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Groups > comp.lang.python > #29196
| Newsgroups | comp.lang.python |
|---|---|
| Date | 2012-09-14 17:51 -0700 |
| References | (8 earlier) <50477cbb$0$29981$c3e8da3$5496439d@news.astraweb.com> <mailman.272.1346885254.27098.python-list@python.org> <50485fca$0$29977$c3e8da3$5496439d@news.astraweb.com> <504875EA.8000603@davea.name> <mailman.289.1346929477.27098.python-list@python.org> |
| Message-ID | <6da9ea89-c3ee-4c2d-9d86-c7b164113bb0@kr6g2000pbb.googlegroups.com> (permalink) |
| Subject | Re: Comparing strings from the back? |
| From | Steve Howell <showell30@yahoo.com> |
On Sep 6, 4:04 am, Oscar Benjamin <oscar.j.benja...@gmail.com> wrote:
> On Thu, 06 Sep 2012 06:07:38 -0400, Dave Angel <d...@davea.name> wrote:
> > For random strings (as defined below), the average compare time is
> > effectively unrelated to the size of the string, once the size
> passes
> > some point.
> > Define random string as being a selection from a set of characters,
> with
> > replacement. So if we pick some set of characters, say 10 (or 256,
> it
> > doesn't really matter), the number of possible strings is 10**N.
> > The likelihood of not finding a mismatch within k characters is
> > (1/10)**k The likelihood of actually reaching the end of the
> random
> > string is (1/10)**N. (which is the reciprocal of the number of
> strings,
> > naturally)
> > If we wanted an average number of comparisons, we'd have to
> calculate a
> > series, where each term is a probability times a value for k.
> > sum((k * 9*10**-k) for k in range(1, 10))
> > Those terms very rapidly approach 0, so it's safe to stop after a
> few.
> > Looking at the first 9 items, I see a value of 1.1111111
> > This may not be quite right, but the value is certainly well under
> 2 for
> > a population of 10 characters, chosen randomly. And notice that N
> > doesn't really come into it.
>
> It's exactly right. You can obtain this result analytically from
> Johannes' formula above. Just replace 256 with 10 to get that the
> expected number of comparisons is
>
> (10/9)*(1 - 10**(-N))
The math here is simple, but of course the average running time is
also driven by usage patterns.
Let's say you have two independently produced strings with N number of
random bits. The average amount of bit comparisons that it takes to
determine that the strings are different is between 1 and 2 checks for
any value of N, for the same reason that the average number of times
you need to flip a coin before either coming up heads or exhausting
your N tries is always less than 2, and pretty darn close to 2 for
even relatively small values of N.
def compute_checks_for_different_strings(n):
x = 1
p = 0.5
for i in range(0, n):
x += p * i
p = p * 0.5
print n, x
for n in [10, 100, 1000, 10000]:
compute_checks_for_different_strings(n)
10 1.9892578125
100 2.0
1000 2.0
10000 2.0
Now let's say your string comparison function is used to verify 100-
bit passwords, and 99% of the time the password is from a legit user
who types it correctly. Except for the 1% of a time when somebody fat
fingers the copy/paste of their password, every strcmp call is gonna
have to compare 100 pairs of characters, or N pairs. If the usage
pattern says that 99% of the time you're simply verifying that two
strings are equal, then the number of bits is the main driving factor
for running time.
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Comparing strings from the back? Roy Smith <roy@panix.com> - 2012-09-03 21:54 -0400
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-04 12:07 +1000
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-04 02:17 +0000
Re: Comparing strings from the back? Dan Sommers <dan@tombstonezero.net> - 2012-09-03 21:56 -0700
Re: Comparing strings from the back? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-09-04 08:50 +0100
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-04 18:32 +0200
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-04 18:07 +0000
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-05 11:17 +0200
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-05 07:59 +1000
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-05 11:24 +0200
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-05 11:43 +0200
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-05 14:30 +0000
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-05 16:51 +0200
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-05 16:24 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-05 22:47 +0000
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-06 08:33 +0000
Re: Comparing strings from the back? Dave Angel <d@davea.name> - 2012-09-06 06:07 -0400
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-07 04:06 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-07 19:10 +0000
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-08 00:55 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-08 11:53 +0000
RE: Comparing strings from the back? "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-09-13 18:39 +0000
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-13 15:37 -0400
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-13 20:48 -0700
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 00:46 -0400
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-13 21:54 -0700
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 01:38 -0400
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-13 23:06 -0700
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 04:03 -0400
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-14 01:20 -0700
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 04:53 -0400
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-14 03:26 -0700
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 07:36 -0400
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-14 10:16 +0000
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 07:43 -0400
RE: Comparing strings from the back? "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-09-14 22:43 +0000
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-14 23:10 -0400
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-16 18:11 -0700
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-17 14:05 +1000
Re: Comparing strings from the back? alex23 <wuwei23@gmail.com> - 2012-09-16 23:06 -0700
Re: Comparing strings from the back? Ethan Furman <ethan@stoneleaf.us> - 2012-09-17 13:35 -0700
Re: Comparing strings from the back? Neil Hodgson <nhodgson@iinet.net.au> - 2012-09-18 09:14 +1000
Re: Comparing strings from the back? Ethan Furman <ethan@stoneleaf.us> - 2012-09-18 08:12 -0700
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-18 11:55 -0400
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-18 11:59 -0400
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-18 12:17 -0400
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-19 00:47 +0000
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-19 02:20 +1000
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-18 16:40 -0400
Re: Comparing strings from the back? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-09-19 00:48 +0100
Re: Comparing strings from the back? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-09-13 20:53 +0100
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-13 17:06 -0400
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-14 03:39 +0000
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-14 14:15 +1000
Re: Comparing strings from the back? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-09-13 22:17 +0100
Re: Comparing strings from the back? Dwight Hutto <dwightdhutto@gmail.com> - 2012-09-13 17:35 -0400
RE: Comparing strings from the back? "Prasad, Ramit" <ramit.prasad@jpmorgan.com> - 2012-09-14 21:32 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-07 19:40 +0000
Re: Comparing strings from the back? Gelonida N <gelonida@gmail.com> - 2012-09-08 17:52 +0200
Re: Comparing strings from the back? Duncan Booth <duncan.booth@invalid.invalid> - 2012-09-10 08:59 +0000
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-10 13:45 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-10 14:06 +0000
Re: Comparing strings from the back? Duncan Booth <duncan.booth@invalid.invalid> - 2012-09-11 09:51 +0000
Re: Comparing strings from the back? Terry Reedy <tjreedy@udel.edu> - 2012-09-11 11:55 -0400
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-11 00:26 +1000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-10 14:32 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-10 14:43 +0000
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-11 00:56 +1000
Re: Comparing strings from the back? Duncan Booth <duncan.booth@invalid.invalid> - 2012-09-11 09:41 +0000
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-06 12:04 +0100
Re: Comparing strings from the back? Steve Howell <showell30@yahoo.com> - 2012-09-14 17:51 -0700
Re: Comparing strings from the back? Roy Smith <roy@panix.com> - 2012-09-06 08:13 -0400
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-06 22:29 +1000
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 15:43 +0200
Re: Comparing strings from the back? Dave Angel <d@davea.name> - 2012-09-06 10:23 -0400
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 16:33 +0200
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 16:42 +0200
Re: Comparing strings from the back? Dave Angel <d@davea.name> - 2012-09-06 11:54 -0400
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 16:34 +0200
Re: Comparing strings from the back? Gelonida N <gelonida@gmail.com> - 2012-09-08 17:50 +0200
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 15:37 +0200
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-07 00:39 +1000
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 17:36 +0200
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-06 17:44 +0200
Re: Comparing strings from the back? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2012-09-07 04:07 +0000
Re: Comparing strings from the back? Terry Reedy <tjreedy@udel.edu> - 2012-09-04 01:13 -0400
Re: Comparing strings from the back? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2012-09-04 08:56 +0100
Re: Comparing strings from the back? Alain Ketterlin <alain@dpt-info.u-strasbg.fr> - 2012-09-04 11:58 +0200
Re: Comparing strings from the back? Neil Hodgson <nhodgson@iinet.net.au> - 2012-09-05 12:18 +1000
Re: Comparing strings from the back? MRAB <python@mrabarnett.plus.com> - 2012-09-05 03:39 +0100
Re: Comparing strings from the back? Roy Smith <roy@panix.com> - 2012-09-04 22:48 -0400
Re: Comparing strings from the back? Johannes Bauer <dfnsonfsduifb@gmx.de> - 2012-09-05 16:33 +0200
Re: Comparing strings from the back? Peter Otten <__peter__@web.de> - 2012-09-05 10:29 +0200
Re: Comparing strings from the back? Chris Angelico <rosuav@gmail.com> - 2012-09-05 18:33 +1000
Re: Comparing strings from the back? Peter Otten <__peter__@web.de> - 2012-09-05 11:48 +0200
Re: Comparing strings from the back? Peter Otten <__peter__@web.de> - 2012-09-05 17:45 +0200
Re: Comparing strings from the back? Dan Goodman <dg.gmane@thesamovar.net> - 2012-09-10 18:07 +0200
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-10 16:33 +0000
Re: Comparing strings from the back? Dan Goodman <dg.gmane@thesamovar.net> - 2012-09-10 19:32 +0200
Re: Comparing strings from the back? Dan Goodman <dg.gmane@thesamovar.net> - 2012-09-10 19:44 +0200
Re: Comparing strings from the back? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-09-10 21:52 +0000
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