lang comparison: in-place algorithm for reversing a list in Perl, Python, Lisp

Xah Lee xahlee at gmail.com
Thu Mar 1 05:07:49 CET 2012


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



More information about the Python-list mailing list