[R] Write a function that allows access to columns of a passeddataframe.
John Sorkin
jsorkin at grecc.umaryland.edu
Tue Dec 6 15:28:07 CET 2016
Over my almost 50 years programming, I have come to believe that if one wants a program to be useful, one should write the program to do as much work as possible and demand as little as possible from the user of the program. In my opinion, one should not ask the person who uses my function to remember to put the name of the data frame column in quotation marks. The function should be written so that all that needs to be passed is the name of the column; the function should take care of the quotation marks.
Jihny
> 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)
> On Dec 6, 2016, at 3:17 AM, Rui Barradas <ruipbarradas at sapo.pt> wrote:
>
> Hello,
>
> Just to say that I wouldn't write the function as John did. I would get
> rid of all the deparse/substitute stuff and instinctively use a quoted
> argument as a column name. Something like the following.
>
> myfun <- function(frame, var){
> [...]
> col <- frame[, var] # or frame[[var]]
> [...]
> }
>
> myfun(mydf, "age") # much better, simpler, no promises.
>
> Rui Barradas
>
> Em 05-12-2016 21:49, Bert Gunter escreveu:
>> Typo: "lazy evaluation" not "lay evaluation."
>>
>> -- 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 1:46 PM, Bert Gunter <bgunter.4567 at gmail.com> wrote:
>>> 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.
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