[hidden email] wrote:

> I am just curious. Every once and a while I see an attribute attached to an object called "assign". What meaning does this have? For example:

>

> dist ~ speed, data=cars

>

> forms a matrix like:

>

> num [1:50, 1:2] 1 1 1 1 1 1 1 1 1 1 ...

> - attr(*, "dimnames")=List of 2

> ..$ : chr [1:50] "1" "2" "3" "4" ...

> ..$ : chr [1:2] "(Intercept)" "speed"

> - attr(*, "assign")= int [1:2] 0 1

>

> The "dimnames" attribute is fairly self-explanatory. I just am not sure what the "assign" attribute means.

It has to do with the mapping between terms of the formula and columns

of the design matrix:

> str(model.matrix(Ozone~factor(Month)+Wind,data=airquality))

num [1:116, 1:6] 1 1 1 1 1 1 1 1 1 1 ...

- attr(*, "dimnames")=List of 2

..$ : chr [1:116] "1" "2" "3" "4" ...

..$ : chr [1:6] "(Intercept)" "factor(Month)6" "factor(Month)7"

"factor(Month)8" ...

- attr(*, "assign")= int [1:6] 0 1 1 1 1 2

- attr(*, "contrasts")=List of 1

..$ factor(Month): chr "contr.treatment"

I.e. columns 2:5 belong to the first non-intercept term, factor(Month).

(Notice that it is implicitly assumed that you have the corresponding

terms() output to hand, likewise for the "contrasts" attribute.)

--

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(*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918

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