Compare a value to null. Why is this true?

问题

Why is

isTRUE(NULL != 2)
[1] FALSE

And how would I receive TRUE?

In my real case I have variables and I want to process something, if the values differ. However, when one value is NULL I don't recognize them as different!

回答1:

As @Roland pointed out, we can't perform any logical operations directly on NULL object. To compare them we might need to perform an additional check of is.null and then perform the logical comparison.

We can use identical instead to compare values which handles integers as well as NULL.

identical(4, 2) 
#FALSE

identical(NULL, 2) 
#FALSE

identical(2, 2) 
#TRUE

回答2:

In order to answer the why part of your question:

Comparing NULL with other types will give you logical(0) (i.e., a logical vector of length zero). So,

isTRUE(NULL != 2)

actually is

isTRUE(logical(0))

which is FALSE.

In order to compare the values where you might have NULL values also, you could also do something like this (using short circuit logical operator):

a <- 2
b <- 2
!is.null(a) && !is.null(b) && a==b
#[1] TRUE

a <- 3
b <- 2
!is.null(a) && !is.null(b) && a==b
#[1] FALSE

a <- 2
b <- NULL
!is.null(a) && !is.null(b) && a==b
#[1] FALSE

来源：https://stackoverflow.com/questions/44774483/compare-a-value-to-null-why-is-this-true