Casting a Date matrix?
问题 I have a data frame containing the following fields: a, b, c. a and b are identifiers and c is a Date. Not all identifier combinations have a date. There are some duplicate (a,b)s in the data. I only need the last c. I want to create a table where the levels of a form the rows and the levels of b form the columns. If there is a c matching the levels of a and b, it should end up in the corresponding cell (t[a,b] = c). (I want to cluster the events with the table as a basis for a distance