From: stefan-lucks@see-the.signature
Subject: Re: Inefficient algorithms
Date: Sat, 11 Sep 2010 16:28:20 +0200
Date: 2010-09-11T16:28:20+02:00 [thread overview]
Message-ID: <Pine.LNX.4.64.1009111612280.19046@medsec1.medien.uni-weimar.de> (raw)
In-Reply-To: <1f6f73cd-f5ff-4408-a1d7-c9ca7dfa70ee@m35g2000prn.googlegroups.com>
On Fri, 10 Sep 2010, Rick wrote:
> I am working on a class in algorithmic efficiency and, to make a
> point, need an algorithm that runs O(2^n) - a factoring algorithm
> perhaps. All I can find is 'C' code I can't decipher. Can someone
> please help with an Ada example.
Technically,all polynomial-time algorithms are in O(2^n), since the
"big-Oh" means something like "not faster than".
But if you really need (worst-case) exponential time, here is such an
algorithm for for factoring a number n (or for detecting if n is prime):
function Factor(C: Positive) return (Positive or "is prime") is
-- pre C>1
begin
for I in 2 .. C-1 loop
if I divides C then
return I
end if
end loop
return "is prime"
end Factor;
This is pseudo-code with an Ada-like syntax, and shouldn't be hard to
translate into Ada. (Hey, this is *your* homework!)
Note that if C is a prime, this the loop iterates C-2=Theta(C) times. If C
is an n-bit prime, the number of iterations is
2**(log_2(C)) - 2 > 2**(n-1) - 2.
--
------ Stefan Lucks -- Bauhaus-University Weimar -- Germany ------
Stefan dot Lucks at uni minus weimar dot de
------ I love the taste of Cryptanalysis in the morning! ------
next prev parent reply other threads:[~2010-09-11 14:28 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-11 4:24 Inefficient algorithms Rick
2010-09-11 6:51 ` J-P. Rosen
2010-09-13 3:45 ` Robert A Duff
2010-09-11 6:54 ` Niklas Holsti
2010-09-11 7:07 ` Niklas Holsti
2010-09-11 9:07 ` Rick
2010-09-11 15:05 ` Niklas Holsti
2010-09-17 5:26 ` Rick
2010-09-11 9:20 ` Ludovic Brenta
2010-09-11 9:23 ` Ludovic Brenta
2010-09-11 11:20 ` Niklas Holsti
2010-09-11 18:29 ` Peter C. Chapin
2010-09-11 14:28 ` stefan-lucks [this message]
2010-09-12 1:04 ` Wilson
2010-09-12 1:53 ` Rick
2010-09-12 8:35 ` Georg Bauhaus
2010-09-12 11:56 ` stefan-lucks
2010-09-15 1:11 ` BrianG
-- strict thread matches above, loose matches on Subject: below --
2010-09-15 8:51 Rick
2010-09-15 21:45 ` John B. Matthews
2010-09-16 12:05 ` Chad R. Meiners
2010-09-16 20:19 ` John B. Matthews
2010-09-17 5:24 ` Rick
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