R: NULL, NA, and NaN

• NaN (“Not a Number”) means 0/0
• NA (“Not Available”) is generally interpreted as a missing value and has various forms – NA_integer_, NA_real_, etc.
• Therefore, NaN ≠ NA and there is a need for NaN and NA.
• is.na() returns TRUE for both NA and NaN, however is.nan() return TRUE for NaN (0/0) and FALSE for NA.
• NULL represents that the value in question simply does not exist, rather than being existent but unknown.
• REF:
• http://www.cookbook-r.com/Basics/Working_with_NULL_NA_and_NaN/
• http://stackoverflow.com/questions/7031127/data-frames-and-is-nan
• http://www.r-bloggers.com/difference-between-na-and-nan-in-r/
• http://help.scilab.org/docs/5.5.2/en_US/isnan.html
• http://www.quantlego.com/howto/special-missing-values-in-r/