[R] design matrix construction question

[Ben Bolker]

  with the following simple data frame
dd = structure(list(z = structure(c(1L, 1L, 1L, 1L, 2L, 2L, 2L, 2L
), .Label = c("a", "b"), class = "factor"), x = c(0.3, 0.2, 0.1,
0, 0, 0, 0.2, 0.3)), .Names = c("z", "x"), row.names = c(NA,
-8L), class = "data.frame")

  I would like know if it's possible to use model.matrix()
to construct the following design matrix in some sensible way:

  za zb
1  1  0
2  1  0
3  1  0
4  0  0
5  0  0
6  0  0
7  0  1
8  0  1

  In other words, I want column 1 to be (z=="a" & x>0) and
column 2 to be (z=="b" & x>0).  I can construct this matrix
using

sweep(model.matrix(~z-1,dd),1,dd$x>0,"*")

and then stick it into lm.fit -- but is there a more
elegant way to do this in general?  I haven't found a formula combining
(z-1) and I(x>0) that works, although I can imagine there is one.

 thanks
  Ben Bolker


-- 
Ben Bolker
Associate professor, Biology Dep't, Univ. of Florida
bolker at ufl.edu / www.zoology.ufl.edu/bolker
GPG key: www.zoology.ufl.edu/bolker/benbolker-publickey.asc

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