On 25.02.2012 19:16, Paul Johnson wrote:
Hello, Everybody:
This may not be a "bug", but for me it is an unexpected outcome. A
factor variable's levels
do not retain their ordering after the levels function is used. I
supply an example in which
a factor with values "BC" "AD" (in that order) is unintentionally
re-alphabetized by the levels
function.
To me, this is very bad behavior. Would you agree?
# Paul Johnson 2012-02-05
x<- c("AD","BC","AD","BC","AD","BC")
xf<- factor(x, levels=c("BC", "AD"), labels=c("Before Christ","After Christ"))
y<- rnorm(6)
m1<- lm (y ~ xf )
plot(y ~ xf)
abline (m1)
## Just a little problem the line does not "go through" the box
## plot in the right spot because contrasts(xf) is 0,1 but
## the plot uses xf in 1,2.
xlevels<- levels(xf)
newdf<- data.frame(xf=xlevels)
ypred<- predict(m1, newdata=newdf)
##Watch now: the plot comes out "reversed", AC before BC
plot(ypred ~ newdf$xf)
## Ah. Now I see:
levels(newdf$xf)
## Why doesnt newdf$xf respect the ordering of the levels?
Because xlevels was a character and you coerced it to a factor by
calling data.frame(xf=xlevels) on it without telling anything about the
orderiung, hence it got sorted lexicographically.
Uwe Ligges
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