[R] enclosing with() in a function

[thmsfuller066 at gmail.com]

On Mon, Aug 8, 2011 at 6:08 PM, peter dalgaard <pdalgd at gmail.com> wrote:
>
> 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)

I'm trying to understand 'substitute', is there a way to visualize the
parsed tree returned by 'substitute'?

> substitute(mean(x))
mean(x)
> str(substitute(mean(x)))
 language mean(x)


> 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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>>> 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
> "Døden skal tape!" --- Nordahl Grieg
>
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>



-- 
Tom