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

David Carlson dcarlson at tamu.edu
Fri May 3 17:38:20 CEST 2013

A logical operation involving NA returns NA, never TRUE or FALSE:

See the 8th Circle of the R Inferno (8.1.4):


> num <- 1
> num==NA
[1] NA
> is.na(num)

David L Carlson
Associate Professor of Anthropology
Texas A&M University
College Station, TX 77840-4352

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On
Behalf Of jpm miao
Sent: Friday, May 3, 2013 10:25 AM
To: r-help
Subject: [R] Why can't R understand if(num!=NA)?

I have a program, when I write


it yields an error message.

However, if I write


it works.

Why doesn't the first statement work?



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