group without recursion
in [Lisp]
|
From: Tamas K Papp on 12 Mar 2010 04:31 Hi, I am trying to rewrite GROUP from On Lisp without recursion (just as an exercise). My first attempt is below, I am wondering if there is a nicer (more idiomatic, etc) way to do it. (defun group (list n) "Return elements of LIST as a list of lists in groups of N." (check-type n (integer 1)) (let (sublist result (i 0)) (dolist (element list) (push element sublist) (incf i) (when (= i n) (push (nreverse sublist) result) (setf i 0 sublist nil))) (assert (zerop i) () "~A could not be broken up to sublists of ~A elements" list n) (nreverse result))) Thanks, Tamas
From: Rainer Joswig on 12 Mar 2010 04:44 In article <7vufvgFifkU1(a)mid.individual.net>, Tamas K Papp <tkpapp(a)gmail.com> wrote: > Hi, > > I am trying to rewrite GROUP from On Lisp without recursion (just as an > exercise). My first attempt is below, I am wondering if there is a > nicer (more idiomatic, etc) way to do it. > > (defun group (list n) > "Return elements of LIST as a list of lists in groups of N." > (check-type n (integer 1)) > (let (sublist > result > (i 0)) > (dolist (element list) > (push element sublist) > (incf i) > (when (= i n) > (push (nreverse sublist) result) > (setf i 0 > sublist nil))) > (assert (zerop i) () "~A could not be broken up to sublists of ~A elements" list n) > (nreverse result))) > > Thanks, > > Tamas Nicer? Let's see... (defun group (list n) "Return elements of LIST as a list of lists in groups of N." (check-type n (integer 1)) (loop with list1 = list while list1 collect (loop repeat n unless list1 do (error "~A could not be broken up to sublists of ~A elements" list n) collect (pop list1)))) -- http://lispm.dyndns.org/
From: Tamas K Papp on 12 Mar 2010 06:17 On Fri, 12 Mar 2010 10:44:54 +0100, Rainer Joswig wrote: > In article <7vufvgFifkU1(a)mid.individual.net>, > Tamas K Papp <tkpapp(a)gmail.com> wrote: > >> Hi, >> >> I am trying to rewrite GROUP from On Lisp without recursion (just as an >> exercise). My first attempt is below, I am wondering if there is a >> nicer (more idiomatic, etc) way to do it. >> >> (defun group (list n) >> "Return elements of LIST as a list of lists in groups of N." >> (check-type n (integer 1)) >> (let (sublist >> result >> (i 0)) >> (dolist (element list) >> (push element sublist) >> (incf i) >> (when (= i n) >> (push (nreverse sublist) result) >> (setf i 0 >> sublist nil))) >> (assert (zerop i) () "~A could not be broken up to sublists of ~A >> elements" list n) (nreverse result))) >> >> Thanks, >> >> Tamas > > Nicer? Let's see... > > > (defun group (list n) > "Return elements of LIST as a list of lists in groups of N." > (check-type n (integer 1)) > (loop with list1 = list > while list1 collect > (loop repeat n > unless list1 > do (error "~A could not be broken up to sublists of ~A > elements" list n) collect (pop list1)))) Very neat, thanks! I didn't think of popping elements like that, but it makes sense. Tamas
From: Rainer Joswig on 12 Mar 2010 06:38 In article <7vum6aFifkU2(a)mid.individual.net>, Tamas K Papp <tkpapp(a)gmail.com> wrote: > On Fri, 12 Mar 2010 10:44:54 +0100, Rainer Joswig wrote: > > > In article <7vufvgFifkU1(a)mid.individual.net>, > > Tamas K Papp <tkpapp(a)gmail.com> wrote: > > > >> Hi, > >> > >> I am trying to rewrite GROUP from On Lisp without recursion (just as an > >> exercise). My first attempt is below, I am wondering if there is a > >> nicer (more idiomatic, etc) way to do it. > >> > >> (defun group (list n) > >> "Return elements of LIST as a list of lists in groups of N." > >> (check-type n (integer 1)) > >> (let (sublist > >> result > >> (i 0)) > >> (dolist (element list) > >> (push element sublist) > >> (incf i) > >> (when (= i n) > >> (push (nreverse sublist) result) > >> (setf i 0 > >> sublist nil))) > >> (assert (zerop i) () "~A could not be broken up to sublists of ~A > >> elements" list n) (nreverse result))) > >> > >> Thanks, > >> > >> Tamas > > > > Nicer? Let's see... > > > > > > (defun group (list n) > > "Return elements of LIST as a list of lists in groups of N." > > (check-type n (integer 1)) > > (loop with list1 = list > > while list1 collect > > (loop repeat n > > unless list1 > > do (error "~A could not be broken up to sublists of ~A > > elements" list n) collect (pop list1)))) > > Very neat, thanks! I didn't think of popping elements like that, but it > makes sense. > > Tamas Slightly improved: (defun group (list n &aux (orig-list list)) "Return elements of LIST as a list of lists in groups of N." (check-type n (integer 1)) (loop while list collect (loop repeat n unless list do (error "~A could not be broken up to sublists of ~A elements" orig-list n) collect (pop list)))) -- http://lispm.dyndns.org/
From: Jochen Schmidt on 12 Mar 2010 08:40
(defun group (list n) (loop for e in list for i upfrom 1 collect e into group if (zerop (mod i 5)) collect (copy-list group) and do (setf group nil))) -- Jochen Schmidt CRISPYLOGICS Uhlandstr. 9, 90408 Nuremberg Fon +49 (0)911 517 999 82 Fax +49 (0)911 517 999 83 mailto:(format nil "~(~36r@~36r.~36r~)" 870180 1680085828711918828 16438) http://www.crispylogics.com |