R help - Jul 2010 - Question about a perceived irregularity in R syntax

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"])

Thanks
Pat


-- 
Patrick Schmitz
Graduate Student
Plant Biology
1206 West Gregory Drive
RM 1500

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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



-- 
Peter Dalgaard
Center for Statistics, Copenhagen Business School
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com