[Rd] Puzzled about a new method for "[".

Duncan Murdoch murdoch@dunc@n @end|ng |rom gm@||@com
Mon Nov 4 22:54:46 CET 2019

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
>>> (e.g.
>>> https://urldefense.proofpoint.com/v2/url?u=https-3A__vctrs.r-2Dlib.org_articles_s3-2Dvector.html-23cached-2Dsum&d=DwICAg&c=eRAMFD45gAfqt84VtBcfhQ&r=BK7q3XeAvimeWdGbWY_wJYbW0WYiZvSXAJJKaaPhzWA&m=pqLHzHYLUeyQnxA1K_XhSbKJql6r9wK1RXcDG2tuZ6s&s=kPUlNqBPr6j4lPvqkIj8w2Gl5JYGLqJ7ws6wH5tpGcw&e=
>>> ). 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.

Duncan Murdoch

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