Example: From 'The R Book' 2nd ed. ~ 2.3.2 Generating Factor Levels...

[1] "# From 'The R Book' 2nd ed. ~ 2.3.2 Generating factor levels..."
   Temp   Soft M.user Brand
1   Low   Hard      N     X
2   Low   Hard      N     M
3  High   Hard      N     X
4  High   Hard      N     M
5   Low   Hard      Y     X
6   Low   Hard      Y     M
7  High   Hard      Y     X
8  High   Hard      Y     M
9   Low Medium      N     X
10  Low Medium      N     M
11 High Medium      N     X
12 High Medium      N     M
13  Low Medium      Y     X
14  Low Medium      Y     M
15 High Medium      Y     X
16 High Medium      Y     M
17  Low   Soft      N     X
18  Low   Soft      N     M
19 High   Soft      N     X
20 High   Soft      N     M
21  Low   Soft      Y     X
22  Low   Soft      Y     M
23 High   Soft      Y     X
24 High   Soft      Y     M