R devel - Feb 2012 - Unexpected behavior in factor level ordering

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?




-- 
Paul E. Johnson
Professor, Political Science
1541 Lilac Lane, Room 504
University of Kansas

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

>
>
>