R help - Jan 2005 - Subsetting a data frame by a factor, using the level that occurs the most times

I think that title makes sense... I hope it does...

I have a data frame, one of the columns of which is a factor.  I want
the rows of data that correspond to the level in that factor which
occurs the most times.  

I can get a list by doing:

by(data,data$pattern,subset)

And go through each element of the list counting the rows, to find the
maximum....

BUT I can't help thinking there's a more elegant way of doing this....

The second part is figuring out the rows which have the maximum number
of consecutive patterns which are the same... Now that I would love some
help with... :-)

Thanks
Mick

newdata <- subset(mydata, mydata$myfact == 
names(which.max(table(mydata$myfact))))

michael watson (IAH-C) wrote:
> I think that title makes sense... I hope it does...
> 
> I have a data frame, one of the columns of which is a factor.  I want
> the rows of data that correspond to the level in that factor which
> occurs the most times.  
> 
> I can get a list by doing:
> 
> by(data,data$pattern,subset)
> 
> And go through each element of the list counting the rows, to find the
> maximum....
> 
> BUT I can't help thinking there's a more elegant way of doing
this....
> 
> The second part is figuring out the rows which have the maximum number
> of consecutive patterns which are the same... Now that I would love some
> help with... :-)
> 
> Thanks
> Mick
> 
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On Jan 20, 2005, at 8:57 AM, michael watson ((IAH-C)) wrote:
> I think that title makes sense... I hope it does...
>
> I have a data frame, one of the columns of which is a factor.  I want
> the rows of data that correspond to the level in that factor which
> occurs the most times.
>
> I can get a list by doing:
>
> by(data,data$pattern,subset)
>
see ?split.

data.split <- split(data,data$pattern)

> And go through each element of the list counting the rows, to find the
> maximum....
>
sort(sapply(data.split,nrow))
> BUT I can't help thinking there's a more elegant way of doing
this....
We'll see what the other responses are....

Sean

michael watson (IAH-C) wrote:
> I think that title makes sense... I hope it does...
> 
> I have a data frame, one of the columns of which is a factor.  I want
> the rows of data that correspond to the level in that factor which
> occurs the most times.  
So first you want to determine the mode (in the sense of the most 
frequently occuring value) of the factor.   One way to do this is

names(which.max(table(fac)))

Use this comparison for the subset as

subset(data, pattern == names(which.max(table(pattern))))

> From: Douglas Bates
> 
> michael watson (IAH-C) wrote:
> > I think that title makes sense... I hope it does...
> > 
> > I have a data frame, one of the columns of which is a 
> factor.  I want
> > the rows of data that correspond to the level in that factor which
> > occurs the most times.  
> 
> So first you want to determine the mode (in the sense of the most 
> frequently occuring value) of the factor.   One way to do this is
> 
> names(which.max(table(fac)))
> 
> Use this comparison for the subset as
> 
> subset(data, pattern == names(which.max(table(pattern))))
Just be careful that if there are ties (i.e., more than one level having the
max) which.max() will randomly pick one of them.  That may or may not be
what's desired.  If that is a possibility, Mick will need to think what he
wants in such cases.

Andy

 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
> 
>

> From: Douglas Bates
> 
> Liaw, Andy wrote:
> >>From: Douglas Bates
> >>
> >>michael watson (IAH-C) wrote:
> >>
> >>>I think that title makes sense... I hope it does...
> >>>
> >>>I have a data frame, one of the columns of which is a 
> >>
> >>factor.  I want
> >>
> >>>the rows of data that correspond to the level in that factor
which
> >>>occurs the most times.  
> >>
> >>So first you want to determine the mode (in the sense of the most 
> >>frequently occuring value) of the factor.   One way to do this is
> >>
> >>names(which.max(table(fac)))
> >>
> >>Use this comparison for the subset as
> >>
> >>subset(data, pattern == names(which.max(table(pattern))))
> > 
> > 
> > Just be careful that if there are ties (i.e., more than one 
> level having the
> > max) which.max() will randomly pick one of them.  That may 
> or may not be
> > what's desired.  If that is a possibility, Mick will need 
> to think what he
> > wants in such cases.
> 
> According to the documentation it picks the first one.  Also, that's 
> what Martin Maechler told me and he wrote the code so I trust him on 
> that.  I figure that if you have to trust someone to be 
> meticulous and 
> precise then a German-speaking Swiss is a good choice.
My apologies!  I got it mixed up with max.col, which does the tie-breaking. 

Andy