You can use function ifelse
:
x <- data.frame(medv=rnorm(10, mean=30), rm=rnorm(10, mean=7))
x$CATMEDV <- ifelse(x$medv>30.0 & x$rm>7, 1, 0)
x
medv rm CATMEDV
1 30.59482 8.139964 1
2 29.64808 6.565482 0
3 30.99625 5.875143 0
4 31.46609 7.848523 1
5 29.65703 7.875010 0
6 29.47165 7.023216 0
7 28.26122 8.046844 0
8 31.62395 6.007356 0
9 32.10948 7.423388 1
10 29.53155 6.098616 0