How to get statistics from a sub-set of factors?

Question

Using by I can get statistics for a desired column based on a factor column.

For instance, if I want to know the ratio of Sepal.Width/Sepal.Length per Species, in the iris dataframe, I'd go like this:

by(iris$Sepal.Width/iris$Sepal.Length, iris$Species, mean)
iris$Species: setosa
[1] 0.6842483
------------------------------------------------------------ 
iris$Species: versicolor
[1] 0.4676804
------------------------------------------------------------ 
iris$Species: virginica
[1] 0.4533956

So far so good. Now, the question is, how can I the same but for only a sub-set of levels. e.g setosa and versicolor only?

I have a complex data.frame with thousands of factors. I am playing a bit with table in order to create sub-sets of factors based on different statistics. I would like to then go back to my original data.frame and create more numbers for my desired sub-set of factors.

Thanks

Answer 1

with( droplevels( subset(iris, Species %in% c("setosa",  "versicolor") ) ), 
     by(Sepal.Width/Sepal.Length, Species, mean) )