Question about a perceived irregularity in R syntax

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Peter Dalgaard
Sent: Thursday, July 22, 2010 3:13 PM
To: Pat Schmitz
Cc: r-help at r-project.org
Subject: Re: [R] Question about a perceived irregularity in R syntax

Pat Schmitz wrote:
    

Both vector query's can select the values from the data.frame as
      

written,
    

however in the first form assigning a value to said selected numbers
      

fails.
    

 Can you explain the reason this fails?

dat <- data.frame(index = 1:10, Value = c(1:4, NA, 6, NA, 8:10))

dat$Value[dat$Value == "NA"] <- 1 #Why does this  fails to work,
dat$Value[dat$Value %in% NA] <- 1 #While this does work?


#Particularly when str() results in an equivalent class
dat <- data.frame(index = 1:10, Value = c(1:4, NA, 6, NA, 8:10))
str(dat$Value[dat$Value %in% NA])
str(dat$Value[dat$Value == "NA"])
      

1. NA and "NA" are very different things
2. checkout is.na() and its help page


    

I also would have suggested is.na to do the replacement.  What surprised me was that 

dat$Value[dat$Value %in% NA] <- 1 

actually worked.  I guess I always assumed that if 

  

NA == NA
    

[1] NA

then an attempt to compare NA to elements in a vector would also return NA, but not so.

  

NA %in% c(1,NA,3)
    

[1] TRUE


Learned something new today,