Duncan Murdoch wrote:
> On 29/04/2009 6:41 PM, Steven McKinney wrote:
>>
>>> foo <- matrix(1:12, nrow = 3)
>>> bar <- data.frame(foo)
>>> bar$NewCol <- foo[foo[, 1] == 4, 4]
>>> bar
>> X1 X2 X3 X4 NewCol
>> 1 1 4 7 10 <NA>
>> 2 2 5 8 11 <NA>
>> 3 3 6 9 12 <NA>
>> Warning message:
>> In format.data.frame(x, digits = digits, na.encode = FALSE) :
>> corrupt data frame: columns will be truncated or padded with NAs
>> Is this a bug in the data.frame machinery?
>> If an attempt is made to add a new column
>> to a data frame, and the new object does
>> not have length = number of rows of data frame,
>> or cannot be made to have such length via recycling,
>> shouldn't an error be thrown?
>>
>> Instead in this example I end up with a
>> "corrupt data frame" having one zero-length column.
>>
>>
>> Should this be reported as a bug, or did I misinterpret
>> the documentation?
>
> I don't think "$" uses any data.frame machinery. You are working at a
> lower level.
well, there is the function `$<-.data.frame`. why does
bar$NewCol <- ...
*not* dispatch to $<-.data.frame? $<- is used on bar, and bar is a data
frame:
is(bar)
# "data.frame" ...
trace('$<-.data.frame')
bar$foo <- 1
# no output
trace('$<-')
bar$foo <- 1
# trace: `$<-`(`*tmp*`, foo, value = 1)
(still with the ugly *tmp*-hack)
and, actually, ?'$<-.data.frame' says:
" ## S3 replacement method for class 'data.frame':
x$i <- value"
>
> If you had added the new column using
>
> bar <- data.frame(bar, NewCol=foo[foo[, 1] == 4, 4])
>
> you would have seen the error:
>
> Error in data.frame(bar, NewCol = foo[foo[, 1] == 4, 4]) :
> arguments imply differing number of rows: 3, 0
>
> But since you treated it as a list,
he has *not*: he has used the "S3 replacement method for class
'data.frame'". the fact that it didn't work as expected seems to be a
consequence of a bug in the dispatch mechanism.
> it let you go ahead and create something that was labelled as a
> data.frame but wasn't.
wasn't? what wasn't what? after bar$NewCol <- integer(0), bar is
labelled as a data frame, and it seems to actually *be* a data frame;
data frame operations seem to work on bar, and the warning from print
bar talks about a corrupt data frame, not a non-data frame.
or do you mean that bar is not a data frame internally? that would be a
semantic weirdo where a user successfully performs an operation on a
data frame and gets a zombie. in any case, looks like a bug.
> This is one of the reasons some people prefer S4 methods: it's easier
> to protect against people who mislabel things.
it's *R* that mislabels things here. i can't see the user doing any
explicit labelling; the only stuff used was data.frame() and '$<-.',
which should dispatch to '$<-.data.frame'. the resulting zombie object
is clearly R's, not the user's, fault.
vQ
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