[R] design matrix construction question

[Ken Knoblauch]

> On Nov 2, 2009, at 10:40 PM, Ben Bolker wrote:
> >  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

How about something like this?

dd$xf <- (dd$x > 0) + 0
model.matrix(~ z:xf - 1, dd)
  za:xf zb:xf
1     1     0
2     1     0
3     1     0
4     0     0
5     0     0
6     0     0
7     0     1
8     0     1
attr(,"assign")
[1] 1 1
attr(,"contrasts")
attr(,"contrasts")$z
[1] "contr.treatment"

> > thanks
> >  Ben Bolker
> >
> >

-- 
Ken Knoblauch
Inserm U846
Stem-cell and Brain Research Institute
Department of Integrative Neurosciences
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