"Strictly speaking", Greg is correct, Bert. https://cran.r-project.org/doc/manuals/r-release/R-lang.html#List-objects
Lists in R are vectors. What we colloquially refer to as "vectors" are more precisely referred to as "atomic vectors". And without a doubt, this "vector" nature of lists is a key underlying concept that explains why adding a dim attribute creates a matrix that can hold data frames. It is also a stumbling block for programmers from other languages that have things like linked lists. On July 9, 2021 2:36:19 PM PDT, Bert Gunter <bgunter.4...@gmail.com> wrote: >"1. a column, when extracted from a data frame, *is* a vector." >Strictly speaking, this is false; it depends on exactly what is meant >by "extracted." e.g.: > >> d <- data.frame(col1 = 1:3, col2 = letters[1:3]) >> v1 <- d[,2] ## a vector >> v2 <- d[[2]] ## the same, i.e >> identical(v1,v2) >[1] TRUE >> v3 <- d[2] ## a data.frame >> v1 >[1] "a" "b" "c" ## a character vector >> v3 > col2 >1 a >2 b >3 c >> is.vector(v1) >[1] TRUE >> is.vector(v3) >[1] FALSE >> class(v3) ## data.frame >[1] "data.frame" >## but >> is.list(v3) >[1] TRUE > >which is simply explained in ?data.frame (where else?!) by: >"A data frame is a **list** [emphasis added] of variables of the same >number of rows with unique row names, given class "data.frame". If no >variables are included, the row names determine the number of rows." > >"2. maybe your question is "is a given function for a vector, or for a > data frame/matrix/array?". if so, i think the only way is reading > the help information (?foo)." > >Indeed! Is this not what the Help system is for?! But note also that >the S3 class system may somewhat blur the issue: foo() may work >appropriately and differently for different (S3) classes of objects. A >detailed explanation of this behavior can be found in appropriate >resources or (more tersely) via ?UseMethod . > >"you might find reading ?"[" and ?"[.data.frame" useful" > >Not just 'useful" -- **essential** if you want to work in R, unless >one gets this information via any of the numerous online tutorials, >courses, or books that are available. The Help system is accurate and >authoritative, but terse. I happen to like this mode of documentation, >but others may prefer more extended expositions. I stand by this claim >even if one chooses to use the "Tidyverse", data.table package, or >other alternative frameworks for handling data. Again, others may >disagree, but R is structured around these basics, and imo one remains >ignorant of them at their peril. > >Cheers, >Bert > > >Bert Gunter > >"The trouble with having an open mind is that people keep coming along >and sticking things into it." >-- Opus (aka Berkeley Breathed in his "Bloom County" comic strip ) > >On Fri, Jul 9, 2021 at 11:57 AM Greg Minshall <minsh...@umich.edu> >wrote: >> >> Kai, >> >> > one more question, how can I know if the function is for column >> > manipulations or for vector? >> >> i still stumble around R code. but, i'd say the following (and look >> forward to being corrected! :): >> >> 1. a column, when extracted from a data frame, *is* a vector. >> >> 2. maybe your question is "is a given function for a vector, or for >a >> data frame/matrix/array?". if so, i think the only way is >reading >> the help information (?foo). >> >> 3. sometimes, extracting the column as a vector from a data >frame-like >> object might be non-intuitive. you might find reading ?"[" and >> ?"[.data.frame" useful (as well as ?"[.data.table" if you use >that >> package). also, the str() command can be helpful in >understanding >> what is happening. (the lobstr:: package's sxp() function, as >well >> as more verbose .Internal(inspect()) can also give you insight.) >> >> with the data.table:: package, for example, if "DT" is a >data.table >> object, with "x2" as a column, adding or leaving off quotation >marks >> for the column name can make all the difference between ending up >> with a vector, or with a (much reduced) data table: >> ---- >> > is.vector(DT[, x2]) >> [1] TRUE >> > str(DT[, x2]) >> num [1:9] 32 32 32 32 32 32 32 32 32 >> > >> > is.vector(DT[, "x2"]) >> [1] FALSE >> > str(DT[, "x2"]) >> Classes ‘data.table’ and 'data.frame': 9 obs. of 1 variable: >> $ x2: num 32 32 32 32 32 32 32 32 32 >> - attr(*, ".internal.selfref")=<externalptr> >> ---- >> >> a second level of indexing may or may not help, mostly depending >on >> the use of '[' versus of '[['. this can sometimes cause >confusion >> when you are learning the language. >> ---- >> > str(DT[, "x2"][1]) >> Classes ‘data.table’ and 'data.frame': 1 obs. of 1 variable: >> $ x2: num 32 >> - attr(*, ".internal.selfref")=<externalptr> >> > str(DT[, "x2"][[1]]) >> num [1:9] 32 32 32 32 32 32 32 32 32 >> ---- >> >> the tibble:: package (used in, e.g., the dplyr:: package) also >> (always?) returns a single column as a non-vector. again, a >> second indexing with double '[[]]' can produce a vector. >> ---- >> > DP <- tibble(DT) >> > is.vector(DP[, "x2"]) >> [1] FALSE >> > is.vector(DP[, "x2"][[1]]) >> [1] TRUE >> ---- >> >> but, note that a list of lists is also a vector: >> > is.vector(list(list(1), list(1,2,3))) >> [1] TRUE >> > str(list(list(1), list(1,2,3))) >> List of 2 >> $ :List of 1 >> ..$ : num 1 >> $ :List of 3 >> ..$ : num 1 >> ..$ : num 2 >> ..$ : num 3 >> >> etc. >> >> hth. good luck learning! >> >> cheers, Greg >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide >http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > >______________________________________________ >R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >https://stat.ethz.ch/mailman/listinfo/r-help >PLEASE do read the posting guide >http://www.R-project.org/posting-guide.html >and provide commented, minimal, self-contained, reproducible code. -- Sent from my phone. Please excuse my brevity. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
