R help - Apr 2011 - Help in kmeans

Hi All,

  I was using the following command for performing kmeans for Iris dataset.

Kmeans_model<-kmeans(dataFrame[,c(1,2,3,4)],centers=3)

This was giving proper results for me. But, in my application we generate
the R commands dynamically and there was a requirement that the column names
will be sent instead of column indices to the R commands.Hence, to
incorporate this, i tried using the R commands in the following way.

kmeans_model<-kmeans((SepalLength+SepalWidth+PetalLength+PetalWidth),centers=3)

or

kmeans_model<-kmeans(as.matrix(SepalLength,SepalWidth,PetalLength,PetalWidth),centers=3)

In both the ways, we found that the results are different from what we saw
with the first command (with column indices).

can you please let  us know what is going wrong here.If so, can you please
let us know how the column names can be used in kmeans to obtain the correct
results?

Many thanks,
Raji 

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I'm not going to comment on column names, but this is just to make you 
aware that the results of k-means depend on random initialisation.

This means that it is possible that you get different results if you run 
it several times. It basically gives you a local optimum and there may be 
more than one of these.
Use set.seed to see whether this explains your problem.

Best regards,
Christian

On Wed, 6 Apr 2011, Raji wrote:
> Hi All,
>
>  I was using the following command for performing kmeans for Iris dataset.
>
> Kmeans_model<-kmeans(dataFrame[,c(1,2,3,4)],centers=3)
>
> This was giving proper results for me. But, in my application we generate
> the R commands dynamically and there was a requirement that the column
names
> will be sent instead of column indices to the R commands.Hence, to
> incorporate this, i tried using the R commands in the following way.
>
>
kmeans_model<-kmeans((SepalLength+SepalWidth+PetalLength+PetalWidth),centers=3)
>
> or
>
>
kmeans_model<-kmeans(as.matrix(SepalLength,SepalWidth,PetalLength,PetalWidth),centers=3)
>
> In both the ways, we found that the results are different from what we saw
> with the first command (with column indices).
>
> can you please let  us know what is going wrong here.If so, can you please
> let us know how the column names can be used in kmeans to obtain the
correct
> results?
>
> Many thanks,
> Raji
>
> --
> View this message in context:
http://r.789695.n4.nabble.com/Help-in-kmeans-tp3430433p3430433.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
*** --- ***
Christian Hennig
University College London, Department of Statistical Science
Gower St., London WC1E 6BT, phone +44 207 679 1698
chrish at stats.ucl.ac.uk, www.homepages.ucl.ac.uk/~ucakche

Hi All,

 For executing kmeans for Iris, we found that there were 2 different ways.

dataFrame <- read.csv("c:/Iris.csv",header=T)

1. kmeans_model<-kmeans(dataFrame[1:5],size=3)
   *This gave an error as it had Species which is a String column as one of
the inputs*

2.attach(dataFrame)
 
kmeans_model<-kmeans(cbind(SepalLength,SepalWidth,PetalLength,PetalWidth,Species),3)

* But this command worked and gave output.*

Does this mean that kmeans can accept String inputs also?

Can you please let me know how the second command works? 

Thanks in advance.

Regards,
Raji

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