Rui Barradas
2016-Dec-05 17:53 UTC
[R] Write a function that allows access to columns of a passed dataframe.
Hello, Inline. Em 05-12-2016 17:09, David Winsemius escreveu:> >> On Dec 5, 2016, at 7:29 AM, John Sorkin <jsorkin at grecc.umaryland.edu> wrote: >> >> Rui, >> I appreciate your suggestion, but eliminating the deparse statement does not solve my problem. Do you have any other suggestions? See code below. >> Thank you, >> John >> >> >> mydf <- data.frame(id=c(1,2,3,4,5),sex=c("M","M","M","F","F"),age=c(20,34,43,32,21)) >> mydf >> class(mydf) >> >> >> myfun <- function(frame,var){ >> call <- match.call() >> print(call) >> >> >> indx <- match(c("frame","var"),names(call),nomatch=0) >> print(indx) >> if(indx[1]==0) stop("Function called without sufficient arguments!") >> >> >> cat("I can get the name of the dataframe as a text string!\n") >> #xx <- deparse(substitute(frame)) >> print(xx) >> >> >> cat("I can get the name of the column as a text string!\n") >> #yy <- deparse(substitute(var)) >> print(yy) >> >> >> # This does not work. >> print(frame[,var]) >> >> >> # This does not work. >> print(frame[,"var"]) >> >> >> >> >> # This does not work. >> col <- xx[,"yy"] >> >> >> # Nor does this work. >> col <- xx[,yy] >> print(col) >> } >> >> >> myfun(mydf,age) > > > When you use that calling syntax, the system will supply the values of whatever the `age` variable contains. (And if there is no `age`-named object, you get an error at the time of the call to `myfun`.Actually, no, which was very surprising to me but John's code worked (not the function, the call). And with the change I've proposed, it worked flawlessly. No errors. Why I don't know. Rui Barradas You need either to call it as:> > myfun( mydf , "age") > > > # Or: > > age <- "age" > myfun( mydf, age) > > Unless your value of the `age`-named variable was "age" in the calling environment (and you did not give us that value in either of your postings), you would fail. >
David Winsemius
2016-Dec-05 20:34 UTC
[R] Write a function that allows access to columns of a passed dataframe.
> On Dec 5, 2016, at 9:53 AM, Rui Barradas <ruipbarradas at sapo.pt> wrote: > > Hello, > > Inline. > > Em 05-12-2016 17:09, David Winsemius escreveu: >> >>> On Dec 5, 2016, at 7:29 AM, John Sorkin <jsorkin at grecc.umaryland.edu> wrote: >>> >>> Rui, >>> I appreciate your suggestion, but eliminating the deparse statement does not solve my problem. Do you have any other suggestions? See code below. >>> Thank you, >>> John >>> >>> >>> mydf <- data.frame(id=c(1,2,3,4,5),sex=c("M","M","M","F","F"),age=c(20,34,43,32,21)) >>> mydf >>> class(mydf) >>> >>> >>> myfun <- function(frame,var){ >>> call <- match.call() >>> print(call) >>> >>> >>> indx <- match(c("frame","var"),names(call),nomatch=0) >>> print(indx) >>> if(indx[1]==0) stop("Function called without sufficient arguments!") >>> >>> >>> cat("I can get the name of the dataframe as a text string!\n") >>> #xx <- deparse(substitute(frame)) >>> print(xx) >>> >>> >>> cat("I can get the name of the column as a text string!\n") >>> #yy <- deparse(substitute(var)) >>> print(yy) >>> >>> >>> # This does not work. >>> print(frame[,var]) >>> >>> >>> # This does not work. >>> print(frame[,"var"]) >>> >>> >>> >>> >>> # This does not work. >>> col <- xx[,"yy"] >>> >>> >>> # Nor does this work. >>> col <- xx[,yy] >>> print(col) >>> } >>> >>> >>> myfun(mydf,age) >> >> >> When you use that calling syntax, the system will supply the values of whatever the `age` variable contains. (And if there is no `age`-named object, you get an error at the time of the call to `myfun`. > > Actually, no, which was very surprising to me but John's code worked (not the function, the call). And with the change I've proposed, it worked flawlessly. No errors. Why I don't know.I see. Must be one of those "promise" things. It appears that if you don't actually require the value you can just pass a name with no value? Thanks for the correction. -- David.> > Rui Barradas > > You need either to call it as: >> >> myfun( mydf , "age") >> >> >> # Or: >> >> age <- "age" >> myfun( mydf, age) >> >> Unless your value of the `age`-named variable was "age" in the calling environment (and you did not give us that value in either of your postings), you would fail. >>David Winsemius Alameda, CA, USA
Bert Gunter
2016-Dec-05 21:34 UTC
[R] Write a function that allows access to columns of a passed dataframe.
Inline. -- 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 Mon, Dec 5, 2016 at 9:53 AM, Rui Barradas <ruipbarradas at sapo.pt> wrote:> Hello, > > Inline. > > Em 05-12-2016 17:09, David Winsemius escreveu: >> >> >>> On Dec 5, 2016, at 7:29 AM, John Sorkin <jsorkin at grecc.umaryland.edu> >>> wrote: >>> >>> Rui, >>> I appreciate your suggestion, but eliminating the deparse statement does >>> not solve my problem. Do you have any other suggestions? See code below. >>> Thank you, >>> John >>> >>> >>> mydf <- >>> data.frame(id=c(1,2,3,4,5),sex=c("M","M","M","F","F"),age=c(20,34,43,32,21)) >>> mydf >>> class(mydf) >>> >>> >>> myfun <- function(frame,var){ >>> call <- match.call() >>> print(call) >>> >>> >>> indx <- match(c("frame","var"),names(call),nomatch=0) >>> print(indx) >>> if(indx[1]==0) stop("Function called without sufficient arguments!") >>> >>> >>> cat("I can get the name of the dataframe as a text string!\n") >>> #xx <- deparse(substitute(frame)) >>> print(xx) >>> >>> >>> cat("I can get the name of the column as a text string!\n") >>> #yy <- deparse(substitute(var)) >>> print(yy) >>> >>> >>> # This does not work. >>> print(frame[,var]) >>> >>> >>> # This does not work. >>> print(frame[,"var"]) >>> >>> >>> >>> >>> # This does not work. >>> col <- xx[,"yy"] >>> >>> >>> # Nor does this work. >>> col <- xx[,yy] >>> print(col) >>> } >>> >>> >>> myfun(mydf,age) >> >> >> >> When you use that calling syntax, the system will supply the values of >> whatever the `age` variable contains. (And if there is no `age`-named >> object, you get an error at the time of the call to `myfun`. > > > Actually, no, which was very surprising to me but John's code worked (not > the function, the call). And with the change I've proposed, it worked > flawlessly. No errors. Why I don't know. > > Rui Barradas > > You need either to call it as: >> >> >> myfun( mydf , "age") >> >> >> # Or: >> >> age <- "age" >> myfun( mydf, age) >> >> Unless your value of the `age`-named variable was "age" in the calling >> environment (and you did not give us that value in either of your postings), >> you would fail. >> > > ______________________________________________ > R-help at 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.
Bert Gunter
2016-Dec-05 21:46 UTC
[R] Write a function that allows access to columns of a passed dataframe.
Sorry, hit "Send" by mistake. Inline. On Mon, Dec 5, 2016 at 1:34 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:> Inline. > > -- 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 Mon, Dec 5, 2016 at 9:53 AM, Rui Barradas <ruipbarradas at sapo.pt> wrote: >> Hello, >> >> Inline. >> >> Em 05-12-2016 17:09, David Winsemius escreveu: >>> >>> >>>> On Dec 5, 2016, at 7:29 AM, John Sorkin <jsorkin at grecc.umaryland.edu> >>>> wrote: >>>> >>>> Rui, >>>> I appreciate your suggestion, but eliminating the deparse statement does >>>> not solve my problem. Do you have any other suggestions? See code below. >>>> Thank you, >>>> John >>>> >>>> >>>> mydf <- >>>> data.frame(id=c(1,2,3,4,5),sex=c("M","M","M","F","F"),age=c(20,34,43,32,21)) >>>> mydf >>>> class(mydf) >>>> >>>> >>>> myfun <- function(frame,var){ >>>> call <- match.call() >>>> print(call) >>>> >>>> >>>> indx <- match(c("frame","var"),names(call),nomatch=0) >>>> print(indx) >>>> if(indx[1]==0) stop("Function called without sufficient arguments!") >>>> >>>> >>>> cat("I can get the name of the dataframe as a text string!\n") >>>> #xx <- deparse(substitute(frame)) >>>> print(xx) >>>> >>>> >>>> cat("I can get the name of the column as a text string!\n") >>>> #yy <- deparse(substitute(var)) >>>> print(yy) >>>> >>>> >>>> # This does not work. >>>> print(frame[,var]) >>>> >>>> >>>> # This does not work. >>>> print(frame[,"var"]) >>>> >>>> >>>> >>>> >>>> # This does not work. >>>> col <- xx[,"yy"] >>>> >>>> >>>> # Nor does this work. >>>> col <- xx[,yy] >>>> print(col) >>>> } >>>> >>>> >>>> myfun(mydf,age) >>> >>> >>> >>> When you use that calling syntax, the system will supply the values of >>> whatever the `age` variable contains. (And if there is no `age`-named >>> object, you get an error at the time of the call to `myfun`. >> >> >> Actually, no, which was very surprising to me but John's code worked (not >> the function, the call). And with the change I've proposed, it worked >> flawlessly. No errors. Why I don't know.See ?substitute and in particular the example highlighted there. The technical details are explained in the R Language Definition manual. The key here is the use of promises for lay evaluations. In fact, the expression in the call *is* available within the functions, as is (a pointer to) the environment in which to evaluate the expression. That is how substitute() works. Specifically, quoting from the manual, ***** It is possible to access the actual (not default) expressions used as arguments inside the function. The mechanism is implemented via promises. When a function is being evaluated the actual expression used as an argument is stored in the promise together with a pointer to the environment the function was called from. When (if) the argument is evaluated the stored expression is evaluated in the environment that the function was called from. Since only a pointer to the environment is used any changes made to that environment will be in effect during this evaluation. The resulting value is then also stored in a separate spot in the promise. Subsequent evaluations retrieve this stored value (a second evaluation is not carried out). Access to the unevaluated expression is also available using substitute. ******** -- Bert>> >> Rui Barradas >> >> You need either to call it as: >>> >>> >>> myfun( mydf , "age") >>> >>> >>> # Or: >>> >>> age <- "age" >>> myfun( mydf, age) >>> >>> Unless your value of the `age`-named variable was "age" in the calling >>> environment (and you did not give us that value in either of your postings), >>> you would fail. >>> >> >> ______________________________________________ >> R-help at 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.