Duncan Murdoch wrote: > On 12/12/2008 3:41 AM, Wacek Kusnierczyk wrote: >> >> but this 'general rule' is not really adhered to in r! one example >> already discussed here at length is subset: >> >> subset(data.frame(...), select=a) >> >> what will be selected? column named "a", or columns named by the >> components of the vector a? this is an example of how you can't say >> what an expression means in a context-independent manner. > > From which you might conclude that I don't like the design of subset, > and you'd be right. However, I don't think this is a counterexample > to my general rule. In the subset function, the select argument is > treated as an unevaluated expression, and then there are rules about > what to do with it. (I.e. try to look up name `a` in the data frame, > if that fails, ...) > > For the requested behaviour to similarly fall within the general rule, > we'd have to treat all indices to all kinds of things (vectors, > matrices, dataframes, etc.) as unevaluated expressions, with special > handling for the particular symbol `end`. But Mike wanted an End > function, so presumably he wanted the old behaviour of indexing, but > to have a function whose value depended on where it was called from. > We do have those (e.g. the functions for examining the stack that Mike > wanted to make use of), and they're needed for debugging and a few > special cases, but as a general rule they should be avoided. > > > and this is >> an ubiquitous problem in r. > > I don't think so. >
i'd think that neither the 'evaluate' nor the 'deparse' approaches to establishing the values of arguments are particularly rare in r. vQ ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
