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MathGroup Archive 2000

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Re: Could this be improved?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21763] Re: [mg21741] Could this be improved?
  • From: "David Park" <djmp at earthlink.net>
  • Date: Thu, 27 Jan 2000 22:56:43 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Jordan,

This might be somewhat simpler. We reverse the vector, pad it on the ends, and then
Take a sliding section:

myMtx[v_?VectorQ] :=
  Module[{w, l = Length[v]},
    w = Join[Table[0, {l - 1}], Reverse[v], Table[0, {l - 1}]];
    Table[Take[w, {2l - i, 3l - 1 - i}], {i, 1, 2l - 1}]
    ]

myMtx[{1, 2, 3}]
{{1, 0, 0}, {2, 1, 0}, {3, 2, 1}, {0, 3, 2}, {0, 0, 3}}

myMtx[{1, 2, 3, 4}]
{{1, 0, 0, 0}, {2, 1, 0, 0}, {3, 2, 1, 0}, {4, 3, 2, 1}, {0, 4, 3, 2}, {0, 0,
    4, 3}, {0, 0, 0, 4}}

David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/




>Hi all,
>
>I wrote the following code which works correctly.  I was wondering, however,
>if there was a way of doing the same thing that had more of a Mathematica
>approach.  I am new to Mathematica and am still trying to get a grasp on how
>to program effectively within the environment.
>
>myMtx[v_] := Module[
>    {nCols, nRows, vPadded, c},
>    nCols = Length[v];
>    nRows = 2nCols - 1;
>    c = ZeroMatrix[nRows, nCols];
>    vPadded = PadRight[v, nRows, 0];
>    For[i = 1, i <= nCols, i++,
>      c[[All, i]] = vPadded;
>      vPadded = RotateRight[vPadded]
>      ];
>    c
>    ]
>
>For example, myMtx[{1,2,3}] takes the vector {1,2,3} and turns it into the
>matrix {{1, 0, 0}, {2, 1, 0}, {3, 2, 1}, {0, 3, 2}, {0, 0, 3}} which looks
>like
>
>[ 1 0 0 ]
>[ 2 1 0 ]
>[ 3 2 1 ]
>[ 0 3 2 ]
>[ 0 0 3 ]
>
>
>Thanks,
>
>Jordan
>




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