问题
this is a lame question I guess, but i don't understand what's is happining. If I go:
sum(is.na(census$wd))
It returns 4205
But if I go with:
sum(census$wd == NA)
It returns "NA"
I just would like to understand whats is happening. If I do str(census), wd shows up as:
$ wd : num NA 0.65 0.65 0.65 0.78 0.78 0.78 0.78 0.78 0.78 ...
Can anyone explains why the codes return different outputs? Thank you!
回答1:
== in R is a comparison. But you can not compare something to NA in Ras the following quote from ?Comparison states:
Missing values (NA) and NaN values are regarded as non-comparable even to themselves, so comparisons involving them will always result in NA.
In contrast is.na indicates which elements are missing regardless of their type.
So it returns a vector of TRUE and FALSE entries.
> a <- c(NA,1,2,3)
> a == NA
[1] NA NA NA NA
> is.na(a)
[1] TRUE FALSE FALSE FALSE
this is why sum is working with is.na (interpreting TRUE=1 and FALSE=0 but it cannot sum up a vector of NA's (generated by ==NA)
来源：https://stackoverflow.com/questions/25274518/something-weird-about-returning-nas