2008/11/8 Bert Gunter [EMAIL PROTECTED]:
Is the following intended or not?
func- function(y) match.call()
z - func(y =2)
z
func(y = 2)
z[[a]] - 5
z
func(y = 2, 5) ## Note that the second argument **is not** named
## BUT...
z - func(y =2)
z$a - 5
z
func(y = 2, a = 5) ## The second argument **is** named
### End of example code ###
The reason I ask is that the man page for [[ specifically says:
**
Both [[ and $ select a single element of the list. The main difference is
that $ does not allow computed indices, whereas [[ does. x$name is
equivalent to x[[name, exact = FALSE]]. Also, the partial matching
behavior of [[ can be controlled using the exact argument.
[ and [[ are sometimes applied to other recursive objects such as calls and
expressions. Pairlists are coerced to lists for extraction by [, but all
three operators can be used for replacement.
I (mis?)read this as saying the behavior in the code snippets above should
produce identical results.
I note that the above inconsistency can be trivially avoided by first
coercing the call object to a list, modifying it either way, and then
coercing it back to a call object.
I too have been caught by this apparent inconsistency a few times.
By the way, another way to get around it is
z - quote(func(y = 2))
z[a] - list(5)
Another inconsistency between the indexing of lists and calls is
recursive indexing, e.g.
## recursive indexing of lists works
foo - list(a = list(1, 2, 3), b = list(4, 5, 6))
foo[[c(2,2)]]
[1] 5
## recursive indexing of calls fails
foocall - quote(func(a = list(1, 2, 3), b = list(4, 5, 6)))
foocall[[c(3, 2)]]
Error in foocall[[c(3, 2)]] : attempt to select more than one element
OK, this is pretty obscure, and it is easy enough to write a function
to do it (as I have done), but it would be nice to have in the long
run, so that indexing is more standardised.
--
Felix Andrews / 安福立
http://www.neurofractal.org/felix/
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