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 Program in Computational Biology Division of Public Health Sciences Fred Hutchinson Cancer Research Center 1100 Fairview Ave. N, M1-B514 P.O. Box 19024 Seattle, WA 98109-1024 E-mail: hpa...@fredhutch.org Phone: (206) 667-5791 Fax: (206) 667-1319 ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
