       Re: check if a square matrix is diagonal

• To: mathgroup at smc.vnet.net
• Subject: [mg115314] Re: check if a square matrix is diagonal
• From: "Sjoerd C. de Vries" <sjoerd.c.devries at gmail.com>
• Date: Fri, 7 Jan 2011 04:10:38 -0500 (EST)
• References: <ig3pkm\$jh5\$1@smc.vnet.net>

```d is a list,so you can't use it as a loop bound. ConstantArray can use
it as it is. Also: better use := instead of = in your definition

checkIfDiagonalMatrix[m_] :=
Module[{d, mtemp}, d = Dimensions[m];
mtemp = DiagonalMatrix[Diagonal[m]] - m;
If[mtemp == ConstantArray[0, d], True, False]]

Cheers -- Sjoerd

On Jan 6, 8:05 am, benyaya <xiao... at gmail.com> wrote:
> What I try to do is extract the diagonal, subtract if from the matrix,
> then compare the new matrix with 0.
> My code doesn't work out though, can anyone help? thanks a lot.
>
> checkIfDiagonalMatrix[m_] = Module[{d, mtemp},
>   d = Dimensions[m];
>   mtemp = DiagonalMatrix[Diagonal[m]] - m;
>   If[mtemp == Table[Table[0, {i, 1, d}], {i, 1, d}],
>    True,
>    False]
>   ]

```

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