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Re: How to construct symmetric matrix from just a one half matrix

• To: mathgroup at smc.vnet.net
• Subject: [mg110371] Re: How to construct symmetric matrix from just a one half matrix
• From: Julian <julianhaw at gmail.com>
• Date: Wed, 16 Jun 2010 05:35:39 -0400 (EDT)
• References: <hv76li$k4j$1@smc.vnet.net>

On Jun 15, 2:30 am, dragec <culino... at gmail.com> wrote:
> please help me with simple to apply function that will construct
> symmetric matrix from given just a half matrix with diagonal.
> Eg:
>
> From:
> 1  0  0  0
> 2  3  0  0
> 4  9  5  0
> 2  2  3  4
>
> To give:
>
> 1  2  4  2
> 2  3  9  2
> 4  9  5  3
> 2  2  3  4

Let's say the matrix with the zeros is called A.  A possibly clunky
way to achieve your goal is to do

f[A_?MatrixQ]:= A + Transpose[A] - DiagonalMatrix[Diagonal[A]]