[R] removing factor level represented by less than x rows

[Frank E Harrell Jr]

Mikkel Grum wrote:
> In a number of different situations I'm trying to
> remove factor levels that are represented by less than
> a certain number of rows, e.g. if I had the dataset aa
> below and wanted to remove the species that are
> represented in less than 2 rows:
> 
> data(iris)
> aa <- iris[1:101,]
> 
> In this case, since I can see that the species
> virginica only has one row, I can write:
> 
> table(aa$Species)
> setosa versicolor  virginica 
>         50         50          1 
> 
> aa[aa$Species != "virginica", ]
> 
> but:
> 
> aa[aa$Species == names(table(aa$Species)> 2),]
> 
> does not work.
> 
> This must be a fairly common task with a straight
> forward solution that I can't see. Any ideas?
> 
> Best wishes,
> Mikkel

library(Hmisc)
?combine.levels

This doesn't remove levels but combines infrequent ones though.

Frank

-- 
Frank E Harrell Jr   Professor and Chair           School of Medicine
                      Department of Biostatistics   Vanderbilt University