R help - Sep 2009 - Computing the sum of cross products of rows of a matrix

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

Dear Stephan,
Try

crossprod(x)

HTH,
Jorge


On Sat, Sep 19, 2009 at 11:42 PM, Stephan Lindner <> wrote:
> 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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>
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