partial match gotcha

Hi

The following gotcha took me a long time to catch:

f <- function(x, main0="red", ...){
  par(col.axis=main0)
  plot(x,...)
}

f(1:10,main="title here")
f(1:10,main="title here",main0="blue")


I can't quite succinctly summarize why the second case works but the
first one
doesn't.

You need to write

f <- function(x, ..., main0="red"){
   par(col.axis=main0)
   plot(x,...)
}

See e.g. S Programming p.40 or the Draft R Language Definition (section 
'Argument matching').  Yes, it is a trap for the unwary, which is why 
knowing the exact rules is important.

Inserting print(match.call()) can help: in your first case it gives
f(x = 1:10, main0 = "title here")

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595