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
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