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# st: RE: MATA function that gives the TRUE indices of a logical object

 From Nick Cox To "'statalist@hsphsun2.harvard.edu'" Subject st: RE: MATA function that gives the TRUE indices of a logical object Date Mon, 7 Feb 2011 16:12:55 +0000

```I am not aware of anything built-in to Mata [sic; not MATA].

Here is something rough:

mata:

real matrix which(real matrix x, scalar s) {

real scalar i, j
real matrix out
out = J(0,2,.)

for(i = 1; i <= rows(x); i++) {
for (j = 1; j <= cols(x); j++) {
if (x[i,j] == s) out = out \ (i,j)
}
}

return(out)
}

end

This just does what your example does, modulo some convention about row first or column first. Testing for equality with some non-integer value is a whole heap more problematic.

Nick
n.j.cox@durham.ac.uk

r.a.nobel@umail.leidenuniv.nl

I have the following question on a function which a want to use in MATA. I think
it's best illustrated with an example. In the R-language you have the which()
function, which works as follows. Suppose you have a matrix Z:

0 0 0
1 0 0
0 1 0
9 9 1
9 9 9
9 9 9

then using which(Z==9, arr.ind = T)
finds entries with Z=9 and returns an array with coordinates as follows:

row col
4 1
5 1
6 1
4 2
5 2
6 2
5 3
6 3

Does a similar function exist in MATA?

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```