[R] Unexpected R Behavior: Adding 4 to Large Numbers/IDs Containing Current Year

[Christopher T. Moore]

Hello,

I have encountered some unexpected behavior in R that seems to occur as a 
result of having the current year embedded in a number:

> ########################################
> 
> #Some large numbers, representing IDs.
> IDs <- c(41255689815201100, 41255699815201100, 41255709815201100)
> 
> #In scientific notation
> IDs 
[1] 4.125569e+16 4.125570e+16 4.125571e+16
> 
> #Change penalty.
> options(scipen = 5)
> 
> #Why does R add 4?
> IDs
[1] 41255689815201104 41255699815201104 41255709815201104
> 
> #Changing from numeric to character makes no difference.
> as.character(IDs)
[1] "41255689815201104" "41255699815201104" "41255709815201104"
> 
> #What happens if I treat the numbers as characters?
  IDs.character <- c("41255689815201100", "41255699815201100", 
"41255709815201100")
> 
> #No change.
> IDs.character
[1] "41255689815201100" "41255699815201100" "41255709815201100"
> 
> #R adds 4 upon converting to numeric.
> as.numeric(IDs.character)
[1] 41255689815201104 41255699815201104 41255709815201104
> 
  #Is this problem occurring because the current year is embedded in the 
number?
> IDs <- c(41255689815201100, 41255699815201000, 41255709815201200)
> 
> #R is no longer adding 4 to the numbers without "2011".
> IDs
[1] 41255689815201104 41255699815201000 41255709815201200
> 
> ########################################

Am I doing something wrong? Any insight on how I can avoid the problem of R 
changing numbers on its own? Are others able to replicate this example? Is 
this some kind of bug? Am I right that this problem is occurring because 
the current year is embedded in the number? I discovered this when trying 
to merge two data sets, one with IDs stored numbers and one with IDs as 
characters. I have replicated this in Windows XP with R 2.12 and Windows 7 
with R 2.13 (both 32- and 64-bit versions).

Thanks,
Chris

-- 
Christopher T. Moore, M.P.P.
Doctoral Student
Quantitative Methods in Education
University of Minnesota
44.9785°N, 93.2396°W
moor0554 at umn.edu
http://umn.edu/~moor0554