On Sun, May 30, 2010 at 4:31 PM, Duncan Murdoch <murdoch.dun...@gmail.com> wrote: > As far as I know, there is none. It is reserved simply because it follows > the pattern of "..n". It would not be hard to make ..0 specially unreserved, > but what would be the point?
Here is the case I had in mind. The gsubfn function in the gsubfn package is like gsub except the replacement string can be a replacement function such that it passes the back references in the pattern as successive args to the function. Here it would pass two args to the function represented by (.) and (.) in the regular expression and then replace the match with the output of the function which in this case is the sum of the digits surrounded with angle brackets: > gsubfn("(.)#(.)", function(...) paste0("<", as.numeric(..1) + > as.numeric(..2), ">"), "1#2 3#4") [1] "<3> <7>" There is also an option to pass the entire match followed by the back references to the user function. In that case it would be nice for the user to be able to write the user function as follows where ..0 means the entire match. Here we replace the match with an angle bracket, the entire match, an equal sign, the sum and an end angle bracket: > gsubfn("(.)#(.)", function(..0, ...) paste0("<", ..0, "=", as.numeric(..1) + > as.numeric(..2), ">"), "1#2 3#4") [1] "<1#2=3> <3#4=7>" Note the use of ..0 to mean the entire match is in this context and how natural it is. To do the above the best would be if we could just use ..0 as an ordinary variable. Barring that we could replace all occurrences of ..0 with X..0, say, so that: function(..0, ...) list(..0, ..1) would be transformed to the function function(X..0, ...) list(X..0, ..1) and that could work now but that would depend on R not being changed in the future to issue an error when a function such as f above that uses ..0 is created. (Although not illustrated here there is also a formula notation which allows one to specify the function as a formula whose body is taken to be the formula's RHS and the args are reconstructed from the RHS free variables. In that case the ..0 notation becomes even more useful.) ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel