On Mar 29, 2012, at 1:34 PM, Duncan Murdoch wrote:
On 29/03/2012 1:02 PM, Julio Sergio wrote:
I learnt that functions can be handled as objects, the same way the
variables
are. So, the following is perfectly valid:
> f = function(a, b) {
+ print(a)
+ print(b)
+ }
>
> f1 = function(foo) {
+ foo(1,2)
+ }
>
> f1(f)
[1] 1
[1] 2
>
I also know that operators are functions, so, I can call:
> '+'(1,2)
[1] 3
>
However, when I want to pass the '+' function to the previous f1
function, it
doesn't work:
> f1('+')
Error en f1("+") : no se pudo encontrar la función "foo"
>
(Error in f1("+") : the function "foo" cannot be found)
Do you have any comments on this?
You are seeing the effects of the evaluator and parser being
helpful. When you say
foo(1,2)
the evaluator looks for an object that appears to be a function and
evaluates it. Since foo is a character vector in your last example,
it skips over that one, and never finds another.
When you say
'+'(1,2)
the parser sees a string constant being used in a context where a
function name is needed, so it helpfully converts it to an object
name, and then the evaluator looks up that name, and finds the
addition function.
You can get the results you want by using f1(`+`), i.e. being
explicit about passing the name of the addition function. If you
have a string containing a function name, you need to explicitly use
get() to retrieve it. Use get(foo, mode="function") to skip over
non-functions.
There is also the `do.call` function to construct proper calls from a
character that matches the name of a function
f1 = function(foo) {
do.call(foo, list(1,2))
}
f1("+")
[1] 3
--
David Winsemius, MD
West Hartford, CT
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