[R] Why does R replace all row values with NAs

[Duncan Murdoch]

On 27/02/2015 9:49 AM, Dimitri Liakhovitski wrote:
> So, Duncan, do I understand you correctly:
> 
> When I use x$x<6, R doesn't know if it's TRUE or FALSE, so it returns
> a logical value of NA.

Yes, when x$x is NA.  (Though I think you meant x$c.)

> When this logical value is applied to a row, the R says: hell, I don't
> know if I should keep it or not, so, just in case, I am going to keep
> it, but I'll replace all the values in this row with NAs?

Yes.  Indexing with a logical NA is probably a mistake, and this is one
way to signal it without actually triggering a warning or error.

BTW, I should have mentioned that the example where you indexed using
-which(x$c>=6) is a bad idea:  if none of the entries were 6 or more,
this would be indexing with an empty vector, and you'd get nothing, not
everything.

Duncan Murdoch


> 
> On Fri, Feb 27, 2015 at 9:13 AM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
>> On 27/02/2015 9:04 AM, Dimitri Liakhovitski wrote:
>>> I know how to get the output I need, but I would benefit from an
>>> explanation why R behaves the way it does.
>>>
>>> # I have a data frame x:
>>> x = data.frame(a=1:10,b=2:11,c=c(1,NA,3,NA,5,NA,7,NA,NA,10))
>>> x
>>> # I want to toss rows in x that contain values >=6. But I don't want
>>> to toss my NAs there.
>>>
>>> subset(x,c<6) # Works correctly, but removes NAs in c, understand why
>>> x[which(x$c<6),] # Works correctly, but removes NAs in c, understand why
>>> x[-which(x$c>=6),] # output I need
>>>
>>> # Here is my question: why does the following line replace the values
>>> of all rows that contain an NA # in x$c with NAs?
>>>
>>> x[x$c<6,]  # Leaves rows with c=NA, but makes the whole row an NA. Why???
>>> x[(x$c<6) | is.na(x$c),] # output I need - I have to be super-explicit
>>>
>>> Thank you very much!
>>
>> Most of your examples (except the ones using which()) are doing logical
>> indexing.  In logical indexing, TRUE keeps a line, FALSE drops the line,
>> and NA returns NA.  Since "x$c < 6" is NA if x$c is NA, you get the
>> third kind of indexing.
>>
>> Your last example works because in the cases where x$c is NA, it
>> evaluates NA | TRUE, and that evaluates to TRUE.  In the cases where x$c
>> is not NA, you get x$c < 6 | FALSE, and that's the same as x$c < 6,
>> which will be either TRUE or FALSE.
>>
>> Duncan Murdoch
>>
> 
> 
>