[R] filtering lines in a table

Douglas Bates bates at stat.wisc.edu
Sat Dec 9 22:34:08 CET 2000

Alexandre Fayolle <Alexandre.Fayolle at free.fr> writes:

> I realize this is probably a dumb question, but I think there must be an
> easy way to do this with R and I was not able to find it in the doc. 
> I have a table (read from a file with read.table), with two factor
> columns, table$A and table$B, and a numerical column, table$X. I wish to
> make a boxplot of  (table$X ~ table$A) for a given value of B. Is there
> a way to express this with a formula, or is it necessary to manually
> extract the right lines in the table? In that case, what is the best way
> to do so?

The "subset" argument allows you to specify an expression that
determines the subset of interest.  You would use something like

 boxplot( X ~ A, data = table, subset = B =="Bvalue" )

By the way, it is not a good idea to call the data "table" because
there is a function called table that may need to be used in boxplot.
R is smart about this and looks specifically for a function when a
function is needed but it is still better to avoid name conflicts if
you can.
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