repeated searching of no-missing values

Perhaps... But plyr works only on **basic** data structures, and I referred
to all **possible** data strucures (deliberately); so I stand by my
statement and note that you did not contradict it.

To me the basic structures are vectors, matrices and arrays; lists;
and data frames (that these are not mutually exclusive categories).
If you master these three basic types, you can master any more complex
structure because it must be some combination of the simpler types.
Similarly, while each plyr function only works with one of the types,
you should be able to deal with any arbitrarily complex data structure
by combining multiple functions.

This approach doesn't generalise so well with truly recursive data
structures (e.g. trees) but these are rarely encountered in their
traditional CS form in R.

Hadley

http://had.co.nz/