obtaining output from an evaluated expression
On Fri, 19 Dec 2008, Dieter Menne wrote:
Jack Bowden <jack.bowden <at> mrc-bsu.cam.ac.uk> writes:
dU1dtheta <- deriv(~ xi-(alpha0+alpha1*gi), c("alpha0","alpha1"))
eval(dU1dtheta)
(Intercept)
-0.2547153
attr(,"gradient")
alpha0 alpha1
[1,] -1 0
I want to extract the output gradient values of -1 and 0 but I don't
know how to access them. The only thing I can access is the intercept
term via.
eval(dU1dtheta)[1].
It probably easiest to ask for a function to be returned using function.arg, as the example in deriv shows. Having been bitten by deriv in the past, I prefer to use it to give me the function or expression, and paste it explicitly into my code, doing some sanity check first. It's not very general, but tells you better what is happening.
But he is stil going to need to learn about attr(): attr(eval(dU1dtheta)[1], "gradient") does the job (I presume, we don't have all the data in this message, nor I think in the previous one although you inexplicably silently removed the line that said gi = 0).
Dieter
## function with defaulted arguments:
(fx <- deriv(y ~ b0 + b1 * 2^(-x/th), c("b0", "b1", "th"),
function(b0, b1, th, x = 1:7){} ) )
fx(2,3,4)
attr(fx(2,3,4),"gradient")
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