[R] "strange" behaviour: recognition of decimal numbers by 'which'

[J.delasHeras at ed.ac.uk]

It is a strange behaviour in that I did not expect it... but I am sure  
there is a simple explanation for it and it'll have to do with the way  
numbers are stored in R, but it's caught me by surprise and I don't  
find it obvious.

Here's a simplified example reproducing the behaviour I encountered:

I create an empty vector, and I fill it with a sequence of numbers: 0,  
0.005, 0.01, 0.015, 0.02, etc... until 0.1.
(In my real code this is just a way to store certain values when  
certain conditions are met etc etc)
Then I ask which element contains teh value 0.01. Ok. Same with 0.02.  
But when I ask about 0.03 or above... the answer is none. But I can  
see "0.03" in my vector, it is there!

This must be because of the value stored internally. Indeed, if I do:

v-0.03, when I reach the value that appeared to contain 0.03, I don't  
obtain zero, but "3.47e-18"

why is this?
what is the recommended way to overcome this?
Interestingly, if I store the values as "character", then I can match  
them fine...

Below is the code, and my sessionInfo() output.

Thanks!

Jose de las Heras


code example:
#v<-vector(mode="character")
v<-c()
i<-0
while (i <= 0.1)
   {
   v<-c(v,i)
   i<-i+0.005
   }
v
which(v==0.01)
which(v==0.02)
which(v==0.03)



> sessionInfo()
R version 2.10.0 (2009-10-26)
i386-pc-mingw32

locale:
[1] LC_COLLATE=English_United Kingdom.1252
[2] LC_CTYPE=English_United Kingdom.1252
[3] LC_MONETARY=English_United Kingdom.1252
[4] LC_NUMERIC=C
[5] LC_TIME=English_United Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] biomaRt_2.2.0

loaded via a namespace (and not attached):
[1] RCurl_1.3-1  tools_2.10.0 XML_2.6-0



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