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Topic: computation precision and display
Replies: 3   Last Post: Dec 4, 2012 12:32 PM

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Steven Lord

Posts: 17,944
Registered: 12/7/04
Re: computation precision and display
Posted: Dec 4, 2012 12:32 PM
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"Nasser M. Abbasi" <nma@12000.org> wrote in message
news:k9jmbo$ila$1@speranza.aioe.org...
> On 12/3/2012 7:01 PM, Jose wrote:
>> Hi,
>>
>> It just happens in my linux 64 bits PC, running Matlab 2010a (on format
>> long g) that when I calculate
>>

>>>> A=(0.714844838298060 + 0.699283102297360i)*conj(0.714844838298060 +
>>>> 0.699283102297360i)

>>
>> A =
>>
>> 1.000000000000000
>>
>> Now when I calculate
>>
>>

>>>> B=(0.757342604591773 + 0.653017748051421i)*conj(0.757342604591773 +
>>>> 0.653017748051421i)

>>
>> B =
>>
>> 1
>>
>> And the difference is:
>>

>>>> A-B
>>
>> ans =
>>
>> -1.11022302462516e-16
>>
>> This is important because when I try to plot A and B as elements of a
>>matrix with a colormap using imagesc they get very different colors.

>
> that is strange, since (A-B) is less than eps


Why is that strange?

Remember that eps() [== eps(1)] is the smallest number such that 1+eps is
greater than 1. It is perfectly fine to have numbers smaller than eps (for
example, REALMIN is many orders of magnitude smaller) and in particular the
difference between 1 and the next SMALLER number is eps/2.

format hex
a = 1
b = 1-(eps/2)
d = eps(b)
(a == (b+d))

--
Steve Lord
slord@mathworks.com
To contact Technical Support use the Contact Us link on
http://www.mathworks.com




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