[R] Problem with do.call().

[Rolf Turner]

On 29/03/14 01:34, peter dalgaard wrote:
>
> On 28 Mar 2014, at 02:37 , Rolf Turner <r.turner at auckland.ac.nz> wrote:
>>>
>>> So there you are.  Feel enlightened?
>>
>> Somewhat, actually, but not to such an extent as to have reached nirvana.  "Promises" blow me away.
>>
>>>
>>> Here's the most useful part of the post:  to get what you want, use
>>>
>>> do.call(plot, list(x=x, y=y, xlab="x", ylab="y"))
>>
>> Yes.  That's effectively what I wound up doing --- in a slightly more complicated context.
>>
>> Thomas's idea is sexier, but I of course understand even less than you profess to do.
>
> Well, let me try (famous last words, I know)
>
> x <- 1
> do.call("plot", list(x=x))
>
> is equivalent to
>
> plot(x=1)
>
> because the list argument is a list with one element, called 'x' and having the value '1'. do.call is not going out to look for the difference between that and, say,
>
> y <- 1
> l <- list(x=y)
> do.call("plot", l)
>
> The thing that is going on inside do.call is that first we construct the call, effectively using
>
> as.call(c(as.name("plot"), list(x=x)))
>
> which you can verify results in the unevalued expression
>
> plot(x = 1)
>
> Then, the expression is evaluated, and at this stage, plot obviously has no idea that the argument 1 was called x at some earlier stage.
>
> Thomas' trick amounts to
>
> as.call(c(as.name("plot"), list(x=quote(x))))
>
> which is
>
> plot(x = x)
>
> which obviously does the usual thing when evaluated. The main part of the magic lies in the conversion of lists to call objects and vice versa.
>
> (One curiosity:
>
>> do.call(quote(plot),list(quote(x)))
> Error in do.call(quote(plot), list(quote(x))) :
>    'what' must be a character string or a function
>
> which sees a bit overzealous. I see no reason that it shouldn't work.)
>

Thanks Peter.  I shall meditate upon this and try to absorb it.  Takes a 
while to sink in, but.

cheers,

Rolf