[R] Symmetric matrix

[Martin Maechler]

>>>>> "DR" == Dimitris Rizopoulos <d.rizopoulos at erasmusmc.nl>
>>>>>     on Sun, 21 Sep 2008 19:58:44 +0200 writes:

    DR> try the following
    DR> a <- matrix(rnorm(36), 6)
    DR> ind <- lower.tri(a)
    DR> a[ind] <- t(a)[ind]
    DR> a

Yes, indeed, it needs the t(.) trick.

Note that 'Matrix' package has a function  forceSymmetric(.) to
do this for you (faster, using C code):

 A <- forceSymmetric(Matrix(rnorm(36), 6))

is all you'd need {if can afford to trash half of the random
       	     	  numbers generated}

Martin Maechler, ETH Zurich


    DR> I hope it helps.

    DR> Best,
    DR> Dimitris


    DR> Megh Dal wrote:
    >> I have following matrix :
    >> 
    >> a = matrix(rnorm(36), 6)
    >> 
    >> Now I want to replace the lower-triangular elements with it's upper-triangular elements. That is I want to make a symmetric matrix from a. I have tried with lower.tri() and upper.tri() function, but got desired result. Can anyone please tell me how to do that?