a valid value for that component of
dimnames (x) . For a matrix or array this is either
NULL or a character vector of non-zero length equal to the
Unfortunately only for first item is actually true. The data.frame() and as.data.frame() methods try to do some conversions to make more of the items in the above list are usually true. We know data.frame is implemented as a list of columns, but the idea is the class data.frame overrides a lot of operators and should be able to maintain some useful invariants for us.