Xah Lee wrote: > fun example. > > in-place algorithm for reversing a list in Perl, Python, Lisp > http://xahlee.org/comp/in-place_algorithm.html > > plain text follows > ---------------------------------------- > > What's In-place Algorithm? > > Xah Lee, 2012-02-29 > > This page tells you what's In-place algorithm, using {python, perl, > emacs lisp} code to illustrate. > > Here's Wikipedia In-place algorithm excerpt: > > In computer science, an in-place algorithm (or in Latin in situ) is an > algorithm which transforms input using a data structure with a small, > constant amount of extra storage space. The input is usually > overwritten by the output as the algorithm executes. An algorithm > which is not in-place is sometimes called not-in-place or out-of- > place. > > Python > > Here's a python code for reversing a list. Done by creating a new > list, NOT using in-place: > > # python > # reverse a list > > list_a = ["a", "b", "c", "d", "e", "f", "g"] > > list_length = len(list_a) > list_b = [0] * list_length > > for i in range(list_length): > list_b[i] = list_a[list_length -1 - i] > > print list_b > Here's in-place algorithm for reversing a list: > > # python > # in-place algorithm for reversing a list > > list_a = ["a", "b", "c", "d", "e", "f", "g"] > > list_length = len(list_a) > > for i in range(list_length/2): > x = list_a[i] > list_a[i] = list_a[ list_length -1 - i] > list_a[ list_length -1 - i] = x > > print list_a > Perl > > Here's a perl code for reversing a list. Done by creating a new list, > NOT using in-place: > > # perl > > use strict; > use Data::Dumper; > > my @listA = qw(a b c d e f g); > > my $listLength = scalar @listA; > my @listB = (); > > for ( my $i = 0; $i < $listLength; $i++ ) { > $listB[$i] = $listA[ $listLength - 1 - $i]; > } > > print Dumper(\@listB); > > # perl > # in-place algorithm for reversing a list. > > use strict; > use Data::Dumper; > use POSIX; # for floor > > my @listA = qw(a b c d e f g); > > my $listLength = scalar @listA; > > for ( my $i = 0; $i < floor($listLength/2); $i++ ) { > my $x = $listA[$i]; > $listA[$i] = $listA[ $listLength - 1 - $i]; > $listA[ $listLength - 1 - $i] = $x; > } > > print Dumper(\@listA); > __END__ > > emacs lisp > > ;; emacs lisp > ;; reverse a array > > (setq arrayA ["a" "b" "c" "d" "e" "f" "g"]) > > (setq arrayLength (length arrayA)) > > (setq arrayB (make-vector arrayLength 0)) > > (dotimes (i arrayLength ) > (aset arrayB i (aref arrayA (- (1- arrayLength) i)) ) > ) > > (print (format "%S" arrayB)) > ;; emacs lisp > ;; in-place algorithm for reversing a array > > (setq arrayA ["a" "b" "c" "d" "e" "f" "g"]) > > (setq arrayLength (length arrayA)) > > (dotimes (i (floor (/ arrayLength 2))) > (let (x) > (setq x (aref arrayA i)) > (aset arrayA i (aref arrayA (- (1- arrayLength) i))) > (aset arrayA (- (1- arrayLength) i) x) ) ) > > (print (format "%S" arrayA)) > > Xah
NewLisp: > (setq lst '(2 3 5 8)) (2 3 5 8) > (reverse lst) (8 5 3 2) > lst (8 5 3 2) -- http://mail.python.org/mailman/listinfo/python-list
