R devel - Apr 2016 - boxplot with formula involving two-factor levels

Hi,

I noticed two seemingly equivalent call to boxplot will give different plots (in
the way how the combined factor levels are arranged on the x-axis):

x = factor(rep(c("a", "b", "c"), each=2));
y = rep(factor(c("one", "two")), each=3);
r = 3;
n = r * 6;
x = rep(x, 3);
y = rep(y, 3);
z = rnorm(n);

par(mfrow=c(2,1));

## The following two seeming equivalent calls to boxplot give different results
boxplot(z~x:y);

f = x:y;
boxplot(z~f);

This is puzzling to me. Is this normal? 

Thanks!

Best,
Yanming
	[[alternative HTML version deleted]]

> On 29 Apr 2016, at 23:54 , diy at stat.oregonstate.edu wrote:
> 
> Hi,
> 
> I noticed two seemingly equivalent call to boxplot will give different
plots (in the way how the combined factor levels are arranged on the x-axis):
> 
> x = factor(rep(c("a", "b", "c"), each=2));
> y = rep(factor(c("one", "two")), each=3);
> r = 3;
> n = r * 6;
> x = rep(x, 3);
> y = rep(y, 3);
> z = rnorm(n);
> 
> par(mfrow=c(2,1));
> 
> ## The following two seeming equivalent calls to boxplot give different
results
> boxplot(z~x:y);
> 
> f = x:y;
> boxplot(z~f);
> 
> This is puzzling to me. Is this normal? 
Normal, but a little odd. The root cause is the difference between x:y and
interaction(x,y), documented on the help page for the latter.

-pd

> Thanks!
> 
> Best,
> Yanming
> 	[[alternative HTML version deleted]]
> 
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