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MathGroup Archive 1999

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Re: Replacing Part of a Matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg20260] Re: Replacing Part of a Matrix
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Sun, 10 Oct 1999 00:04:09 -0400
  • References: <7tl0bj$40p@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

A = Table[0, {7}, {7}];

B = Table[y, {3}, {3}];

A[[Range[3, 5], Range[3, 5]]] = B;

A // MatrixForm

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   0   y   y   y   0   0

0   0   y   y   y   0   0

0   0   y   y   y   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0

This only works with a name (here A) for the matrix to be changed, and it
does change the value of A.

{{0, 0, 0, 0}, {0, 0, 0, 0}, {0, 0, 0, 0}, {0, 0, 0, 0}} [[{2, 3}, {1,
          2}]] = {{a, a}, {a, a}};

Set::"setps": "\!\({\(\({0, 0, 0, 0}\)\), \(\({0, 0, 0, 0}\)\), \(\({0, 0,
0, \
0}\)\), \(\({0, 0, 0, 0}\)\)}\) in assignment of part is not a symbol."

A = Table[0, {7}, {7}];

B = Table[y, {3}, {3}];

But we can use a temporay variable (here A1) to avoid these features

Block[{A1 = A},
    A1[[Range[3, 5], Range[2, 4]]] = B; A1
    ] // MatrixForm

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   y   y   y   0   0   0

0   y   y   y   0   0   0

0   y   y   y   0   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0

A // MatrixForm

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0

0   0   0   0   0   0   0


Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565


<flippy652 at my-deja.com> wrote in message news:7tl0bj$40p at smc.vnet.net...
> Hello all,
>
> I have a n x n matrix, call it A.
>
> I have a (n - 2)x(n - 2) matrix, call it B.
>
> I would like to keep the outer rows and columns
> of A, but repalce the inner rows and columns with
> thos of B to get a new C.
>
> I looked at the ReplacePart and other matrix and
> list manipulation rules and couldn't quite find
> one.
>
> I wrote a very gross module to do it.
>
> Does anyone have such a module in Mathematica they would
> like to share.
>
> For example,
>
> c = replace_a_b[a, b]
>
> would return the desired result.
>
> Example:
>
> A = {{1,2,3,4},{5,6,7,8},{9,10,11,12},
> {13,14,15,16}}
> B = {{20,30,},{40,50}}
>
> C = replace_a_b[A,B], would produce:
>
> C = {{1,2,3,4},{5,20,30,8},{9,40,50,12},
> {13,14,15,16}}
>
> Basically, this function just replaces the
> interior (n-2)x(n-2) in the larger matrix.
>
> Can anyone help?
>
> By the way, thank you to all who responded with
> such an easy and useful rectangular grid
> generator.
>
> Thank you.
>
> Wilson Figueroa
> flip at safebunch.com
> http://safebunch.com
>
>
> Sent via Deja.com http://www.deja.com/
> Before you buy.
>



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