[R] getting data frame rows out of a by object

[Julian Taylor]

Ed L Cashin wrote:
> 
> Hi.  I can quickly create a by object that selects rows from a data
> frame.  After that, though, I don't know how to merge the rows back
> into a data frame that I can use.
> 
> Here is an example where there is a data frame with three columns, a,
> b, and c.  I update it so that there are two rows for each combination
> of a and b.  I use by to select the subgroups of rows that share the
> same a and b values, and then I take only the row with the highest c
> value.
> 
> I can see little data frames inside the by object, but I can't get a
> new data frame containing only the rows with the highest c value.  In
> the example below most of the by object is NULL, but it contains data
> frames with the rows I'm interested in selecting.
> 
>   > d <- data.frame(a=1:4,b=4:1,c=31:34)
>   > d
>     a b  c
>   1 1 4 31
>   2 2 3 32
>   3 3 2 33
>   4 4 1 34
>   > b <- by(d, list(d$a,d$b,d$c), function(x) x)
>   > d <- data.frame(a=1:4,b=4:1,c=31:34)
>   > d <- rbind(d, data.frame(a=1:4,b=4:1,c=41:44))
>   > b <- by(d, list(d$a,d$b,d$c), function(x) x[x$c == max(x$c),])

You are better off using other tools to give you the right subsets. Try

d <- do.call("rbind", lapply(split(d, factor(paste(d$a, d$b, sep =
""))), 
                            function(el) el[el$c == max(el$c), ]))

HTH,
Jules

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Julian Taylor			phone: +61 8 8303 6751
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University of Adelaide/SARDI    email: julian.taylor at adelaide.edu.au
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