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

[Pages, Herve]

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

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