[R] (newbie) Accessing the pieces of a 'by' object

[Gabor Grothendieck]

Try this:

iris.by <- do.call("rbind", by(iris[,-5], iris[,5,drop=FALSE],colSums))
do.call("rbind", iris.by)

On 3/7/06, Vivek Satsangi <vivek.satsangi at gmail.com> wrote:
> Folks,
> I know that I can do the following using a loop. That's been a lot
> easier for me to write and understand. But I am trying to force myself
> to use more vectorized / matrixed code so that eventually I will
> become a better R programmer.
>
> I have a dataframe that has some values by Year, Quarter and Ranking.
> The variable of interest is the return (F3MRet), to be weighted
> averaged within the year, quarter and ranking. At the end, we want to
> end up with a table like this:
> year  quarter ranking1 ranking2 ... ranking10
> 1987 1 1.33 1.45 ... 1.99
> 1987 2 6.45 3.22 ... 8.33
> .
> .
> 2005 4 2.22 3.33 ... 1.22
>
> The dataset is too large to post and I can't come up with a small
> working example very easily.
>
> I tried the Reshape() package and also the aggregate and reshape
> functions. Those don't work too well becuase of the need to pass
> weighted.mean a weights vector. I tried the by() function, but now I
> don't know how to coerce the returned object into a matrix so that I
> can reshape it.
>
> > fvs_weighted.mean <- function(y) weighted.mean(y$F3MRet, y$IndexWeight, na.rm=T);
> > tmp_byRet <- by(dfReturns,
>     list(dfReturns$Quarter,dfReturns$Year,dfReturns$Ranking),
>     fvs_weighted.mean);
>
> And various other ways to get the tmp_byRet object into a matrix were
> tried, eg. unlist(), a loop like this:
> dfRet <- data.frame(tmp_byRet);
> for(i in 1:dim(dfRet)[2]){
>       dfRet[ ,i] <- as.vector(dfRet[ ,i]);
> }
> In each case, I got some error or the other.
>
> So, please help me get unstuck. How can I get the tmp_byRet() object
> into a matrix or a dataframe?
>
> --
> -- Vivek Satsangi
> Rochester, NY USA
> "No amount of sophistication is going to allay the fact that all your
> knowledge is about the past and all your decisions are about the
> future." -- Ian Wilson
>
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