[R] Conversion from expression to numeric

[Gabor Grothendieck]

On 11/23/06, Paul Smith <phhs80 at gmail.com> wrote:
> On 23 Nov 2006 18:20:17 +0100, Peter Dalgaard <p.dalgaard at biostat.ku.dk> wrote:
> > > I am trying to convert from the type "expression" to the type
> > > "numeric". The following works:
> > >
> > > > x <- expression(6.2)
> > > > as.numeric(as.character(x))
> > > [1] 6.2
> > >
> > > However, the following does not work:
> > >
> > > > x <- expression(62/100)
> > > > as.numeric(as.character(x))
> > > [1] NA
> > > Warning message:
> > > NAs introduced by coercion
> > >
> > > Any idea about how to deal with the second case?
> >
> > Well, you could start by watching where you are going...
> >
> > > x <- expression(62/100)
> > > x
> > expression(62/100)
> > > as.character(x)
> > [1] "62/100"
> >
> > and as.numeric wouldn't know dashes about strings that contain
> > slashes...
> >
> > eval(x) might be closer to the mark.
>
> Thanks, Peter. I did not know about the existence of the command
> 'eval', in spite of the fact of having searched the Internet for a
> solution for my problem.
>
> 'eval' seems not working in connection with Ryacas:
>
> > library(Ryacas)
> > x <- yacas("2/3*5/7")
> > x
> expression(10/21)
> > eval(x)
> expression(10/21)
> >
>
> To work, one has to do the following:
> > eval(x)
> expression(10/21)
> > eval(as.expression(x))
> [1] 0.4761905

That's because yacas returns an object of class "yacas", not an expression.
See documentation in ?yacas and try this:

> eval(yacas("1/2")[[1]])
[1] 0.5