Dictionaries and pairs

Question

In R I was wondering if I could have a dictionary (in a sense like python) where I have a pair (i, j) as the key with a corresponding integer value. I have not

Answer 1

R matrices allow you to do this. There are both sparse and dense version. I beleive the tm-package uses a variation on sparse matrices to form its implementation of dictionaries. This shows how to extract the [i,j] elements of matrix M where [i,j] is represented as a a two-column matrix.

M<- matrix(1:20, 5, 5)
ij <- cbind(sample(1:5), sample(1:5) )

> ij
     [,1] [,2]
[1,]    4    4
[2,]    1    2
[3,]    5    3
[4,]    3    1
[5,]    2    5
> M[ij]
[1] 19  6 15  3  2

@Justin also points out that you could use lists which can be indexed by position:

 L <- list( as.list(letters[1:5] ), as.list( paste(2,letters[1:5] ) ) )

> L[[1]][[2]]
[1] "b"
> L[[2]][[2]]
[1] "2 b"