[Tutor] Tutor Digest, Vol 81, Issue 39
marupalli charan
marupalli.charan at gmail.com
Wed Nov 10 13:28:27 CET 2010
Thanks, but first one not working!!!
On 11/10/10, tutor-request at python.org <tutor-request at python.org> wrote:
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> Today's Topics:
>
> 1. Re: List comprehension question (Richard D. Moores)
> 2. Re: query result set (bob gailer)
> 3. Re: Columnar Transposition Cipher question (Stefan Behnel)
> 4. Re: Columnar Transposition Cipher question (Eike Welk)
> 5. Re: List comprehension question (Steven D'Aprano)
> 6. Re: Columnar Transposition Cipher question (Steven D'Aprano)
> 7. Re: List comprehension question (Albert-Jan Roskam)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Tue, 9 Nov 2010 08:17:55 -0800
> From: "Richard D. Moores" <rdmoores at gmail.com>
> To: "Martin A. Brown" <martin at linux-ip.net>
> Cc: tutor at python.org
> Subject: Re: [Tutor] List comprehension question
> Message-ID:
> <AANLkTi=-=zYvQA7z6H6fNg3MGonJCL9BrCJxJ3_fFZfv at mail.gmail.com>
> Content-Type: text/plain; charset=UTF-8
>
> On Tue, Nov 9, 2010 at 05:51, Martin A. Brown <martin at linux-ip.net> wrote:
>>
>> Hello,
>>
>> ?: def proper_divisors_sum(n):
>> ?: ? ? pd_list = []
>> ?: ? ? for x in range(1, int(n**.5)+1):
>> ?: ? ? ? ? if n % x == 0:
>> ?: ? ? ? ? ? ? factor = int(x)
>> ?: ? ? ? ? ? ? pd_list.append(factor)
>> ?: ? ? ? ? ? ? factor2 = int(n/factor)
>> ?: ? ? ? ? ? ? pd_list.append(factor2)
>> ?: ? ? pd_list = list(set(pd_list))
>> ?: ? ? pd_list.sort()
>> ?: ? ? pd_list = pd_list[:-1]
>> ?: ? ? return sum(pd_list)
>>
>> A few questions--noting, of course, that I'm not reading this with
>> an eye toward performance, which it seems you are, but these occur
>> to me:
>>
>> ?* Why use a list (pd_list = []), when you want unique divisors?
>> ? ?Why not, pd = set() ? ?Then, instead of pd_list.append(factor),
>> ? ?pd.add(factor) or something like pd.update((factor,factor2))?
>> ? ?(See also my suggestion below.)
>>
>> ?* Also, since you are throwing away the divisor n, anyway,
>> ? ?why not skip it from the beginning? ?Like this:
>>
>> ? ? ?pd_list = [1,]
>> ? ? ?for x in range(2, int(n**.5)+1):
>>
>> ?* So, I'm not terribly math aware, but I don't think I have
>> ? ?substantially changed the meaning of your program. ?I find
>> ? ?breaking out the functions makes things a bit clearer to
>> ? ?somebody who might be reading my code later.
>>
>> Combining these suggestions and splitting into separate functions
>> (you don't really need to sort before summing), I end up with the
>> below. ?Note, that I stuff the proper divisor 1 in the set at
>> creation time. ?This is probably not worth it from an optimization
>> perspective, but as I have learned, the only way to know is to time
>> it (and I didn't time it).
>>
>> ? ? ?def proper_divisors_sum(n):
>> ? ? ? ? ?return sum(proper_divisors(n))
>>
>> ? ? ?def proper_divisors_sorted(n):
>> ? ? ? ? ?return sorted(proper_divisors(n))
>>
>> ? ? ?def proper_divisors(n):
>> ? ? ? ? ?pd = set((1,))
>> ? ? ? ? ?for x in range(2, int(n**.5)+1):
>> ? ? ? ? ? ? ?factor, mod = divmod(n,x)
>> ? ? ? ? ? ? ?if mod == 0:
>> ? ? ? ? ? ? ? ? ?pd.update((x,factor))
>> ? ? ? ? ?return list(pd)
>
> Thanks for your ideas, Martin. I adopted your idea of starting at 2,
> with an initial list of [1]. It resulted in a small increase in
> speed. And you're right, I don't need to sort.
>
> Speed is important, because the function is the heart of my amiable
> numbers script. Did you happen to see that post of mine?
>
> #Here's my fastest version before seeing Martin's post:
> def proper_divisors_sum1(n):
> pd_list = []
> for x in range(1, int(n**.5)+1):
> if n % x == 0:
> pd_list.append(x)
> pd_list.append(n//x)
> pd_list = list(set(pd_list))
> pd_list.sort()
> pd_list = pd_list[:-1]
> return sum(pd_list)
>
> #This uses several of Martin's ideas
> # It's now my fastest version
> def proper_divisors_sum2(n):
> pd = set((1,))
> for x in range(2, int(n**.5)+1):
> if n % x == 0:
> pd.update((x, n//x))
> return sum(list(pd))
>
> Here's what I put together from all your points, except for the
> splitting off of several small functions:
>
> # This incorporates all of Martin's ideas
> # Seems that divmod() is a speed killer
> def proper_divisors_sum3(n):
> pd = set((1,))
> for x in range(2, int(n**.5)+1):
> factor, mod = divmod(n,x)
> if mod == 0:
> pd.update((x,factor))
> return sum(list(pd))
>
> It may be that I've haven't done your suggestions justice, but that
> function is 76% slower than proper_divisors_sum2().
>
> See <http://tutoree7.pastebin.com/R82876Eg> for a speed test with n =
> 100,000 and 100,000 loops
>
> Thanks again, Martin.
>
> Dick
>
>
> ------------------------------
>
> Message: 2
> Date: Tue, 09 Nov 2010 12:00:56 -0500
> From: bob gailer <bgailer at gmail.com>
> To: tutor at python.org
> Subject: Re: [Tutor] query result set
> Message-ID: <4CD97E48.9080604 at gmail.com>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> When starting a new subject, please start with a new email. Do not
> "hijack" an existing one, as that causes your query to appear as part of
> the hijacked thread.
>
>
> --
> Bob Gailer
> 919-636-4239
> Chapel Hill NC
>
>
>
> ------------------------------
>
> Message: 3
> Date: Tue, 09 Nov 2010 18:36:43 +0100
> From: Stefan Behnel <stefan_ml at behnel.de>
> To: tutor at python.org
> Subject: Re: [Tutor] Columnar Transposition Cipher question
> Message-ID: <ibc0rb$g96$1 at dough.gmane.org>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> Steven D'Aprano, 09.11.2010 05:01:
>> http://pypi.python.org/pypi/obfuscate
>
> Hmm - why is the Windows installer on that page called "linux-i686"?
>
> Stefan
>
>
>
> ------------------------------
>
> Message: 4
> Date: Tue, 9 Nov 2010 19:37:28 +0100
> From: Eike Welk <eike.welk at gmx.net>
> To: tutor at python.org
> Subject: Re: [Tutor] Columnar Transposition Cipher question
> Message-ID: <201011091937.28881.eike.welk at gmx.net>
> Content-Type: Text/Plain; charset="iso-8859-1"
>
> On Tuesday 09.11.2010 18:36:43 Stefan Behnel wrote:
>> Steven D'Aprano, 09.11.2010 05:01:
>> > http://pypi.python.org/pypi/obfuscate
>>
>> Hmm - why is the Windows installer on that page called "linux-i686"?
>
> It was probably created on Linux. Python's Distutils create installers for
> Windows even on Linux. They get file names that contain "linux-". You must
> rename them, otherwise your users get confused.
>
> I did never try them, because I have no Windows computer. Do these Windows
> installers work when they are created on Linux?
>
>
> Eike.
>
>
> ------------------------------
>
> Message: 5
> Date: Wed, 10 Nov 2010 07:54:24 +1100
> From: Steven D'Aprano <steve at pearwood.info>
> To: tutor at python.org
> Subject: Re: [Tutor] List comprehension question
> Message-ID: <4CD9B500.4090605 at pearwood.info>
> Content-Type: text/plain; charset=UTF-8; format=flowed
>
> Richard D. Moores wrote:
>
>> See <http://tutoree7.pastebin.com/R82876Eg> for a speed test with n =
>> 100,000 and 100,000 loops
>
> As a general rule, you shouldn't try to roll your own speed tests. There
> are various subtleties that can throw your results right out. Timing
> small code snippets on modern multi-tasking computers is fraught with
> difficulties.
>
> Fortunately Python comes with that battery already included: the timeit
> module. You can use it from the shell like this:
>
> python -m timeit -s "setup code goes here" "test code goes here"
>
> At the Python interactive interpreter, the most useful pattern I've
> found is:
>
> from timeit import Timer
> t = Timer("proper_divisors(n)",
> "from __main__ import proper_divisors; n = 100000")
> min(t.repeat(repeat=5, number=100000))
>
>
> Or if you're happy with Python's defaults, that last line can be just:
>
> min(t.repeat())
>
>
>
> --
> Steven
>
>
> ------------------------------
>
> Message: 6
> Date: Wed, 10 Nov 2010 08:29:51 +1100
> From: Steven D'Aprano <steve at pearwood.info>
> To: tutor at python.org
> Subject: Re: [Tutor] Columnar Transposition Cipher question
> Message-ID: <4CD9BD4F.3000309 at pearwood.info>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> Eike Welk wrote:
>> On Tuesday 09.11.2010 18:36:43 Stefan Behnel wrote:
>>> Steven D'Aprano, 09.11.2010 05:01:
>>>> http://pypi.python.org/pypi/obfuscate
>>> Hmm - why is the Windows installer on that page called "linux-i686"?
>>
>> It was probably created on Linux.
>
> That would be it.
>
> > Python's Distutils create installers for
>> Windows even on Linux. They get file names that contain "linux-". You must
>>
>> rename them, otherwise your users get confused.
>
> I didn't realise that.
>
>> I did never try them, because I have no Windows computer. Do these Windows
>>
>> installers work when they are created on Linux?
>
> I'm pretty sure I tried it, once, but I might be confabulating.
>
>
>
> --
> Steven
>
>
>
> ------------------------------
>
> Message: 7
> Date: Tue, 9 Nov 2010 13:58:22 -0800 (PST)
> From: Albert-Jan Roskam <fomcl at yahoo.com>
> To: Steven D'Aprano <steve at pearwood.info>, tutor at python.org
> Subject: Re: [Tutor] List comprehension question
> Message-ID: <968733.36678.qm at web110711.mail.gq1.yahoo.com>
> Content-Type: text/plain; charset="us-ascii"
>
> For larger blocks of code, cProfile may also be useful for speed tests.
>
> Cheers!!
> Albert-Jan
>
>
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> All right, but apart from the sanitation, the medicine, education, wine,
> public
> order, irrigation, roads, a fresh water system, and public health, what have
> the
> Romans ever done for us?
> ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>
>
>
>
> ________________________________
> From: Steven D'Aprano <steve at pearwood.info>
> To: tutor at python.org
> Sent: Tue, November 9, 2010 9:54:24 PM
> Subject: Re: [Tutor] List comprehension question
>
> Richard D. Moores wrote:
>
>> See <http://tutoree7.pastebin.com/R82876Eg> for a speed test with n =
>> 100,000 and 100,000 loops
>
> As a general rule, you shouldn't try to roll your own speed tests. There are
> various subtleties that can throw your results right out. Timing small code
> snippets on modern multi-tasking computers is fraught with difficulties.
>
> Fortunately Python comes with that battery already included: the timeit
> module.
> You can use it from the shell like this:
>
> python -m timeit -s "setup code goes here" "test code goes here"
>
> At the Python interactive interpreter, the most useful pattern I've found
> is:
>
> from timeit import Timer
> t = Timer("proper_divisors(n)",
> "from __main__ import proper_divisors; n = 100000")
> min(t.repeat(repeat=5, number=100000))
>
>
> Or if you're happy with Python's defaults, that last line can be just:
>
> min(t.repeat())
>
>
>
> -- Steven
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