example code:

data=c(1,NA,2,NA,3,4,NA)
which(is.na(data))

对啊,突然短路了,问了个这么初级的问题,惭愧惭愧

话说,这又是什么情况呢
> any(is.nan(n))
[1] FALSE
> mean(n)
[1] NaN

that's abit weird, the only thing i could think of is that n is a null vector or list.
example:

>data=vector(length=0)
>data
logical(0)
>any(is.nan(data))
[1] FALSE
>mean(data)
[1] NaN

tested in R