Try this:
summary(res[res[,"class"] == "set1",])
Update:
res[row,column]
- genrally 1st value is row index, 2nd value is column index, so:
res[,"class"]
- select "class" column from res.
res[,"class"] == "set1"
- compare "class" column values with string "set1", this will give TRUE, FALSE values.
res[res[,"class"] == "set1",]
- TRUE, FALSE values define which rows to return.