问题
I am just starting out on learning R and came across a piece of code as follows
vec_1 <- c("a","b", NA, "c","d")
# create a subet of all elements which equal "a"
vec_1[vec_1 == "a"]
The result from this is
## [1] "a" NA
Im just curious, since I am subsetting vec_1 for the value "a", why does NA also show up in my results?
回答1:
This is because the result of anything == NA is NA. Even NA == NA is NA.
Here's the output of vec_1 == "a" -
[1] TRUE FALSE NA FALSE FALSE
and NA is not TRUE or FALSE so when you subset anything by NA you get NA. Check this out -
vec_1[NA]
[1] NA NA NA NA NA
When dealing with NA, R tries to provide the most informative answer i.e. T | NA returns TRUE because it doesn't matter what NA is. Here are some more examples -
T | NA
[1] TRUE
F | NA
[1] NA
T & NA
[1] NA
F & NA
[1] FALSE
R has no way to test equality with NA. In your case you can use %in% operator -
5 %in% NA
[1] FALSE
"a" %in% NA
[1] FALSE
vec_1[vec_1 %in% "a"]
[1] "a"
来源：https://stackoverflow.com/questions/52809897/r-subsetting-specific-value-also-returns-na