converting "by" to a data.frame?

Since I don't have your by.df to test with I may not have it exactly 
right, but something along these lines should work:

byFits <- lapply(split(by.df,paste(by.df$A,by.df$B)),
                  FUN=function(data.) {
                     tmp <- coef(lm(y~x,data.))
                     data.frame(A=unique(data.$A),
                                B=unique(data.$B),
                                intercept=tmp[1],
                                slope=tmp[2])
                    })

byFitsDF <- do.call('rbind',byFits)

That's assuming I've got all the closing parantheses in the right 
places, since my email software (Eudora) doesn't do R syntax checking!

This approach can get rather slow if by.df is big, or when the 
computations in FUN are extensive (or both).

If by.df$A has mode character (as opposed to being a factor), then 
replacing A=unique(data.$A) with A=I(unique(data.$A)) might improve 
performance. You want to avoid character to factor conversions when 
using an approach like this.

-Don

Dear R-Help:

  I want to (a) subset a data.frame by several columns, (b) 
fit a model to each subset, and (c) store a vector of results from 
the fit in the columns of a data.frame.  In the past, I've used 
"for" loops do do this.  Is there a way to use "by"?

  Consider the following example:

 > byFits <- by(by.df, list(A=by.df$A, B=by.df$B),

+  function(data.)coef(lm(y~x, data.)))

 > byFits

A: A1
B: B1
  (Intercept)             x
 3.333333e-01 -1.517960e-16
------------------------------------------------------------
A: A2
B: B1
NULL
------------------------------------------------------------
A: A1
B: B2
NULL
------------------------------------------------------------
A: A2
B: B2
 (Intercept)            x
6.666667e-01 3.282015e-16

#############################
Desired output:

data.frame(A=c("A1","A2"), B=c("B1", "B2"),
.Intercept.=c(1/3, 2/3), x=c(-1.5e-16, 3.3e-16))

What's the simplest way to do this?
Thanks,
Spencer Graves

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://www.stat.math.ethz.ch/mailman/listinfo/r-help

--------------------------------------
Don MacQueen
Environmental Protection Department
Lawrence Livermore National Laboratory
Livermore, CA, USA