R help - Nov 2012 - How to get the result of eval()

I just discover the deriv function but I have a minor problem at the end 
when using its result:
For example:

dx2x <- deriv(~ A*x^2, "x") ; dx2x

# it works fine:
# expression({
# .value <- A * x^2
# .grad <- array(0, c(length(.value), 1L), list(NULL, c("x")))
# .grad[, "x"] <- A * (2 * x)
# attr(.value, "gradient") <- .grad
# .value
# })

A <- 3
x <- 23

eval(dx2x)

# great 138 is well the derivative at the point 23
# [1] 1587
# attr(,"gradient")
# x
# [1,] 138


But I don?t know how to get the 138 value.

Any idea ?

Thanks a lot

Hello,
Try the following.

attr(eval(dx2x), "gradient")

Hope this helps,

Rui Barradas
Em 16-11-2012 11:02, Marc Girondot escreveu:
> I just discover the deriv function but I have a minor problem at the 
> end when using its result:
> For example:
>
> dx2x <- deriv(~ A*x^2, "x") ; dx2x
>
> # it works fine:
> # expression({
> # .value <- A * x^2
> # .grad <- array(0, c(length(.value), 1L), list(NULL, c("x")))
> # .grad[, "x"] <- A * (2 * x)
> # attr(.value, "gradient") <- .grad
> # .value
> # })
>
> A <- 3
> x <- 23
>
> eval(dx2x)
>
> # great 138 is well the derivative at the point 23
> # [1] 1587
> # attr(,"gradient")
> # x
> # [1,] 138
>
>
> But I don?t know how to get the 138 value.
>
> Any idea ?
>
> Thanks a lot
>
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