You can find the names of the columns of a dataframe using
colnames(my.df)
A dataframe is a value just as much as a number is, and as such,
doesn't _have_ a name. However, when you call a function in R,
the arguments are not evaluated, and their forms can be recovered,
just as "plot" does. In fact, looking at plot to see how it does
that is a good idea.
demo <- function (df) {
list(name = deparse(substitute(df)), cols = colnames(df))
}
my.df <- data.frame(x = c(1,2), y = c(3,4))
demo(my.df)
$name
[1] "my.df"
$cols
[1] "x" "y"
On Sun, 2 Jun 2019 at 13:43, Sorkin, John <jsor...@som.umaryland.edu> wrote:
> Colleagues,
>
> Despite Bert having tried to help me, I am still unable to perform a
> simple act with a function. I want to pass the names of the columns of a
> dataframe along with the name of the dataframe, and use the parameters to
> allow the function to access the dataframe and modify its contents.
>
> I apologize multiple postings regarding this question, but it is a
> fundamental concept that one who wants to program in R needs to know.
>
> Thank you,
> John
>
> # Create a toy dataframe.
> df <- data.frame(a=c(1:20),b=(20:39))
> df
>
>
> # Set up a function that will access the first and second columns of the
> # data frame, print the columns of the dataframe and add the columns
> demo <- function(first,second,df)
> {
> # None of the following work
> print(df[,all.vars(first)])
> print(df[,first])
> print(df[,"first"])
>
> print(df[,all.vars(second)])
> print(df[,second])
> print(df[,"second"])
>
> df[,"sum"] <- print(df[,first])+print(df[,second])
>
> }
> demo(a,b, df)
>
>
>
>
>
> John David Sorkin M.D., Ph.D.
> Professor of Medicine
> Chief, Biostatistics and Informatics
> University of Maryland School of Medicine Division of Gerontology and
> Geriatric Medicine
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> (Phone) 410-605-7119
> (Fax) 410-605-7913 (Please call phone number above prior to faxing)
>
>
> ________________________________
> From: Bert Gunter <bgunter.4...@gmail.com>
> Sent: Wednesday, May 29, 2019 11:27 PM
> To: Sorkin, John
> Cc: r-help@r-project.org
> Subject: Re: [R] Tying to underdressed the magic of lm redux
>
> Depends on how you want to specify variables. You are not clear (to me) on
> this. But, for instance:
>
> demo <- function(form,df)
> {
> av <- all.vars(form)
> df[,av]
> }
> demo(~a+b, df)
> demo(a~b,df)
>
> ?all.vars, ?all.names for details
>
> Bert Gunter
>
>
> On Wed, May 29, 2019 at 7:33 PM Sorkin, John <jsor...@som.umaryland.edu
> <mailto:jsor...@som.umaryland.edu>> wrote:
> Bert,
> Thank you for your reply. You are correct that your code will print the
> contents of the data frame. While it works, it is not as elegant as the lm
> function. One does not have to pass the independent and dependent variables
> to lm In parentheses.
>
> Fit1<-lm(y~x,data=mydata)
>
> None of the parameters to lm are passed in quotation marks. Somehow, using
> deparse(substitute()) and other magic lm is able to get the data in the
> dataframe mydata. I want to be able to do the same magic in functions I
> write; pass a dataframe and column names, all without quotation marks and
> be able to write code that will provide access to the columns of the
> dataframe without having to pass the column names in quotation marks.
> Thank you,
> John
>
> John David Sorkin M.D., Ph.D.
> Professor of Medicine
> Chief, Biostatistics and Informatics
> University of Maryland School of Medicine Division of Gerontology and
> Geriatric Medicine
> Baltimore VA Medical Center
> 10 North Greene Street
> GRECC (BT/18/GR)
> Baltimore, MD 21201-1524
> (Phone) 410-605-711<tel:410-605-7119>9
> (Fax) 410-605-7913<tel:410-605-7913> (Please call phone number above prior
> to faxing)
>
> On May 29, 2019, at 9:59 PM, Bert Gunter <bgunter.4...@gmail.com<mailto:
> bgunter.4...@gmail.com>> wrote:
>
> Basically, huh?
>
> > df <- data.frame(a = 1:3, b = letters[1:3])
> > nm <- names(df)
> > print(df[,nm[1]])
> [1] 1 2 3
> > print(df[,nm[2]])
> [1] a b c
> Levels: a b c
>
> This can be done within a function, of course:
>
> > demo <- function(df, colnames){
> + print(df[,colnames])
> + }
> > demo(df,c("a","b"))
> a b
> 1 1 a
> 2 2 b
> 3 3 c
>
> Am I missing something? (Apologies, if so).
>
> Bert Gunter
>
>
>
> On Wed, May 29, 2019 at 6:40 PM Sorkin, John <jsor...@som.umaryland.edu
> <mailto:jsor...@som.umaryland.edu>> wrote:
> Thanks to several kind people, I understand how to use
> deparse(substitute(paramter)) to get as text strings the arguments passed
> to an R function. What I still can't do is put the text strings recovered
> by deparse(substitute(parameter)) back together to get the columns of a
> dataframe passed to the function. What I want to do is pass a column name
> to a function along with the name of the dataframe and then, within the
> function access the column of the dataframe.
>
> I want the function below to print the columns of the dataframe testdata,
> i.e. testdata[,"FSG"] and testdata[,"GCM"]. I have tried several ways to
> tell the function to print the columns; none of them work.
>
> I thank everyone who has helped in the past, and those people who will
> help me now!
>
> John
>
> testdata <- structure(list(FSG = c(271L, 288L, 269L, 297L, 311L, 217L,
> 235L,
>
> 172L, 201L, 162L), CGM = c(205L, 273L,
> 226L, 235L, 311L, 201L,
>
> 203L, 155L, 182L, 163L)), row.names =
> c(NA, 10L), class = "data.frame")
>
> cat("This is the data frame")
>
> class(testdata)
>
> testdata
>
>
>
> BAPlot <- function(first,second,indata){
>
> # these lines of code work
>
> col1 <- deparse(substitute(first))
>
> col2 <- deparse(substitute(second))
>
> thedata <- deparse(substitute(third))
>
> print(col1)
>
> print(col2)
>
> print(thedata)
>
> cat("This gets the data, but not as a dataframe\n")
>
> zoop<-paste(indata)
>
> print(zoop)
>
> cat("End This gets the data, but not as a dataframe\n")
>
> # these lines do not work
>
> print(indata[,first])
>
> print(indata[,"first"])
>
> print(thedata[,col1])
>
> paste(zoop[,paste(first)])
>
> paste(zoop[,first])
>
> zap<-paste(first)
>
> print(zap)
>
> }
>
>
>
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org<mailto:R-help@r-project.org> mailing list -- To
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> PLEASE do read the posting guide
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> and provide commented, minimal, self-contained, reproducible code.
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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