What is "assign" attribute?

[rkevinburton]

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.

Thank you.

Kevin

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[Peter Dalgaard]

[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.)
--
    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
  (*) \(*) -- University of Copenhagen   Denmark      Ph:  ([phone]
~~~~~~~~~~ - ([hidden email])              FAX: [phone]

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[rkevinburton]

I am sorry I still don't understand.

In the example you give the 'assign' vecotr is a vector of length 6 and there are indeed 6 columns in the data frame. But the formula only has two variables namely 'Month' and 'Wind'. Where do the values if the 'assign' vector come from? I see '0 1 1 1 1 2'. What is '1' an index to? '2'? Maybe I am get confused with what the term 'factors' does to the formula.

Thanks agiain for your help.

Kevin

---- Peter Dalgaard <[hidden email]> wrote:

> [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.)
> --
>     O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>    c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>   (*) \(*) -- University of Copenhagen   Denmark      Ph:  ([phone]
> ~~~~~~~~~~ - ([hidden email])              FAX: [phone]

______________________________________________
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