[R] different results by re-ordering vector: bug?

[Spencer Graves]

      I just got the same answer both times with R 1.8.1 under Windows 
2000 on an IBM Thinkpad T30.  However, if abs(diff(mean.row[c(3,10)])) 
is not much bigger than .Machine$double.esp*sum(abs(mean.row[c(3,10)])), 
then I would not call that a bug.  Rather, it should be considered a 
warning not to expect exact equality in comparing floating point numbers

      Just now, I checked (pi+x-x) == 4*atan(1) and got TRUE for both in 
R 1.8.1 and S-Plus 2000 with x = 1 but FALSE with x = 100. 

      hope this helps. 
      spencer graves

Carlos Soares wrote:

> Dear R users,
>
> I obtain the following behavior which I cannot understand. Given a 
> file mydata with the following numbers:
> 0.171409662475182
> 0.15817339510258108
> 0.32230311052283256
> 0.14890800794176043
> 0.17074784910655194
> 0.16611515552614162
> 0.41
> 0.16611515552614162
> 0.41760423560555926
> 0.11978821972203839
>
> I read the data and perform some calculations:
> a <- 1-read.table("mydata")$V1
> m <- outer(a, a, "/")
> diag(m) <- NA
> mean.row <- apply(m, 1, mean, na.rm=TRUE)
>
> which yield the same value for indices 6 and 8 of mean.row, as would 
> be expected because values 6 and 8 of the original vector are the same:
> > mean.row[6]==mean.row[8]
> [1] TRUE
>
> However, if I reorder the values as follows:
> a <- 1-read.table("mydata")$V1[c(10,2,8,9,7,3,1,4,5,6)]
>
> and repeat the calculations:
> m <- outer(a, a, "/")
> diag(m) <- NA
> mean.row <- apply(m, 1, mean, na.rm=TRUE)
> mean.row[6]==mean.row[8]
>
> The values for indices 10 and 3 of mean.row, which correspond to 6 and 
> 8 in the previous calculations, are not the same anymore:
> > mean.row[10]==mean.row[3]
> [1] FALSE
>
> I understand that limited precision causes "incorrect" results but I 
> wouldn't expect ordering operations to do the same. I couldn't find 
> any information in the site about this. Maybe it's a bug with my version:
> > R.version
>         _               platform i686-pc-linux-gnu
> arch     i686            os       linux-gnu       system   i686, 
> linux-gnu status                   major    1               minor    
> 7.0             year     2003            month    04              
> day      16              language R              
> Thanks and best regards,
> Carlos
>
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