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col(x, as.factor = FALSE)

dim

x

ij

j

row

# extract an off-diagonal of a matrix ma <- matrix(1:12, 3, 4) ma[row(ma) == col(ma) + 1] # create an identity 5-by-5 matrix x <- matrix(0, nrow = 5, ncol = 5) x[row(x) == col(x)] <- 1

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