[Rd] model.frame strips class as promised, but fails to strip OBJECT in C

[Joris Meys]

Given the documentation in ?is.object and the info in R Internals section
1.1.2, I'd argue that this indeed a bug.

Looking at
https://github.com/wch/r-source/blob/trunk/src/library/stats/src/model.c
(line 220 and following) the function copyMostAttribNoTs is called to copy
the attributes lost after the na.action. This decision makes sense, but
when I look at that functiion copyMostAttribNoTs it sets the object bit to
the original state of the input, whereas in the case of a ts object, both
the class and the tsp attribute were dropped and not restored. More
specifically, line 338 of
https://github.com/wch/r-source/blob/3d0650af456d97648c72de66a40b85d3ec96a497/src/main/attrib.c
is imho the place where the object bit is set on a previous ts object that
lost its attributes, and if I read R Internals correctly, that shouldn't
happen.

Cheers
Joris

On Mon, Mar 5, 2018 at 4:59 PM, Michael Chirico <michaelchirico4 at gmail.com>
wrote:

> Full thread here:
>
> https://github.com/tidyverse/broom/issues/287
>
> Reproducible example:
>
> is.object(freeny$y)
> # [1] TRUE
> attr(freeny$y, 'class')
> # [1] "ts"
> class(freeny$y)
> # [1] "ts"
>
> # ts attribute wiped by model.frame
> class(model.frame(y ~ ., data = freeny)$y)
> # [1] "numeric"
> attr(model.frame(y ~ ., data = freeny)$y, 'class')
> # NULL
>
> # but still:
> is.object(model.frame(y ~ ., data = freeny)$y)
> # [1] TRUE
>
> That is, given a numeric vector with class "ts", model.frame strips the
> "ts" attribute but keeps the is.object property.
>
> This behavior is alluded to in ?model.frame:
>
> Unless na.action = NULL, time-series attributes will be removed from the
> > variables found (since they will be wrong if NAs are removed).
> >
>
> And in fact explicitly setting na.action = NULL prevents dropping the
> class:
>
> class(model.frame(y ~ ., data = freeny, na.action = NULL)$y)
> # [1] "ts"
>
> The reason this looks especially like a bug is that it differs from how
> na.omit behaves:
>
> DF <- data.frame(x = c(1, 2, 3), y = c(0, 10, NA))
> is.object(DF$y)
> # [1] FALSE
> class(DF$y) = 'foo'
> is.object(DF$y)
> # [1] TRUE
> class(na.omit(DF)$y)
> # [1] "numeric"
> is.object(na.omit(DF)$y)
> # [1] FALSE
>
>
> That is, similarly presented with a classed object, na.omit strips the
> class *and* the OBJECT attribute.
>
> Thanks,
> Michael Chirico
>
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>
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>



-- 
Joris Meys
Statistical consultant

Department of Data Analysis and Mathematical Modelling
Ghent University
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