R help - Apr 2011 - Turning a vector into a matrix based on a matrix of vector indices

Hello

I have a matrix (though it could also be a ragged array) called items.used, that
has the item numbers of questions administered to a set of respondents. I also
have a vector of item parameters called b[]. I am looking for an elegant way to
create a matrix b.mat in which each element is the b[x] item parameter, where x
is the item number in items.used. When the number of administered items was
fixed for all respondents, I would do th following:

ni <- 30
iused.vec <- as.vector(t(items.used[,1:ni]))
b.vec <- b[iused.vec]
b.mat <- matrix(b.vec,ncol=ni)

This, however, would not work if the number of items administered to a
respondent varies from respondent to respondent.

Any help would be greatly appreciated.

Barth



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