Hi: To mimic Sarah Goslee's reply within base R, either of these work:
crossprod(t(as.matrix(xtabs( ~ a + b)))) crossprod(t(as.matrix(table(a, b)))) HTH, Dennis On Thu, Nov 4, 2010 at 12:42 PM, cory n <corynis...@gmail.com> wrote: > Let's suppose I have userids and associated attributes... columns a and b > > a <- c(1,1,1,2,2,3,3,3,3) > b <- c("a","b","c","a","d","a", "b", "e", "f") > > so a unique list of a would be > > id <- unique(a) > > I want a matrix like this... > > [,1] [,2] [,3] > [1,] 3 1 2 > [2,] 1 2 1 > [3,] 2 1 4 > > Where element i,j is the number of items in b that id[i] and id[j] share... > > So for example, in element [1,3] of the result matrix, I want to see > 2. That is, id's 1 and 3 share two common elements in b, namely "a" > and "b". > > This is hard to articulate, so sorry for the terrible description > here. The way I have solved it is to do a double loop, looping over > every member of the id column and comparing it to every other member > of id to see how many elements of b they share. This takes forever. > > Thanks > > cn > > ______________________________________________ > 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. > [[alternative HTML version deleted]] ______________________________________________ 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.