Hi everyone,
I would like to compute the sum of cross products of rows of a
matrix. Does anyone know how to do this without an explicit loop for
computational efficiency? Below is a code example.
Thanks!
Stephan
# I.e., if I have a matrix like this
x <- matrix(rep(c(1,2,3),3),ncol=3)
## > x
## [,1] [,2] [,3]
## [1,] 1 1 1
## [2,] 2 2 2
## [3,] 3 3 3
## and a storage matrix like this
cross <- matrix(rep(0,9),ncol=3)
## then use a loop to sum up the cross products of the rows
for (i in 1:nrow(x)){ cross <- cross+tcrossprod(x[i,]) }
## The result
##
## > cross
## [,1] [,2] [,3]
## [1,] 14 14 14
## [2,] 14 14 14
## [3,] 14 14 14
--
-----------------------
Stephan Lindner
University of Michigan
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