R S4类/对象

Question

我有一个S4类对象：

    class(pred)
      [1] "prediction"
      attr(,"package")
      [1] "ROCR"

和pred有以下列

slotNames(pred)
 #[1] "predictions" "labels"      "cutoffs"     "fp"          "tp"          "tn"          "fn"         
 #[8] "n.pos"       "n.neg"       "n.pos.pred"  "n.neg.pred" 

pred有大约10K行。现在我想得到有预测的记录＆gt; 0.5

我如何将其分配？