[R] enclosing with() in a function

[Duncan Murdoch]

On 11-08-08 9:58 PM, thmsfuller066 at gmail.com wrote:
> 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'?

as.list() turns the top level of the tree into a list.  You could 
recursively apply that.  I don't know if there's an existing function to 
do so, but it's not hard to write one:

expand <- function(e) {
	if (is.name(e) || !is.language(e)) return(e)
	lapply(as.list(e), expand)
}

Duncan Murdoch

>
>> 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
>>>>
>>>> ______________________________________________
>>>> 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.
>>>>
>>>
>>> ______________________________________________
>>> 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
>>
>>
>>
>>
>>
>>
>>
>>
>
>
>