R help - Mar 1998 - R-beta: Including an evaluated function in text expression

R-users,

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?

Thanks for any advice,

Matt
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Matthew R. Nelson
Dept. of Human Genetics         
University of Michigan              http://www-personal.umich.edu/~ticul/
4711 Medical Science II             email: ticul at umich.edu
Ann Arbor, MI  48109-0618           phone: (313) 647-3151
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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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