# [R] simplify code for dummy coding of factors

Richard M. Heiberger rmh at temple.edu
Wed Dec 31 01:16:02 CET 2014

```I like this very simple version.  Note that you don't need as.data.frame().

model.matrix(Freq ~ Hair + Eye, data=haireye,
contrasts.arg=list(Hair=diag(4), Eye=diag(4)))

On Tue, Dec 30, 2014 at 6:05 PM, Michael Friendly <friendly at yorku.ca> wrote:
> In a manuscript, I have the following code to illustrate dummy coding of two
> factors in a contingency table.
>
> It works, but is surely obscured by the method I used, involving outer() to
> find equalities and 0+outer()
> to convert to numeric.  Can someone help simplify this code to be more
> comprehensible and give the
> *same* result? I'd prefer a solution that uses base R.
>
> haireye <- margin.table(HairEyeColor, 1:2)
>
> haireye.df <- as.data.frame(haireye)
> dummy.hair <-  0+outer(haireye.df\$Hair, levels(haireye.df\$Hair), `==`)
> colnames(dummy.hair)  <- paste0('h', 1:4)
> dummy.eye <-  0+outer(haireye.df\$Eye, levels(haireye.df\$Eye), `==`)
> colnames(dummy.eye)  <- paste0('e', 1:4)
>
> haireye.df <- data.frame(haireye.df, dummy.hair, dummy.eye)
> haireye.df
>
>> haireye.df
>     Hair   Eye Freq h1 h2 h3 h4 e1 e2 e3 e4
> 1  Black Brown   68  1  0  0  0  1  0  0  0
> 2  Brown Brown  119  0  1  0  0  1  0  0  0
> 3    Red Brown   26  0  0  1  0  1  0  0  0
> 4  Blond Brown    7  0  0  0  1  1  0  0  0
> 5  Black  Blue   20  1  0  0  0  0  1  0  0
> 6  Brown  Blue   84  0  1  0  0  0  1  0  0
> 7    Red  Blue   17  0  0  1  0  0  1  0  0
> 8  Blond  Blue   94  0  0  0  1  0  1  0  0
> 9  Black Hazel   15  1  0  0  0  0  0  1  0
> 10 Brown Hazel   54  0  1  0  0  0  0  1  0
> 11   Red Hazel   14  0  0  1  0  0  0  1  0
> 12 Blond Hazel   10  0  0  0  1  0  0  1  0
> 13 Black Green    5  1  0  0  0  0  0  0  1
> 14 Brown Green   29  0  1  0  0  0  0  0  1
> 15   Red Green   14  0  0  1  0  0  0  0  1
> 16 Blond Green   16  0  0  0  1  0  0  0  1
>>
>
> --
> Michael Friendly     Email: friendly AT yorku DOT ca
> Professor, Psychology Dept. & Chair, Quantitative Methods
> York University      Voice: 416 736-2100 x66249 Fax: 416 736-5814
> 4700 Keele Street    Web:http://www.datavis.ca
> Toronto, ONT  M3J 1P3 CANADA
>
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