Re: Re: Looping
- To: mathgroup at smc.vnet.net
- Subject: [mg95929] Re: [mg95896] Re: Looping
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Fri, 30 Jan 2009 05:42:54 -0500 (EST)
- References: <email@example.com> <200901291056.FAA18117@smc.vnet.net> <96061AD3-433E-4217-AC48-C21E57BB602F@mimuw.edu.pl> <4981EC6A.firstname.lastname@example.org>
On 29 Jan 2009, at 18:50, Adam Strzebonski wrote:
> Andrzej Kozlowski wrote:
>> On 29 Jan 2009, at 11:56, David Bailey wrote:
>>> Jeff Albert wrote:
>>>> I have a program written in Mathematica that has been running now
>>>> for about three days. How can I tell if it's in a loop?
>>> The best approach is to start small, and work your way up to a big
>>> problem like that if necessary. Start by aborting the calculation
>>> then start testing much smaller examples.
>>> Some Mathematica functions - such as Simplify or FullSymplify -
>>> seem to
>>> get stuck in this sort of way - if they do that, they will hang
>>> for ever.
>>> David Bailey
>> I doubt very much that they they ever get "stuck" in the way you
>> describe. Both Simplify and FullSimplify make use of algebraic
>> algorithms some of which have very high complexity (e.g.
>> exponential or even double exponential in the number of variables).
>> Even when it seems that the expression you are simplifying involves
>> only a few variables, its algorithmic complexity may be high
>> because transcendental parts of expressions are often treated as
>> independent variables. Of course, the human time scale: minutes,
>> hours, lifetimes, has not particular place in computer algebra so
>> there is no reason at all why your program should not run for 10
>> years and then suddenly come up with an answer.
>> In fact, I believe Simplify and FullSimplify have some built in
>> protection against infinite loops so they are probably somewhat
>> less likely to fall into them than some other functions. Also, note
>> that both Simplify and FullSimplify have the option TimeConstraint,
>> which can be sometimes useful in dealing with complex expressions.
>> If you run FullSimplify on an expression with TimeConstraint set
>> to, say, an hour, and if it returns to you the same expression that
>> you originally gave to it as input, it won't necessarily mean that
>> it had entered an infinite loop but more likely that it had
>> attempted a transformation or a sequence of transformations which
>> it could not complete before the time limit expired.
>> Andrzej Kozlowski
> Yes, Simplify and FullSimplify have built in protection against
> infinite loops, but no global time limit.
> The TimeConstraint option specifies a time limit allowed for a single
> transformation. If the time limit expires the current transformation
> is aborted, but then (Full)Simplify will attempt other transformations
> with a fresh time allowance for each new transformation. The default
> value of TimeConstraint is 5 minutes for Simplify and Infinity for
> Best Regards,
> Adam Strzebonski
> Wolfram Research
Thanks a lot. I have forgotten about his local limit/global limit
matter. I now recall we have actually discussed it on this forum before.
I think both this time and last time I confused the option
TimeContraint with the Mathematica function TimeConstrained. If I am
not mistaken, that latter will provide global time constraint on the
entire computation but unfortunately, it will return $Aborted rather
than the simplest form of the expression found so far when the time
limit is exceeded. I think I know a rather clunky way to get this
simplest form found by (Full)Simplify before the computation was
aborted (roughly equivalent to doing a Trace) but is there a nice and
efficient way of doing this? If not, would it not be possible and
useful to add this ability to (Full)Simplify?
With best regards
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- From: David Bailey <email@example.com>