[R] Write a function that allows access to columns of a passed dataframe.

[David Winsemius]

> 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`. 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.

> 
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> myfun()
> 
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> 
> 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) 
> 
>>>> Rui Barradas <ruipbarradas at sapo.pt> 12/05/16 10:17 AM >>>
> Hello,
> 
> You don't need xx <- deparse(substitute(...)), since you are passing the 
> data.frame to your function. Just use
> 
> 
> myfun <- function(frame,var){
> 
>   [...]
> 
>   # Nor does this work.
>   col <- frame[,yy]
>   print(col)
> }
> 
> myfun(mydf,age)
> myfun(frame = mydf, var = age)
> [1] 2 3
> I can get the name of the dataframe as a text string!
> [1] "mydf"
> I can get the name of the column as a text string!
> [1] "age"
> [1] 20 34 43 32 21
> 
> 
> Hope this helps,
> 
> Rui Barradas
> 
> 
> 
> Em 05-12-2016 14:44, John Sorkin escreveu:
>> I am trying to write a function which, when passed the name of a dataframe and the name of a column of the dataframe, will allow me to work on the columns of a dataframe. I can not get my code to work. Please see the code below. Any help in getting the function to work would be appreciated.
>> 
>> 
>> 
>> 
>> 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.
>>   col <- xx[,"yy"]
>> 
>> 
>>   # Nor does this work.
>>   col <- xx[,yy]
>>   print(col)
>> }
>> 
>> 
>> myfun(mydf,age)
>> 
>> 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)
>> 
>> 
>> 
>> 
>> 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)
>> 
>> 
>> 
>> 
>> 
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