R:null, NA, and NaN

• NaN ("not a number")  means 0/0
• NA (' not Available ') is Generall Y interpreted as a missing value and has various forms–na_integer_, Na_real_, etc. 
• Therefore, Nan≠na a nd 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 The value in question simply  does not exist , rather than Bei Ng 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/

R:null, NA, and NaN