[R] enclosing with() in a function

[peter dalgaard]

On Aug 9, 2011, at 00:29 , Dennis Murphy wrote:

> Hi:
> 
> Here are a couple of ways; there may well be better ones.
> 
> # (1)  Use the get() function:
> mean_on_element=function(data, elem_name) {
>   with(data, mean(get(elem_name)))
> }
> mean_on_element(data, 'x')

I suspect this goes belly-up if there's a column data$elem_name, though.

Given than with() is essentially evalq() which in turn is eval(quote(...),...), the obvious way to achieve the desired effect would be to omit quoting the argument and do

eval(substitute(mean(elem_name)), data) 

or, to avoid unexpected variable capture:

mean_on_element <- function(data, elem_name) 
   eval(substitute(mean(elem_name)), data, parent.frame()) 

mean_on_element(airquality, Day)

Or rather: this allows variable capture of the same kind that with() allows:

> mean_on_element(airquality, X)
[1] 0.575
> with(airquality, mean(X))
[1] 0.575



> 
> # (2) Lose 'with' and use subscripting instead:
> mean_on_element=function(data, elem_name) {
>   mean(data[[elem_name]])
> }
> mean_on_element(data, 'x')
> 
> Since 'x' is quoted in the function call, you need to use code that
> can convert the string 'x' to extracting the data object with name x.
> 
> HTH,
> Dennis
> 
> On Mon, Aug 8, 2011 at 3:12 PM, thmsfuller066 at gmail.com
> <thmsfuller066 at gmail.com> wrote:
>> Hi All,
>> 
>> I want to enclose with() in a function mean_on_element. Obviously, it
>> is not working. The problem is how to specify the element name with a
>> function body. Does anybody have any suggestion? Thanks!
>> 
>>> data=list(x=1:10)
>>> with(data, mean(x))
>> [1] 5.5
>>> 
>>> mean_on_element=function(data, elem_name) {
>> +   with(data, mean(elem_name))
>> + }
>>> mean_on_element(data, 'x')
>> [1] NA
>> Warning message:
>> In mean.default(elem_name) :
>>  argument is not numeric or logical: returning NA
>> 
>> 
>> --
>> Tom
>> 
>> ______________________________________________
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>> 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.
>> 
> 
> ______________________________________________
> R-help at r-project.org mailing list
> 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.

-- 
Peter Dalgaard, Professor,
Center for Statistics, Copenhagen Business School
Solbjerg Plads 3, 2000 Frederiksberg, Denmark
Phone: (+45)38153501
Email: pd.mes at cbs.dk  Priv: PDalgd at gmail.com
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