[R] Arguments to "personalised" plot()

[Deepayan Sarkar]

On 10/7/07, Ted Harding <Ted.Harding at manchester.ac.uk> wrote:
> Hi Folks,
>
> I'm curious for an explanation of the following -- it's a
> matter of trying to understand how R parses it.
>
> I've written sundry little "helper" variants of functions,
> in particular plot(), to save repetitively typing the same
> options over and over again.
>
> For example:
>
> plotb <- function(x,...){plot(x,pch="+",col="blue",...)}
>
> This does exactly what you'd expect it to do when fed with
> a vector of values to plot, e.g.
>
>   plotb(cos(0.01*2*pi*(0:100)))
>
> namely a plot of the values of cos(..) with x-coordinates
> marked 0, 20, 40, 60, 80, 100, as blue "+".
>
> As expected, one can add other plot options if needed, e.g.
>
>   plotb(cos(0.01*2*pi*(0:100)), xlim=c(0,4*pi))
>
> if one wants. In this case, I'm supposing that the "xlim=c(0,4*pi)"
> goes in under the umbrella of "...", which is what I guessed
> would happen.
>
> Interestingly, though, if I do
>
>   x<-0.01*2*pi*(0:100); plotb(x,cos(x))
>
> I now get it with the x-axis labelled 0,1,2,3,4,5,6 just as
> if I had used the built-in
>
>   x<-0.01*2*pi*(0:100); plot(x,cos(x),pch="+",col="blue")
>
> and I can *also* add "xlim=c(0,4*pi)":
>
>   x<-0.01*2*pi*(0:100); plotb(x,cos(x),xlim=c(0,4*pi))
>
> and it still works! Now the latter is the same "..." mechanism
> as before, I suppose; but this doesn't explain how plotb()
> "sees" x, along with cos(x), and picks it up to do the
> right thing.
>
> So my question -- which is why I'm posting -- is:
>
> How does "x" get in along with "cos(x)" when I do
> "plotb(x,cos(x))", when the definition of the function is
>
>   plotb <- function(x,...){plot(x,pch="+",col="blue",...)}

I'm not sure exactly which part you didn't expect. Given this
definition of plotb, I would expect

plotb(x,cos(x))

to expand to

plot(x, pch="+", col="blue", cos(x))

and as far as I can tell, these give identical results. Is that what
surprises you? Why?

The other possibility is that you are surprised by the behavior of

plot(x, pch="+", col="blue", cos(x))

Remember that named arguments trump positional matching, and
consequently, this is equivalent to

plot(x, cos(x), pch="+", col="blue")

-Deepayan