R help - Dec 2010 - Dynamically build functional expression and taking derivative

Hi!

I'm faced the next problem:
suppose I have a vector of some coefficient and for some purpose from  
these coefficients I need dynamically build a functional expression  
(because, the number of elements in the vector of coefficients might  
differ each time). So, I'm trying to do like this (let the number of  
coefficients be 3):

Building expression
> foo <-
gsub(",","*",toString(paste('(p',1:3,'*x +
> (1-p',1:3,'))',sep="")))
> foo
[1] "(p1*x + (1-p1))* (p2*x + (1-p2))* (p3*x + (1-p3))"

But now, how can I take derivative of this expression w.r.t. "x"?
> dfoo <- D(foo,"x") #doesn't work as well as
> dfoo <- D(as.expression(foo),"x")
May be there are other ways to handle this kind of situation?


Best regards,
Oleksandr

Try this:

 D(parse(text = foo), "x")

On Thu, Dec 9, 2010 at 2:59 PM, Oleksandr Dyklevych
<diklevich@gmail.com>wrote:
> Hi!
>
> I'm faced the next problem:
> suppose I have a vector of some coefficient and for some purpose from these
> coefficients I need dynamically build a functional expression (because, the
> number of elements in the vector of coefficients might differ each time).
> So, I'm trying to do like this (let the number of coefficients be 3):
>
> Building expression
>
>> foo <-
gsub(",","*",toString(paste('(p',1:3,'*x +
(1-p',1:3,'))',sep="")))
>> foo
>>
> [1] "(p1*x + (1-p1))* (p2*x + (1-p2))* (p3*x + (1-p3))"
>
> But now, how can I take derivative of this expression w.r.t. "x"?
>
>  dfoo <- D(foo,"x") #doesn't work as well as
>> dfoo <- D(as.expression(foo),"x")
>>
>
> May be there are other ways to handle this kind of situation?
>
>
> Best regards,
> Oleksandr
>
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>


-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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