Why can't R understand if(num!=NA)?

I have a program, when I write

if(num!=NA)

it yields an error message.

However, if I write

if(is.na(num)==FALSE)

it works.

Why doesn't the first statement work?

Thanks,

Miao

NA is undefined:

NA == NA

[1] NA

NA != NA

[1] NA


Therefore the equality you are attempting does not return a TRUE or FALSE result, it is unknown and NA is returned. ?is.na was designed specifically to test for the presence of an NA value and return a TRUE or FALSE result which can then be tested.

See: http://cran.r-project.org/doc/manuals/r-release/R-intro.html#Missing-values


Regards,

Marc Schwartz