How do I manipulate/access elements of an instance of “dist” class using core R?

Question

A basic/common class in R is called \"dist\", and is a relatively efficient representation of a symmetric distance matrix. Unlike a \"matrix\" object,

Answer 1

You could do this:

d <- function(distance, selection){
  eval(parse(text = paste("as.matrix(distance)[",
               selection, "]")))
}

`d<-` <- function(distance, selection, value){
  eval(parse(text = paste("as.matrix(distance)[",
               selection, "] <- value")))
  as.dist(distance)
}

Which would allow you to do this:

 mat <- matrix(1:12, nrow=4)
 mat.d <- dist(mat)
 mat.d
        1   2   3
    2 1.7        
    3 3.5 1.7    
    4 5.2 3.5 1.7

 d(mat.d, "3, 2")
    [1] 1.7
 d(mat.d, "3, 2") <- 200
 mat.d
          1     2     3
    2   1.7            
    3   3.5 200.0      
    4   5.2   3.5   1.7

However, any changes you make to the diagonal or upper triangle are ignored. That may or may not be the right thing to do. If it isn't, you'll need to add some kind of sanity check or appropriate handling for those cases. And probably others.