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MathGroup Archive 2011

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Re: check if a square matrix is diagonal

  • To: mathgroup at smc.vnet.net
  • Subject: [mg115329] Re: check if a square matrix is diagonal
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Fri, 7 Jan 2011 04:13:36 -0500 (EST)

m = DiagonalMatrix[Range[3]]

{{1, 0, 0}, {0, 2, 0}, {0, 0, 3}}

Note that Dimensions[m] is a list not a number

d = Dimensions[m]

{3, 3}

Select a part

d = Dimensions[m][[1]]

3

You can simplify your Table build

Table[Table[0, {i, 1, d}], {i, 1, d}] == 
 Table[0, {i, 1, d}, {i, 1, d}] ==
 Table[0, {d}, {d}]

True

An If statement is not needed if you are just going to return the boolean

You should also use SetDelayed ( := ) rather than Set ( = )

Clear[checkIfDiagonalMatrix]

checkIfDiagonalMatrix[m_] := Module[{d, mtemp},
  d = Dimensions[m][[1]];
  mtemp = DiagonalMatrix[Diagonal[m]] - m;
  mtemp == Table[0, {d}, {d}]]

checkIfDiagonalMatrix[m]

True


Bob Hanlon

---- benyaya <xiaochu 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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