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

2012-03-02 Thread Xah Lee 

Xah Lee wrote:
«… One easy way to measure it is whether a programer can read and
understand a program without having to delve into its idiosyncrasies.
…»

Chris Angelico wrote:
«Neither the behavior of ints nor the behavior of IEEE floating point
is a quirk or an idiosyncracy. …»

they are computer engineering by-products. Are quirks and
idiosyncracies. Check out a advanced lang such as Mathematica. There,
one can learn how the mathematical concept of integer or real number
are implemented in a computer language, without lots by-products of
comp engineering as in vast majority of langs (all those that chalks
up to some IEEE. (which, sadly, includes C, C++, java, perl,
python, lisp, and almost all. (lisp idiots speak of the jargon “number
tower” instead I.) (part of the reason almost all langs stick to
some I stuff is because it's kinda standard, and everyone
understand it, in the sense that unix RFC (aka really fucking common)
is wide-spread because its free yet technically worst. (in a sense,
when everybody's stupid, there arise a cost to not be stupid..

 Xah
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-03-01 Thread Rainer Weikusat 
Xah Lee xah...@gmail.com writes:

[...]


 # 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);

Better algorithm for that (expects an array reference as first
argument)

sub rev
{
my $a = $_[0];
my ($n0, $n1, $x);

$n0 = 0;
$n1 = $#$a;
while ($n0  $n1) {
$x = $a-[$n0];
$a-[$n0] = $a-[$n1];
$a-[$n1] = $x;

++$n0;
--$n1;
}
}

NB: The fact that a sufficiently sophisticated compiler might be able
to fix this automatically emphasizes the deficiencies of the original
attempt, it doesn't excuse them.
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-03-01 Thread Xah Lee 
On Mar 1, 7:04 am, Kaz Kylheku k...@kylheku.com wrote:
 lisp:
 (floor (/ x y)) --[rewrite]-- (floor x y)

Thanks for this interesting point.

I don't think it's a good lang design, more of a lang quirk.

similarly, in Python 2.x,
x/y
will work when both x and y are integers. Also,
x//y
works too, but that // is just perlish unreadable syntax quirk.

similarly, in perl, either one
require POSIX; floor(x/y);
the require POSIX instead of Math is a quirk. But even, floor should
really be builtin.
or
using a perl hack
int(x/y)

all of the above are quirks. They rely on computer engineering by-
products (such as int), or rely on the lang's idiosyncrasy. One easy
way to measure it is whether a programer can read and understand a
program without having to delve into its idiosyncrasies. Problem with
these lang idioms is that it's harder to understand, and whatever
advantage/optimization they provide is microscopic and temporary.

best is really floor(x/y).

idiomatic programing, is a bad thing. It was spread by perl, of
course, in the 1990s. Idiomatic lang, i.e. lang with huge number of
bizarre idioms, such as perl, is the worst.

 Xah
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-03-01 Thread Rainer Weikusat 
Xah Lee xah...@gmail.com writes:

[...]

 similarly, in perl, either one
 require POSIX; floor(x/y);
 the require POSIX instead of Math is a quirk. But even, floor should
 really be builtin.
 or
 using a perl hack
 int(x/y)

 all of the above are quirks. They rely on computer engineering by-
 products (such as int),

Integral numbers are not 'a computer engineering byproduct'.

 or rely on the lang's idiosyncrasy. One easy way to measure it is
 whether a programer can read and understand a program without having
 to delve into its idiosyncrasies. Problem with these lang idioms is
 that it's harder to understand, and whatever advantage/optimization
 they provide is microscopic and temporary.

It's hard to understand for someone who knows only mathematical
idiosyncrasies and who is also convinced that this should really be
more than enough for a lifetime. But that's not some kind of 'natural
knowledge' people just happen to have but systematically drilled into
pupils from a very early age, despite most of them won't ever have any
use for any of it insofar it goes beyond + - * /. 

[...]

 idiomatic programing, is a bad thing.

If you have to use something (like a particular programming language)
but you resent learning how to use it and rather make lofty excuses,
chances are that you are rather a lazy f*cker than a great philosopher
...
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-03-01 Thread Chris Angelico 
On Fri, Mar 2, 2012 at 9:04 AM, Xah Lee xah...@gmail.com wrote:
 One easy
 way to measure it is whether a programer can read and understand a
 program without having to delve into its idiosyncrasies.

Neither the behavior of ints nor the behavior of IEEE floating point
is a quirk or an idiosyncracy. These are data types with
well-defined semantics, and you need to understand them to use them.
The fact that dividing two positive integers and producing (or casting
to) a third integer rounds the result down is just as much a part of
the definition as is two's complement negatives, which most people can
safely ignore because they just work the way you expect.

Learn what you're working with, if you expect to get decent results from it.

ChrisA
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-03-01 Thread WJ 
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

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

2012-02-29 Thread Xah Lee 
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
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-02-29 Thread Steven D'Aprano 
On Wed, 29 Feb 2012 20:07:49 -0800, Xah Lee wrote:

 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

This is a good example of code written by somebody not very familiar with 
Python idioms. You don't need a temporary variable to swap two values in 
Python. A better way to reverse a list using more Pythonic idioms is:

for i in range(len(list_a)//2):
list_a[i], list_a[-i-1] = list_a[-i-1], list_a[i]


But the best way (even more idiomatic and much, much faster) is this:

list_a.reverse()



-- 
Steven
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-02-29 Thread Dan Stromberg 
On Wed, Feb 29, 2012 at 8:07 PM, Xah Lee xah...@gmail.com 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.

 Aren't in-place reversals rather non-functional?


 Python

The usual way is list_a.reverse().  This is in place.

If you want to be a little weird, you could do this, but it's not in place:
list_a = list_a[::-1]

If you want to pretend you can't do this the easy ways above, you could do
this, which is in place:
length = len(list_a)
for ind in xrange(length // 2):
   other=-ind-1
   list_a[ind], list_a[other] = list_a[other], list_a[ind]

HTH
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-02-29 Thread Xah Lee 
On Feb 29, 9:01 pm, Steven D'Aprano steve
+comp.lang.pyt...@pearwood.info wrote:
 You don't need a temporary variable to swap two values in
 Python. A better way to reverse a list using more Pythonic idioms is:

 for i in range(len(list_a)//2):
     list_a[i], list_a[-i-1] = list_a[-i-1], list_a[i]

forgive me sir, but i haven't been at python for a while. :)
i was, actually, refreshing myself of what little polyglot skills i
have.

 Xah
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-02-29 Thread Evan Driscoll 
On 2/29/2012 23:05, Dan Stromberg wrote:
 
 On Wed, Feb 29, 2012 at 8:07 PM, Xah Lee xah...@gmail.com
 mailto:xah...@gmail.com wrote:
 
 This page tells you what's “In-place algorithm”, using {python, perl,
 emacs lisp} code to illustrate.
 
 Aren't in-place reversals rather non-functional?

There is one place where they're reasonably idiomatic in Lispy
languages, at least by my understanding. That occurs when you are
writing a function that returns a list and there is a natural recursive
way to build up the answer -- but backwards. The idiom then is to build
up a temporary list up backwards, then call an in-place reversal
function. (NREVERSE in Common Lisp. I thought there was a reverse! in
Scheme, but apparently not.)

This doesn't break the external view of a pure function because the list
that's being reversed is a fresh, temporary list, which is why this
idiom would even fit in pretty well in Scheme.

Evan



signature.asc
Description: OpenPGP digital signature
-- 
http://mail.python.org/mailman/listinfo/python-list

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

2012-02-29 Thread WJ 
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))
 

MatzLisp:

a = [2,3,5,8]
==[2, 3, 5, 8]
a.reverse!
==[8, 5, 3, 2]
a
==[8, 5, 3, 2]
-- 
http://mail.python.org/mailman/listinfo/python-list