[R] Extract information from S4 object

[Jay]

The function prediction() returns this:

Formal class 'performance' [package "ROCR"] with 6 slots
  ..@ x.name      : chr "Cutoff"
  ..@ y.name      : chr "Accuracy"
  ..@ alpha.name  : chr "none"
  ..@ x.values    :List of 1
  .. ..$ : Named num [1:89933] Inf 2.23 2.22 2.17 2.16 ...
  .. .. ..- attr(*, "names")= chr [1:89933] "" "36477" "56800"
"41667" ...
  ..@ y.values    :List of 1
  .. ..$ : num [1:89933] 0.5 0.5 0.5 0.5 0.5 ...
  ..@ alpha.values: list()

Now, since I want to match each prediction with its original case, I
need to extract the names, i.e. the information in "- attr(*,
"names")= chr [1:89933] "" "36477" "56800" "41667" ..." so I can use
it with a simple datafile[names,] query.

How do I get these names in plain number formats?