#缺失值 Missing Value
> #NaN不可识别NA
> x <- c(1,NA,2,NA,3)
> is.na(x)
[1] FALSE TRUE FALSE TRUE FALSE
> is.nan(x)
[1] FALSE FALSE FALSE FALSE FALSE
> #NA可识别NaN
> y <- c(1,NaN,2,NaN,3)
> is.na(y)
[1] FALSE TRUE FALSE TRUE FALSE
> is.nan(y)
[1] FALSE TRUE FALSE TRUE FALSE