R help - Sep 2006 - symbolic matrix elements...

Normally, I do symbolics in Maple, or Mathematica, but I'm trying to 
write a simple script for students to handle some *very* simple 
calculations (for other purposes) with matrix or vector elements, where 
the elements are coded symbolically. What I've tried with *partial" 
success is use of the tilde (~) operator. So, for example, consider a 
simple vector:

test=matrix(c(~ x^3-5*x+4, ~log(x^2-4*x)))

Now, when I look at test, I see

 > test
     [,1]     
[1,] Expression
[2,] Expression

Fine. When I try to extract one of the vector elements, I see (for example)

 > test[1]
[[1]]
~x^3 - 5 * x + 4


Fine - but now I'm trying to figure out how to use the extracted matrix 
element for anything else. For example, using D for simple symbolic 
derivatives

f <- test[1];
D(f,"x")

should *in theory* work, but I get the following:

 > D(f,"x");
[1] NA

But, if I try

f <- expression(x^3-5*x+4);
D(f,"x");

works fine.

So, even though it looks as if each element of test is coded as an 
expression, it seems as though it is somehow a different type of 
expression than if I code it explicitly as an expression. I'm *guessing* 
it has to do with the tilde operator not assigning the formula to 
anything, but I'm not sure.

Suggestions? Pointers to the obvious?

Thanks!

-- 
----------------------------------------------------------------------
 Evan Cooch                          e.mail: evan.cooch at cornell.edu
 Department of Natural Resources     voice: 607-255-1368
 Fernow Hall - Cornell University    FAX: 607-255-0349
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Nihilism is best done by professionals. - Iggy Pop

test=matrix(c( expression(x^3-5*x+4), expression(log(x^2-4*x))))
works.

btw. you recieved an error because D expects an expression and you offered  
a list
> class(test[1])
[1] "list"
to get the error relating to the misuse of the tilde operator you have to  
prompt the "correct" extractor "[["
f<-test[[1]]
D(f,"x")


Am Mon, 18 Sep 2006 18:30:57 +0200 schrieb Evan Cooch  
<evan.cooch at cornell.edu>:
> Normally, I do symbolics in Maple, or Mathematica, but I'm trying to
> write a simple script for students to handle some *very* simple
> calculations (for other purposes) with matrix or vector elements, where
> the elements are coded symbolically. What I've tried with
*partial"
> success is use of the tilde (~) operator. So, for example, consider a
> simple vector:
>
> test=matrix(c(~ x^3-5*x+4, ~log(x^2-4*x)))
>
> Now, when I look at test, I see
>
>  > test
>      [,1]
> [1,] Expression
> [2,] Expression
>
> Fine. When I try to extract one of the vector elements, I see (for  
> example)
>
>  > test[1]
> [[1]]
> ~x^3 - 5 * x + 4
>
>
> Fine - but now I'm trying to figure out how to use the extracted matrix
> element for anything else. For example, using D for simple symbolic
> derivatives
>
> f <- test[1];
> D(f,"x")
>
> should *in theory* work, but I get the following:
>
>  > D(f,"x");
> [1] NA
>
> But, if I try
>
> f <- expression(x^3-5*x+4);
> D(f,"x");
>
> works fine.
>
> So, even though it looks as if each element of test is coded as an
> expression, it seems as though it is somehow a different type of
> expression than if I code it explicitly as an expression. I'm
*guessing*
> it has to do with the tilde operator not assigning the formula to
> anything, but I'm not sure.
>
> Suggestions? Pointers to the obvious?
>
> Thanks!
>


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