enclosing with() in a function

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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enclosing with() in a function

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