- To calculate a sum of squares,
`sum(x^2)`or`sum(x*x)`is much faster than`t(x)%*%x.` - rowSums() and colSums() are fast ways to sum over rows or columns.
- Try to use the standard arithmetic operations, +,-,*,/ whenever
possible to do matrix-vector operations. For example, if D is a diagonal
matrix, then can be done in R as
`C/diagD`where`diagD`is a vector of the diagonal elements of D. This is much faster than doing matrix multiplication using`%*%.`

Chris Paciorek 2012-01-21