# Re: [R] symbolic matrix elements...

From: Tony Plate <tplate_at_acm.org>
Date: Mon 18 Sep 2006 - 17:54:10 GMT

> x <- matrix(list( expression(x3-5*x+4), expression(log(x2-4*x))))  > x[1,1]
[]
expression(x3 - 5 * x + 4)

> x[[1,1]]
expression(x3 - 5 * x + 4)
> D(x[[1,1]], "x")
-5
>

The reason c() doesn't work properly here might have something to do with it creating a language object of an unconventional type:

> c( expression(x3-5*x+4), expression(log(x2-4*x)))

```expression(x3 - 5 * x + 4, log(x2 - 4 * x))
> expression(x3-5*x+4)
expression(x3 - 5 * x + 4)
```

>

Using list() with language objects is much safer if you just want to make lists of them.

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

>
> Well, not really (or I'm misunderstanding). Your code enters fine (no
> errors), but I can't access individual elements - e.g., test[1,1] gives
> me an error:
>
> > test=matrix(c( expression(x^3-5*x+4), expression(log(x^2-4*x))))
> > test[1,1]
> Error: matrix subscripting not handled for this type
>
> Meaning...what?
>
>
```>>btw. you recieved an error because D expects an expression and you
>>offered a list
```

>
>
> OK - so why then are each of the elements identified as an expression
> which I print out the vector? Each element is reported to be an
> expression. OK, if so, then I remain puzzled as to how this is a 'list'.
>
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