.gettokensecret()); } and request tokens. dont know how bypass next step. tried directly accessing accesstoken (stupid of me) provider.retrieveaccesstoken(consumer, "mycallback://callback"); no luck, ends in oauth.signpost.exception.oauthnotauthorizedexception: authorization failed (server replied 401). ...
Read more

org.mockito.exceptions.misusing.InvalidUseOfMatchersException: mockito -

getting error org.mockito.exceptions.misusing.invaliduseofmatchersexception: invalid use of argument matchers! 1 matchers expected, 2 recorded. exception may occur if matchers combined raw values: //incorrect: somemethod(anyobject(), "raw string"); when using matchers, arguments have provided matchers. example: //correct: somemethod(anyobject(), eq("string matcher")); at when( getprogramservice .callservice(any(getprogramtype.class))) .thenreturn(jaxbresponse); please explain error , possible resolution the code posted looks fine; don't see problem it. code before or above causing exception. key here: invalid use of argument matchers! 1 matchers expected, 2 recorded. any , used above, doesn't return "special kind of null" or "special kind of getprogramtype"; don't exist. instead, returns null , side effect puts matcher on stack. whe...
Read more

google shop client API returns 400 bad request error while adding an item -

i'm working on solution add items in google shopping market java application. following tutorial real mess because of missing classes of laste release, after setting 1.5-beta maven managed make base example typecheck correctly classes. still can make simple task of adding 1 item. instead of posting whole code i'm posting part creates product feed. private product createproduct(string id) { product product = new product(); product.title = "red wool sweater"; product.content = new content( "text", "comfortable , soft, sweater keep warm on " + "cold winter nights. red , blue stripes."); product.appcontrol = new appcontrol(); product.appcontrol.addrequireddestination("productads"); product.externalid = id; product.lang = "it"; product.country = "it"; product.condition = "nuovo"; product.price = new price("eu...
Read more