[Rd] Puzzled about a new method for "[".
Pages, Herve
hp@ge@ @end|ng |rom |redhutch@org
Mon Nov 4 22:40:19 CET 2019
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.
Best,
H.
>
> cheers,
>
> Rolf
>
--
Hervé Pagès
Program in Computational Biology
Division of Public Health Sciences
Fred Hutchinson Cancer Research Center
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