Obtaining a derivative of nls() SSlogis function

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Hello, I am wondering if someone can help me. I have the following  
function
that I derived using nls() SSlogis. I would like to find its  
derivative. I
thought I had done this using deriv(), but for some reason this isn't
working out for me.

Here is the function:
asym <- 84.951
xmid <- 66.90742
scal <- -6.3

x.seq <- seq(1, 153,, 153)
nls.fn <- asym/((1+exp((xmid-x.seq)/scal)))

try #1
deriv(nls.fn)
#get an Error in .Internal(deriv.default(expr, namevec,  
function.arg, tag,
hessian)) : 'namevec' is missing

try #2
deriv(nls.fn, namevec=c("asym", "xmid", "scal"))
#this doesn't seem to give me the expression, and the gradients are  
zero.
nls.fn is not a function or an expression. It has been evaluated and  
now it's a vectpr, and not what `deriv` is "expecting". If you want to  
use `deriv` or `D` you must first read the help page:

?deriv

And then construct a proper expresssion, call, or function...

 > nls.expr <- expression( 84.951/((1 + exp(( 66.90742- x)/ -6.3))) )
 > D(nls.expr, "x")
-(84.951 * (exp((66.90742 - x)/-6.3) * (1/6.3))/((1 + exp((66.90742 -
     x)/-6.3)))^2)
 > deriv(nls.expr, "x")
expression({
     .expr4 <- exp((66.90742 - x)/-6.3)
     .expr5 <- 1 + .expr4
     .value <- 84.951/.expr5
     .grad <- array(0, c(length(.value), 1L), list(NULL, c("x")))
     .grad[, "x"] <- -(84.951 * (.expr4 * (1/6.3))/.expr5^2)
     attr(.value, "gradient") <- .grad
     .value
})
David.

>
> I've tried to do this with Ryacas as well, but I'm lost.
>
> Can anyone help?
>
> Thank you,
>
> Katrina
>
> 	[[alternative HTML version deleted]]
>
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David Winsemius, MD
West Hartford, CT
Hello, I am wondering if someone can help me. I have the following function
that I derived using nls() SSlogis. I would like to find its derivative. I
thought I had done this using deriv(), but for some reason this isn't
working out for me.

Here is the function:
asym <- 84.951
xmid <- 66.90742
scal <- -6.3

x.seq <- seq(1, 153,, 153)
nls.fn <- asym/((1+exp((xmid-x.seq)/scal)))

try #1
deriv(nls.fn)
#get an Error in .Internal(deriv.default(expr, namevec, function.arg, tag,
hessian)) : 'namevec' is missing

try #2
deriv(nls.fn, namevec=c("asym", "xmid", "scal"))
#this doesn't seem to give me the expression, and the gradients are zero.

I've tried to do this with Ryacas as well, but I'm lost.

Can anyone help?

You can do that with plain R:
e <- quote(asym/((1+exp((xmid-x.seq)/scal))))
for(v in all.vars(e)) cat("deriv wrt", v, "is", format(D(e, v)), "\n")
deriv wrt asym is 1/((1 + exp((xmid - x.seq)/scal)))
deriv wrt xmid is -(asym * (exp((xmid - x.seq)/scal) * (1/scal))/((1 +
exp((xmid -      x.seq)/scal)))^2)
deriv wrt x.seq is asym * (exp((xmid - x.seq)/scal) * (1/scal))/((1 +
exp((xmid -      x.seq)/scal)))^2
deriv wrt scal is asym * (exp((xmid - x.seq)/scal) * ((xmid -
x.seq)/scal^2))/((1 +      exp((xmid - x.seq)/scal)))^2
Statistics & Software Consulting
GKX Group, GKX Associates Inc.
tel: 1-877-GKX-GROUP
email: ggrothendieck at gmail.com
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