[R] Question about a perceived irregularity in R syntax

[Duncan Murdoch]

Nordlund, Dan (DSHS/RDA) wrote:
>> -----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,

I suspect that's not intentional, though I'm not sure it should be 
fixed.  According to the usual convention the result should be a logical NA.

Duncan Murdoch