[datatable-help] := unclarity and possible bug?
Chris Neff
caneff at gmail.com
Thu Aug 4 15:51:36 CEST 2011
I think I understand the difference between DT$y <- TRUE and DT$y <-
rnorm(10) now, and that is that the first is just an atomic element
while the second is a vector. If instead I had did
DT$y <- 0.5
I get a coercion warning. I suppose this is an okay feature if the
warning consistently shows up whenever coercion happens (even if it
coerces successfully with no loss of precision). It is just different
than data.frame and without the warning I didn't understand the logic.
I can see the speedup of not coercing, but from a users point of view
I expect DT$y <- 0.5 and DT$y <- rep(0.5, nrow(DT)) to behave
identically thanks to the magic of vector recycling.
Similarly if I've decided to do DT$y[4] <- .2 and before y was an
integer, I've clearly changed my mind about that aspect and want y to
be a numeric.
However, I'm able to work with the way it is as long as I'm warned
about it. I can see this making terribly confusing bugs for people if
they don't get a warning.
-Chris
On 4 August 2011 09:09, Chris Neff <caneff at gmail.com> wrote:
> I've ran the following 3 different times in new sessions:
>
> install.packages("data.table",
> repos="http://R-Forge.R-project.org",type="source")
>
> and still DT[,z:=5] does nothing. Is there something I check to make
> sure that the latest version is loaded?
>
>
> As for the coercion stuff, I feel that it feels somewhat inconsistent
> right now. For instance:
>
>> DT <- data.table(x=1:10, y=1:10)
>
>> DT$y <- TRUE
>
>> sapply(DT, class)
>
> x y
> "integer" "integer"
>
>> DT$y <- rnorm(10)
>> sapply(DT, class)
> x y
> "integer" "numeric"
>
> So in the first case y silently coerces the logical to an integer
> without warning, but in the second case y happily turns into a numeric
> when need be. Why the difference?
>
> When I do something like DT$y <- foo, I expect that y should turn into
> foo regardless of what y was before. If there is some reason why DT[,
> y:=foo] should be different than DT$y <- foo, that is a secondary
> matter, but I get mightily confused when DT$y <- foo doesn't behave
> like data.frame.
>
> On 4 August 2011 08:50, Matthew Dowle <mdowle at mdowle.plus.com> wrote:
>> Still doesn't seem to be latest version: DT[,z:=5] should add column (and
>> that's tested).
>> Otherwise correct and intended behaviour (although an informative warning
>> needs adding when 5 gets coerced to type of column (i.e. logical) - thanks
>> for spotting). Remember as.logical(5) is TRUE without warning. So, try
>> creating column with NA_integer_ or NA_real_ instead. Once the column type
>> is set, that's it. Columns aren't coerced to match type of RHS, unlike
>> data.frame [which if you think about it is a big hit if the data is large].
>>
>> "Chris Neff" <caneff at gmail.com> wrote in message
>> news:CAAuY0RXT7q+cm91PJ8KGkMwDApwFxM_EALb-Yu=P6ndp+LEfXg at mail.gmail.com...
>> Ignore this second one, restarting and refreshing my data.table
>> install now gives the proper error message when I try that. Sorry I'm
>> not used to being on the bleeding edge of these things and I forget to
>> update. However the first question is still mainly relevant:
>>
>>> DT <- data.table(x=1:10, y=rep(1:2,5))
>>> DT[,z:=5]
>> x y
>> [1,] 1 1
>> [2,] 2 2
>> [3,] 3 1
>> [4,] 4 2
>> [5,] 5 1
>> [6,] 6 2
>> [7,] 7 1
>> [8,] 8 2
>> [9,] 9 1
>> [10,] 10 2
>>> DT[1:nrow(DT),z:=5]
>> Error in `[.data.table`(DT, 1:nrow(DT), `:=`(z, 5)) :
>> Attempt to add new column(s) and set subset of rows at the same
>> time. Create the new column(s) first, and then you'll be able to
>> assign to a subset. If i is set to 1:nrow(x) then please remove that
>> (no need, it's faster without).
>>> DT$z <- NA
>>> DT[, z:=5]
>> x y z
>> [1,] 1 1 TRUE
>> [2,] 2 2 TRUE
>> [3,] 3 1 TRUE
>> [4,] 4 2 TRUE
>> [5,] 5 1 TRUE
>> [6,] 6 2 TRUE
>> [7,] 7 1 TRUE
>> [8,] 8 2 TRUE
>> [9,] 9 1 TRUE
>> [10,] 10 2 TRUE
>>
>>
>>
>> The return on DT[,z:=5] when I haven't initialized DT$z yet is
>> different, but still more uninformative than it is when I do
>> DT[1:nrow(DT), z:=5]. And the DT$z <- NA issue is still there.
>>
>> Thanks!
>>
>>
>> On 4 August 2011 08:18, Chris Neff <caneff at gmail.com> wrote:
>>> A second question while I'm playing with it. It seems from the FRs
>>> that it doesn't support multiple := in one select, but:
>>>
>>> DT <- data.table(x=1:10, y=rep(1:2,10))
>>> DT$a = 0
>>> DT$z = 0
>>>
>>> DT[, list(a := y/sum(y), z := 5)]
>>>
>>> works just fine for me. An error gets thrown but afterwards the
>>> columns are modified as intended. Why the error?
>>>
>>>> DT[,list(z:=5,a:=y/sum(y))]
>>> z
>>> [1] 5
>>> [1] TRUE
>>> a
>>> y/sum(y)
>>> [1] TRUE
>>> Error in data.table(`:=`(z, 5), `:=`(a, y/sum(y))) :
>>> column or argument 1 is NULL
>>>> DT
>>> x y z a
>>> [1,] 1 1 5 0.06666667
>>> [2,] 2 2 5 0.13333333
>>> [3,] 3 1 5 0.06666667
>>> [4,] 4 2 5 0.13333333
>>> [5,] 5 1 5 0.06666667
>>> [6,] 6 2 5 0.13333333
>>> [7,] 7 1 5 0.06666667
>>> [8,] 8 2 5 0.13333333
>>> [9,] 9 1 5 0.06666667
>>> [10,] 10 2 5 0.13333333
>>>
>>> -Chris
>>>
>>> On 4 August 2011 08:12, Chris Neff <caneff at gmail.com> wrote:
>>>> Hi all,
>>>>
>>>> If I do:
>>>>
>>>> DT <- data.table(x=1:10, y=rep(1:2,5))
>>>>
>>>> Then try the following
>>>>
>>>> DT[, z:=5]
>>>>
>>>> I get:
>>>>
>>>>> DT[, z:=5]
>>>> z
>>>> [1] 5
>>>> [1] TRUE
>>>> NULL
>>>>
>>>> and if I were to do DT <- DT[, z:=5], then DT gets set to NULL.
>>>> Alternatively if I do
>>>>
>>>> DT[1:10, z:=5]
>>>>
>>>> I get
>>>>
>>>>> DT=DT[1:nrow(DT),z:=5]
>>>> z
>>>> [1] 5
>>>> [1] 1 2 3 4 5 6 7 8 9 10
>>>> Error in `:=`(z, 5) :
>>>> Attempt to add new column(s) and set subset of rows at the same
>>>> time. Create the new column(s) first, and then you'll be able to
>>>> assign to a subset. If i is set to 1:nrow(x) then please remove that
>>>> (no need, it's faster without).
>>>>
>>>>
>>>> Which is more informative. So I do as it instructs:
>>>>
>>>> DT$z <- NA
>>>>
>>>> DT[, z:=5]
>>>>
>>>> And as output I get:
>>>>
>>>>> DT
>>>> x y z
>>>> [1,] 1 1 TRUE
>>>> [2,] 2 2 TRUE
>>>> [3,] 3 1 TRUE
>>>> [4,] 4 2 TRUE
>>>> [5,] 5 1 TRUE
>>>> [6,] 6 2 TRUE
>>>> [7,] 7 1 TRUE
>>>> [8,] 8 2 TRUE
>>>> [9,] 9 1 TRUE
>>>> [10,] 10 2 TRUE
>>>>
>>>>
>>>> Why isn't z 5 like assigned? I think it is because I assigned it as
>>>> NA, and data table didn't know to change it to integer (although why
>>>> it changed it to logical is another puzzle). If I instead do
>>>>
>>>> DT$z <- 0
>>>>
>>>> DT[, z:=5]
>>>>
>>>> It works fine.
>>>>
>>>> So my two points are:
>>>>
>>>> A) Doing DT[,z:=5] should be as informative as doing DT[1:nrow(DT),
>>>> z:=5] with the error message.
>>>>
>>>> B) What went wrong with the NA assignment I did?
>>>>
>>>> Thanks!
>>>> Chris
>>>>
>>>
>>
>>
>>
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