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

[Pages, Herve]

On 11/4/19 13:54, 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
>>>> (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.

Exactly. More precisely the problem needs to be tackled at the level of 
his objects (i.e. define a [ method for his objects that preserves the 
attributes) and not at the level of the [ method for data frames. The [ 
method for data frames will call his [ method when needed.

H.

> 
> Duncan Murdoch

-- 
Hervé Pagès

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