[Rd] Puzzled about a new method for "[".
r@turner @end|ng |rom @uck|@nd@@c@nz
Mon Nov 4 23:18:56 CET 2019
On 5/11/19 10:54 AM, Duncan Murdoch wrote:
> On 04/11/2019 4:40 p.m., Pages, Herve wrote:
>> Hi Rolf,
>> On 11/4/19 12:28, Rolf Turner wrote:
>>> On 5/11/19 3:41 AM, Hadley Wickham wrote:
>>>> For what it's worth, I don't think this strategy can work in general,
>>>> because a class might have attributes that depend on its data/contents
>>>> ). I
>>>> don't think these are particularly common in practice, but it's
>>>> dangerous to assume that you can restore a class simply by restoring
>>>> its attributes after subsetting.
>>> You're probably right that there are lurking perils in general, but I am
>>> not trying to "restore a class". I simply want to *retain* attributes
>>> of columns in a data frame.
>>> * I have a data frame X
>>> * I attach attributes to certain of its columns;
>>> attr(X$melvin,"clyde") <- 42
>>> (I *don't* change the class of X$melvin.)
>>> * I form a subset of X:
>>> Y <- X[1:100,3:10]
>>> * given that "melvin" is amongst columns 3 through 10 of X,
>>> I want Y$melvin to retain the attribute "clyde", i.e. I
>>> want attr(Y$melvin,"clyde") to return 42
>>> There is almost surely a better approach than the one that I've chosen
>>> (isn't there always?) but it seems to work, and the perils certainly are
>>> not immediately apparent to me.
>> Maybe you've solved the problem for the columns that contain your
>> objects but now you've introduced a potential problem for columns that
>> contain objects with attributes whose value depend on content.
>> Hadley it right that restoring the original attributes of a vector (list
>> or atomic) after subsetting is unsafe.
> Right, so Rolf should only restore attributes that are ones he added in
> the first place. Unknown attributes should be left alone.
Fair point. And that gets fiddly. I guess I'm going to have to rethink
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