使用公式（不包括截距）调用model.matrix（）之后，考虑缺少水平的因素

Question

我正在尝试使用model.matrix()将因数扩展为一组虚拟变量。特别是，我不需要拦截项。但是，当我有多个因素时，其中一个因素会错过水平。例如，

df <- data.frame(cat1 = c("A", "A", "B"), n1 = c(3, 2, 1), cat2 = c("Y", "Y", "N"))
> model.matrix(~cat1 + n1, df)
  (Intercept) cat1B n1
1           1     0  3
2           1     0  2
3           1     1  1
attr(,"assign")
[1] 0 1 2
attr(,"contrasts")
attr(,"contrasts")$cat1
[1] "contr.treatment"

> model.matrix(~cat1 + n1 + 0, df)
  cat1A cat1B n1
1     1     0  3
2     1     0  2
3     0     1  1
attr(,"assign")
[1] 1 1 2
attr(,"contrasts")
attr(,"contrasts")$cat1
[1] "contr.treatment"

但是以下结果使我感到困惑。

> model.matrix(~cat1 + n1 + cat2 + 0, df)
  cat1A cat1B n1 cat2Y
1     1     0  3     1
2     1     0  2     1
3     0     1  1     0
attr(,"assign")
[1] 1 1 2 3
attr(,"contrasts")
attr(,"contrasts")$cat1
[1] "contr.treatment"

attr(,"contrasts")$cat2
[1] "contr.treatment"

为什么没有列cat2N？