R devel - Feb 2005 - double/integer (PR#7687)

Full_Name: Chris Spencer
Version: 2.0.1
OS: Linux
Submission from: (NULL) (163.1.211.93)


Dear R team,

I realise that the following is a bit unsafe (the combination of doubles and
integers), however I wondered whether the following behaviour is expected: 
> #Test R
> test <- vector(length=100000);
> for(i in 1:100000){temp = i/1000; test[i] = (i == temp*1000);}
> table(test);
test
FALSE  TRUE
 1472 98528

For example I can also make the following error:
>  as.integer(259.765*1000);
[1] 259765
>  as.integer(259.763*1000);
[1] 259762

Would you expect this to be the case and if so why?
Thanks for your help.

Chris Spencer

On Fri, 11 Feb 2005 spencer@stats.ox.ac.uk wrote:
> Full_Name: Chris Spencer
> Version: 2.0.1
> OS: Linux
> Submission from: (NULL) (163.1.211.93)
>
>
> Dear R team,
>
> I realise that the following is a bit unsafe (the combination of doubles
and
> integers), however I wondered whether the following behaviour is expected:
>
>> #Test R
>> test <- vector(length=100000);
>> for(i in 1:100000){temp = i/1000; test[i] = (i == temp*1000);}
>> table(test);
> test
> FALSE  TRUE
> 1472 98528
Certainly.  You might even expect worse behaviour than that.

temp can be exactly represented in double precision only when i is a 
multiple of 125.  Depending on exactly how the computations are done and 
how many extra guard digits are carried you could get FALSE for nearly all 
i not a multiple of 125.  The fact that you get TRUE 98% of the time is 
better accuracy than you should expect.


 	-thomas