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Dear R-helpers , I have a simple loop as follows, however, to be more efficient, I would like to use any apply functions (tapply, I suppose) But how can I do this ? I am not very clear about this. # Z is a P * Q matrix # so for each row of Z, I would like to pull out only some of the elements, and save as a separate matrix under a list # index is a vector with length smaller than Q, but the