From murdoch@dunc@n @end|ng |rom gm@||@com  Mon Nov  4 22:54:46 2019
From: murdoch@dunc@n @end|ng |rom gm@||@com (Duncan Murdoch)
Date: Mon, 4 Nov 2019 16:54:46 -0500
Subject: [Rd] Puzzled about a new method for "[".
In-Reply-To: <92d3dc36-f1d8-875b-4250-2cbd70978402@fredhutch.org>
References: <74a6cf10-3266-fef8-9ea0-0ba6d36db807@auckland.ac.nz>
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 <3ccb53ef-a0a7-0449-28f2-fad550b719bf@auckland.ac.nz>
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Message-ID: <ea776fff-03d2-a81c-66f7-3a986c44de5a@gmail.com>

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



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