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?


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().


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