Re: VectorToMatrix

• To: mathgroup at smc.vnet.net
• Subject: [mg4357] Re: VectorToMatrix
• From: rubin at msu.edu (Paul A. Rubin)
• Date: Thu, 11 Jul 1996 01:02:12 -0400
• Organization: Michigan State University
• Sender: owner-wri-mathgroup at wolfram.com

```In article <4qqn5d\$49m at dragonfly.wolfram.com>,
Robert Pratt <rpratt at math.unc.edu> wrote:
->I want to define a function VectorToMatrix[list_, n_] that behaves as
->follows:
->
->VectorToMatrix[{a,b,c,d,e,f,g,h,i,j}, 4]//MatrixForm
->
->0 a b c d
->0 0 e f g
->0 0 0 h i
->0 0 0 0 j
->
->Now n can be determined from Length[list], which will always be equal to
->n choose 2 for some n.  But I would rather input n to save that
->computation.  (I will be doing this for a lot of vectors.)
->
->Any ideas?
->
->Rob Pratt
->Department of Mathematics
->The University of North Carolina at Chapel Hill
->CB# 3250, 331 Phillips Hall
->Chapel Hill, NC  27599-3250
->
->rpratt at math.unc.edu

Another possible solution to this uses the MatrixManipulation standard
package:

<<LinearAlgebra`MatrixManipulation`
utmatrix[ x_List, n_Integer ] :=
Module[
{ a, b, c },
a = Drop[ UpperDiagonalMatrix[ b, n + 1 ], 1 ];
c = DeleteCases[ Flatten[ a ], 0 ];
a /. Thread[ Rule[ c, x ] ]
] /; Positive[ n ] && (Length[ x ] == n(n + 1)/2)
utmatrix[ {a,b,c,d,e,f,g,h,i,j}, 4 ] // MatrixForm

Paul

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```

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