st: implementing an equivalent of egen mean() in Mata.

[Christophe Kolodziejczyk <ck.statalist@gmail.com>]

Dear Stata list members

I want to implement a mata function wich computes group averages over
the columns of a matrix. In other words I want to implement in Mata an
equivalent of egen with mean().
I have not been able to find such a function in the mata manual.

I have written the following function by using panelsetup(). But
because of the loop over the observations it tends to be slow with
large samples. Any idea how to make it work faster? Any other
suggestions to the code is also welcome.
Thanks for your help
Christophe

real matrix meani(real vector id, real matrix x)
{

    real vector info, xi, meanix
    real scalar ninfo, i

    info=panelsetup(id,1)
    ninfo=rows(info)

    meanix=J(rows(id),cols(x),.)

    for (i=1;i<=ninfo;i++) {
        xi = panelsubmatrix(x, i, info)
        meanix[info[i,1]::info[i,2],.]=J(rows(xi),1,mean(xi))
        }

    return(meanix)
}
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