R-beta: Including an evaluated function in text expression
Thomas Lumley
thomas at biostat.washington.edu
Fri Mar 27 20:52:06 CET 1998
On Fri, 27 Mar 1998, Matthew R. Nelson wrote:
> I would like to take advantage of R's mathematical typesetting
> capabilities in the graphical environment using text(...,
> expression(...)), however, I can't figure out how to include in the
> expression some evaluated function. For example,
>
> > x <- 1:8
> > plot(1:10, 1:10)
> > text(5, 5, expression(sum(x[i], i == 1, length(x))) # would like "8"
> # substituted for length(x) here
>
> Is there a way to replace length(x) with the returned value of the
> function?
Yes:
R>text(5,5,parse(text=paste("sum(x[i],i==1,",length(x),")",sep="")))
What we do here is construct a string
R> paste("sum(x[i],i==1,",length(x),")",sep="")
[1] "sum(x[i],i==1,8)"
containing the returned value of length(x), and then
send it to the R parser. This returns a list of calls, ie, an expression.
R> parse(text=paste("sum(x[i],i==1,",length(x),")",sep=""))
expression(sum(x[i], i == 1, 8))
which is what you need to use as the argument to text().
-thomas
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