Why does R replace all row values with NAs

You could define functions like
   is.true <- function(x) !is.na(x) & x
   is.false <- function(x) !is.na(x) & !x
and use them in your selections.  E.g.,

  > x <- data.frame(a=1:10,b=2:11,c=c(1,NA,3,NA,5,NA,7,NA,NA,10))
  > x[is.true(x$c >= 6), ]

      a  b  c
  7   7  8  7
  10 10 11 10


Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Fri, Feb 27, 2015 at 7:27 AM, Dimitri Liakhovitski
<dimitri.liakhovitski at gmail.com> wrote:

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