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

[Dimitri Liakhovitski]

Thank you very much, Duncan.
All this being said:

What would you say is the most elegant and most safe way to solve such
a seemingly simple task?

Thank you!

On Fri, Feb 27, 2015 at 10:02 AM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
> 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
>>>
>>
>>
>>
>



-- 
Dimitri Liakhovitski