Symmetric matrix

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try the following

a <- matrix(rnorm(36), 6)
ind <- lower.tri(a)
a[ind] <- t(a)[ind]
a


I hope it helps.

Best,
Dimitris

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?

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Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus Medical Center

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Dear Megh,
Try this:

a = matrix(rnorm(36), 6)
a[upper.tri(a)]<-a[lower.tri(a)]
a

  
HTH,

  

If you look carefully, you'll see that it doesn't work! Dimitris had the 
better idea.

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  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
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"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

>> 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?