R help - Oct 2015 - writing a function that will refer to the elements of a dataframe much as is done by lm

I am trying to learn how  to write R functions (really to program in R). I want
to write a function that will allow me to refer to the elements of a data frame
by column name, much as is done in lm. I can't seem to get the syntax
correct. I want the function to print the elements of
data2[,"Mo6MONO"]
 
data2 <- data.frame(POSTHHMONO=c(1,2,3,4),Mo6MON=c(10,11,12,13))
data2
doit<- function(pre,post,data) {
  element <- deparse(substitute(pre))
  print(element)
  frame <- deparse(substitute(data))
  print(frame)
  print(frame$element)
}
doit(Mo6MONO,POSTHHMONO,data2)
 
results of running function:
Browse[1]> doit(Mo6MONO,POSTHHMONO,data2)
[1] "Mo6MONO"
[1] "data2"
Error during wrapup: $ operator is invalid for atomic vectors

 
 
I have looked at lm, but don't understand the syntax, and have tried to run
lm so as to learn the syntax by just don't understand . . . .
 
HELP
 
Thanks 
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) 410-605-7119
(Fax) 410-605-7913 (Please call phone number above prior to faxing) 

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Hi John,
Here are a couple of ways to do this. Note that they produce slightly
different results.

data2<-data.frame(POSTHHMONO=c(1,2,3,4),Mo6MON=c(10,11,12,13))
doit<- function(pre,post,data) {
  element <- deparse(substitute(pre))
  print(element)
  print(data[element])
  frame<-deparse(substitute(post))
  print(frame)
  print(data[frame])
}
doit(Mo6MON,POSTHHMONO,data2)

# this produces the same as the $ extractor
doit<- function(pre,post,data) {
  element <- deparse(substitute(pre))
  print(element)
  print(data[,element])
  frame<-deparse(substitute(post))
  print(frame)
  print(data[,frame])
}
doit(Mo6MON,POSTHHMONO,data2)

Jim



On Tue, Oct 13, 2015 at 9:11 AM, John Sorkin <jsorkin at
grecc.umaryland.edu>
wrote:
> I am trying to learn how  to write R functions (really to program in R). I
> want to write a function that will allow me to refer to the elements of a
> data frame by column name, much as is done in lm. I can't seem to get
the
> syntax correct. I want the function to print the elements of
> data2[,"Mo6MONO"]
>
> data2 <- data.frame(POSTHHMONO=c(1,2,3,4),Mo6MON=c(10,11,12,13))
> data2
> doit<- function(pre,post,data) {
>   element <- deparse(substitute(pre))
>   print(element)
>   frame <- deparse(substitute(data))
>   print(frame)
>   print(frame$element)
> }
> doit(Mo6MONO,POSTHHMONO,data2)
>
> results of running function:
> Browse[1]> doit(Mo6MONO,POSTHHMONO,data2)
> [1] "Mo6MONO"
> [1] "data2"
> Error during wrapup: $ operator is invalid for atomic vectors
>
>
>
> I have looked at lm, but don't understand the syntax, and have tried to
> run lm so as to learn the syntax by just don't understand . . . .
>
> HELP
>
> Thanks
> 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) 410-605-7119
> (Fax) 410-605-7913 (Please call phone number above prior to faxing)
>
> Confidentiality Statement:
> This email message, including any attachments, is for ...{{dropped:16}}

On Oct 12, 2015, at 3:11 PM, John Sorkin wrote:
> I am trying to learn how  to write R functions (really to program in R). I
want to write a function that will allow me to refer to the elements of a data
frame by column name, much as is done in lm. I can't seem to get the syntax
correct. I want the function to print the elements of
data2[,"Mo6MONO"]
> 
> data2 <- data.frame(POSTHHMONO=c(1,2,3,4),Mo6MON=c(10,11,12,13))
> data2
> doit<- function(pre,post,data) {
>  element <- deparse(substitute(pre))
>  print(element)
>  frame <- deparse(substitute(data))
>  print(frame)
>  print(frame$element)
> }
> doit(Mo6MONO,POSTHHMONO,data2)
> 
> results of running function:
> Browse[1]> doit(Mo6MONO,POSTHHMONO,data2)
> [1] "Mo6MONO"
> [1] "data2"
> Error during wrapup: $ operator is invalid for atomic vectors
> 
> 
> 
> I have looked at lm, but don't understand the syntax, and have tried to
run lm so as to learn the syntax by just don't understand . . . .
> 
Jim has already given an answer to the specific case of of your example, but lm
doesn't actually have formals that are passed such values. It accepts
formula objects which have a different syntax and evaluation rules. So it was
not clear to me that your preamble matched your example.

When I have done what you did I just passed character vectors and used them
inside "[[" since that is the equivalent of using "$". You
are advised in the help pages not to use $ inside functions. Notice that
deparse(substitute(.)) delivers a character vector.

-- 
David.
> HELP
> 
> Thanks 
> 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) 410-605-7119
> (Fax) 410-605-7913 (Please call phone number above prior to faxing) 
> 
> Confidentiality Statement:
> This email message, including any attachments, is for ...{{dropped:15}}