Question about curve function
As a further example of the trickiness, the "function" method of plot() relies on curve(x, ...) being a request to plot the function x(x) against x. I've added a comment to that effect to the help page.
On Mon, 6 Jun 2011, Prof Brian Ripley wrote:
On Sun, 5 Jun 2011, Abhilash Balakrishnan wrote:
Dear Mr. Murdoch, I find out that still do not understand why the following does not work:
curve(expression(x))
Error in xy.coords(x, y, xlabel, ylabel, log) : 'x' and 'y' lengths differ As here the input to curve is an expression, as documented in the help, and
Not really, and certainly not in the sense you seem to understand it..
'expression(x)' is a call to the expression() function, and that evaluates to
a length-one expression vector. As ?expression says:
?Expression? here is not being used in its colloquial sense, that
of mathematical expressions. Those are calls (see ?call?) in R,
and an R expression vector is a list of calls, symbols etc, for
example as returned by ?parse?.
the expression is simply x.
'Simply' untrue.
What is the y mentioned in the error? There is no y used here.
Yes, there is. Please do read the code for 'curve': y <- eval(expr, envir = list(x = x), enclos = parent.frame()) so you are trying to plot a length-1 expression vector against a length-101 'x'. As others have said, curve() is a convenience function, and its requirements are rather picky. And you have already been given one good solution, curve(I).
Thank you for support. Abhilash B. On Sun, Jun 5, 2011 at 3:39 PM, Duncan Murdoch <murdoch.duncan at gmail.com>wrote:
On 11-06-05 1:07 PM, Abhilash Balakrishnan wrote:
Dear Sirs, I am a new user of the R package. When I try to use the curve function it confuses me. curve(x^2)
Works fine. curve(x)
Makes a complaint I don't understand. Why is x^2 valid and x is not?
curve() is a convenience function, and it tries to guess what you mean. Sometimes it gets it wrong. In the first case, it is clear you want to graph x^2. In the second it guesses you have a function named x and want to graph that. You don't, so it fails. Probably it could try again after the first failure, but I'd guess there will always be strange cases where it does weird things. Duncan Murdoch
I check the documentation of curve, and it says the first argument must be an expression containing x. expression(x)
Is an expression containing x. curve(expression(x))
Makes a different complaint and mentions different lengths of x and y
(but
I
use no y here).
I understand that plotting the function y(x) = x is rather silly, but I
want
to know what I am doing wrong, for the sake of my understanding of how R
works.
Thank you for support.
Abhilash B.
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______________________________________________ R-help at r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
-- Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595
Brian D. Ripley, ripley at stats.ox.ac.uk Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ University of Oxford, Tel: +44 1865 272861 (self) 1 South Parks Road, +44 1865 272866 (PA) Oxford OX1 3TG, UK Fax: +44 1865 272595