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Alex Rivera
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I am calculating sums of matrix columns to each group, where the corresponding group values are contained in matrix columns as well. At the moment I am using a loop as follows: index <- matrix(c("A","A","B","B","B","B","A","A"),4,2) x <- matrix(1:8,4,2) for (i in 1:2) { tapply(x[,i], index[,i], sum) } At the end of the day I need the following result: 1 2 A 3 15 B 7 11 Is there a way to do this using matrix operations without a loop? On top, the real data is large (e.g. 500 x 10000), therefore it has to be fast. Thanks in advance.
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