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

From: Sundar Dorai-Raj <sundar.dorai-raj_at_pdf.com>
Date: Sat 09 Jul 2005 - 00:39:31 EST

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.
>

If you take a look at "table(aa$Species) > 2" you'll see your first mistake. Namely, the names are all still present. Your second mistake is to use "==" to match two names. "==" does not work like that. What you want is "%in%" instead.

I think you want the following:

keep <- levels(aa$Species)[table(aa$Species) > 2] aa <- aa[aa$Species %in% keep, ]

However, the level for "virginica" is still present in the Species variable. If you would like to drop this completely, then try

aa$Species <- aa$Species[drop = TRUE]

HTH, --sundar



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