Using R for Introductory Statistics : John Verzani

Multivariate data 133When a factor is printed, the levels also appear.It is important to realize that these are factors and not numbers. For example, we can’tadd factors.> x + factor(x)[1] NA NA NAWarning message:"+" not meaningful for factors in: Ops.factor(x,factor(x))Adding levels: levels=and levels()When defining a factor, the levels= argument can be used to specify the levels.> factor(x,levels=1:10) # add more levels than 3[1] 1 2 3Levels: 1 2 3 4 5 6 7 8 9 10> factor(x,levels=fac) # get NA if not in levels[1] Levels: a b cThe values of data vector being coerced into a factor should match those specified tolevels=.The levels() function can list the levels of a factor directly and allows us to change oradd to the levels.> x = factor(1:5)> x[1] 1 2 3 4 5Levels: 1 2 3 4 5> levels(x) # character vector[1] "1" "2" "3" "4" "5"The levels() function can also change the levels of a factor. As levels() changes anattribute of the data vector, like names(), it appears on the other side of the assignment.For example:> x = factor(1:3)> x[1] 1 2 3Levels: 1 2 3> levels(x) = letters [1:3]> x[1] a b cLevels: a b c> levels(x) = 1:10 # add morelevels> x[1] 1 2 3Levels: 1 2 3 4 5 6 7 8 9 10