[Rd] documentation error for deriv; MASS scripts ch8; (PR#676)

[kjetilh@umsanet.edu.bo]

Hola!

There is a bug in the documentation file for deriv, which says that
the third argument is unimplemented. It seems only to be implemented
differently than in s-plus. I show whith the MASS R-script
ch8 file as example:

> expn1 <- deriv(y ~ b0 + b1 * 2^(-x/th), c("b0", "b1", "th"),
+                function(b0, b1, th, x) {})
> expn1
function(b0, b1, th, x) {}

which cannnot be used whith nls! But changing to:

> expn1 <- deriv(y ~ b0 + b1 * 2^(-x/th), c("b0", "b1", "th"),
+                c("b0", "b1", "th", "x") )
> expn1
function (b0, b1, th, x) 
{
    .expr3 <- 2^(-x/th)
    .value <- b0 + b1 * .expr3
    .grad <- array(0, c(length(.value), 3), list(NULL, c("b0", 
        "b1", "th")))
    .grad[, "b0"] <- 1
    .grad[, "b1"] <- .expr3
    .grad[, "th"] <- b1 * (.expr3 * (log(2) * (x/th^2)))
    attr(.value, "gradient") <- .grad
    .value
}

gives the desired result. 

Kjetil Halvorsen

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