Format rat and "-1"

[Ben Abbott]

On Tuesday, June 30, 2009, at 03:56PM, "David Bateman" <dbateman at dbateman.org> wrote:
>Eduardo Garcia wrote:
>> Cc: Eduardo Garcia
>> --------
>> Bug report for Octave 3.2.0 configured for i386-apple-darwin8.11.1
>>
>> Description:
>> -----------
>>
>>    When using "format rat" sometimes "-2147483648/-2147483648" is  
>> displayed instead of "-1".
>>    (Same bug is present in Octave 3.0.5 configured for i386-apple- 
>> darwin8.9.1)
>>
>> Repeat-By:
>> ---------
>> BUG:
>> octave-3.2.0:23> a=[1 2;3 5]
>> octave-3.2.0:24> det(a)
>> ans = -1.0000
>> octave-3.2.0:25> format rat
>> octave-3.2.0:26> det(a)
>> ans = -2147483648/-2147483648
>>
>> BUT:
>> octave-3.2.0:29> a=[1 2;1 1]
>> octave-3.2.0:30> format
>> octave-3.2.0:31> det(a)
>> ans = -1
>> octave-3.2.0:32> format rat
>> octave-3.2.0:33> det(a)
>> ans = -1
>>   
>Interesting problem.... What to do for the format rat when the numerator 
>and/or the denominator is outside the valid integer range.. A simpler 
>test example is
>
>octave:1> format rat
>octave:2> 1 - eps
>ans = -2147483648/-2147483648
>
>It would be interesting to know what matlab display for the cases 1 - 
>eps,  1 - 10*eps, 1 - 1e3*eps and maybe a couple of other similar values 
>before looking at a proper fix.
>
>D.

I'm at a loss to come up with a cute punch line ... but the results are below ;-)

>> format rat 
>> 1 - eps

ans =

       1       

>> 1 - 10*eps

ans =

       1       

>> 1 - 1e3*eps

ans =

       1       

Ben