[R] Lazy evaluation in function call

[Thorn]

Bert Gunter <gunter.berton <at> gene.com> writes:

> 
> Inline below.
> 
> -- No.

> f <- function(x, y = x)x+y  ## lazy evaluation enables this

> > f(2)
> [1] 4
> > f(2,3)
> [1] 5

Ok, obviously I was not very precise about what I'd like to do. Sorry. I'm 
aware of this functionality. But I'd like to use the same idea for existing 
function. Take biplot as an example. There you have an argument 
called 'choices' controlling which components you'd like to plot. The standard 
labels of the axis are the names of the two chosen components, PC1 and PC2 
say. If one wants to add the explained variance to the axis notation, one 
could provide the xlab argument:

biplot(pca-object, xlab=paste("PC1", explained.variance.by.pc1))

This works out well, and serves it purpose sufficiently. If it happens that 
you'd like to choose different components, you have to provide the choices 
argument. As a consequence you have to change your manually adjusted axis 
notation as well. Hence, I was wondering, if one could do it in a somewhat 
more automatic fashion:

biplot(pca-object, xlab=paste("PC", choices[1], explained.varaiance.by(choices
[1]))

I've to admit that this is an academic question, because at the time you call 
biplot, you know which components you want to plot. Thus, you could use the 
right values for choices and the labs. But I'm interested whether it can be 
done by telling R that it should evaluate choices[1] not at the prompt level, 
but at the moment it enters the function biplot, where choices will be known 
in any case (since it has default values).

Just to make it clear once again, I'm aware of the possibilities to get this 
specific task done. But I'm in particular interested, wheter it could be done 
using R's lazy evaluation mechanism.

BR, Thorn