Re: floating point issue

• To: mathgroup at smc.vnet.net
• Subject: [mg86796] Re: [mg86748] floating point issue
• From: Chris Scullard <scullard at uchicago.edu>
• Date: Fri, 21 Mar 2008 01:53:59 -0500 (EST)
• References: <20080320080231.D7ZM5.88136.imail@eastrmwml27.mgt.cox.net>

```That simple huh. Thanks Bob. And thanks everyone who replied.

Chris

Bob Hanlon wrote:
> K = Rationalize[{111.5, 10.5, 1.5}, 0];
> g = Rationalize[{-0.7071068, 0., -0.7071068}, 0];
>
> K.Cross[K, g]
>
> 0
>
> K = {111.5`25, 10.5`25, 1.5`25};
> g = {-0.7071068`25, 0.`25, -0.7071068`25};
>
> Chop[K.Cross[K, g], 10^-18]
>
> 0
>
>
> Bob Hanlon
>
> ---- Chris Scullard <scullard at uchicago.edu> wrote:
>
>> Hi everyone,
>>
>> I wonder if I can get some opinions on the best way to deal with this
>> precision issue I am having. I define the vectors:
>>
>> K = {111.5, 10.5, 1.5}
>> g={-0.7071068, 0., -0.7071068}
>>
>> And I need this:
>>
>> K.Cross[K, g]
>>
>> to be 0 in accordance with a vector identity. The answer comes out to
>> around 1.3 x 10^(-13), which is certainly close to 0 but not close
>> enough for what I'm doing. I've tried various things like writing out
>> the cross product explicitly without using the functions but the result
>> is the same. And using N in various places doesn't seem to help either.
>> What's the standard solution for this kind of thing?
>>
>> Thanks,
>> Chris
>>
>>

```

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