On Sat, 29 Aug 2020 21:15:56 +0000
"Sorkin, John" <jsor...@som.umaryland.edu> wrote:
> I am trying to find the minimum of a linear function:
Quadratic function???
> y <- (-0.0263*b) + (0.0010*B^2)
>
> I am having GREAT difficulty with the documentation of the deriv
> function. I have (after playing for two-hours) been able to get the
> following to work:
>
> zoop <- deriv(expression((-0.0263*B)+(0.0010*B^2)),"B",func=TRUE)
> class(zoop)
> zoop(2)
>
> which appears to give me the value of the derivative of my expression
> w.r.t. B (I am not certain what the func arugment does, but it
> appears to be necessary)
It causes deriv() to return a *function* rather than an *expression*.
>
> Following what one learns in calculus 1, I now need to set the
> derivative equal to 0 and solve for B. I have no idea how to do this
>
> Can someone point me in the right direction. Additionally can someone
> suggest documentation for deriv that is easily intelligible to
> someone who wants to learn how to use the function, rather that
> documentation that helps one who is already familiar with the
> function. (I have a need for derivatives that is beyond finding the
> minimum of a function)
>
> Thank you
> John
>
> P.S. Please don�t flame. I spent a good deal of time looking at
> documentation and searching the internet. There may be something on
> line, but I clearly am not using the correct search terms.
Couple of things that you could play around with.
y <- expression(-0.0263*B + 0.0010*B^2)
z <- deriv(y,"B",func=TRUE)
f <- function(x,z){as.vector(attr(z(x),"gradient"))}
(1) uniroot(f,c(5,15),z=z)$root
# 13.15 --- right answer!!! :-)
(2) library(polynom) # You may need to install this package.
p <- poly.calc(x=1:2,y=f(1:2,z=z))
polyroot(p)
# 13.15+0i You can get rid of the extraneous imaginary part
# by using Re(polyroot(p))
HTH
cheers,
Rolf
P.S. It's irritating the way that one has to fiddle about in order to
get a function that returns the value of the derivative, rather than the
value of the function being differentiated!
R.
--
Honorary Research Fellow
Department of Statistics
University of Auckland
Phone: +64-9-373-7599 ext. 88276
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