[R] Bug or feature? sum(c(a, b, c)) != (a + b + c)

[Daniel Lai]

Greetings all,

I'm porting an algorithm from MATLAB to R, and noticed some minor 
discrepancies in small decimal values using rowSums and colSums which 
are exacerbated after heavy iteration and log space transformation. 
This was rather perplexing as both programs claimed and appeared to use 
the IEEE 754 standard for floating point arithmetic (confirmed with 
manual basic operations).  After some tracing and testing, I've managed 
to isolated a minimal working example as follows:

a = 0.812672
b = 0.916541
c = 0.797810
sum(c(a, b, c)) == (a + b + c)
[1] FALSE

Should I attribute this to the woes of working with floating point 
numbers and accept it? i.e.

sprintf("%.30f", sum(c(a, b, c)))
[1] "2.527022999999999797182681504637"
sprintf("%.30f", (a + b + c))
[1] "2.527023000000000241271891354700"

Change the OS or version I'm using?

MAC OSX 10.5.8:
sessionInfo()
R version 2.13.1 (2011-07-08)
Platform: i386-apple-darwin9.8.0/i386 (32-bit)
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

Linux 2.6.34:
R version 2.12.0 (2010-10-15)
Platform: x86_64-unknown-linux-gnu (64-bit)
attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

Or report this as a bug?

Thanks,
Daniel