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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: 17,945
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
> 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
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