[R] Arguments to "personalised" plot()

(Ted Harding) Ted.Harding at manchester.ac.uk
Sun Oct 7 23:48:07 CEST 2007

On 07-Oct-07 21:01:08, Deepayan Sarkar wrote:
> 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?

See below.

> 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,

Ahh, thank you!! I think this is the clue I was looking for.
I had guessed that [A](above) and therefore [B] was what was
really happening, and that therefore some sort of precedence
was working to ensure that things came in in the right "order".
Now I know what it is.

> and consequently, this is equivalent to
> plot(x, cos(x), pch="+", col="blue")

In the sense, I suppose, that in either case the effect is
a) First sort out (*,pch="+",col="blue",*) (since these are
named), leaving (x,                       cos(x))

(as it were).

> -Deepayan

Thanks, Deepayan.

E-Mail: (Ted Harding) <Ted.Harding at manchester.ac.uk>
Fax-to-email: +44 (0)870 094 0861
Date: 07-Oct-07                                       Time: 22:48:05
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