[R] Nominal variables in SVM?

[Erik Iverson]

Noah, depending on what function you use, it might do this automatically for you if you give the function a formula containing a factor.  Otherwise, see ?model.matrix.  

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Noah Silverman
Sent: Wednesday, August 12, 2009 3:59 PM
Cc: r help
Subject: Re: [R] Nominal variables in SVM?

That makes sense.

I my data is already nominal, I need to "expand" a single column into 
several binary ones.

Is there an easy function to do this in R, or do I need to create 
something from scratch?  (If I have to create my own, any suggestions?)

Thanks!

-N

On 8/12/09 1:55 PM, Steve Lianoglou wrote:
> Hi,
>
> On Aug 12, 2009, at 2:53 PM, Noah Silverman wrote:
>
>> Hi,
>>
>> The answers to my previous question about nominal variables has lead 
>> me to a more important question.
>>
>> What is the "best practice" way to feed nominal variable to an SVM.
>>
>> For example:
>> color = ("red, "blue", "green")
>>
>> I could translate that into an index so I wind up with
>> color= (1,2,3)
>>
>> But my concern is that the SVM will now think that the values are 
>> numeric in "range" and not discrete conditions.
>>
>> Another thought would be to create 3 binary variables from the single 
>> color variable, so I have:
>>
>> red = (0,1)
>> blue = (0,1)
>> green = (0,1)
>>
>> A example fed to the SVM would have one positive and two negative 
>> values to indicate the color value:
>> i.e. for a blue example:
>> red = 0, blue =1 , green = 0
>
> Do it this way.
>
> So, imagine if the features for your examples were color and height, 
> your "feature matrix" for N examples would be N x 4
>
> 0,1,0,15  # blue object, height 15
> 1,0,0,10  # red object, height 10
> 0,0,1,5 # green object, height 5
> ...
>
> -steve
>
> -- 
> Steve Lianoglou
> Graduate Student: Computational Systems Biology
>   |  Memorial Sloan-Kettering Cancer Center
>   |  Weill Medical College of Cornell University
> Contact Info: http://cbio.mskcc.org/~lianos/contact
>

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