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Re: assignment of matrix elements

  • To: mathgroup at
  • Subject: [mg17264] Re: [mg17206] assignment of matrix elements
  • From: "Tomas Garza" <tgarza at>
  • Date: Fri, 30 Apr 1999 02:34:46 -0400
  • Sender: owner-wri-mathgroup at

Richard Finley [rfinley at] wrote:

> This seems like it should be straightforward but I am too tired to think
> about it anymore:
> If you have a matrix say mat = Array[m, {4,4}]
> and a list of indices say v = {{1,2},{1,3},{2,3},{3,1}}
> with another list of the same length giving the values of the matrix
> elements at
> those indices: w = {1,5,-2,1}
> How do you assign values to the matrix elements with those
> so
> that m[1,2]=1, m[1,3]=5 etc...
> Obviously you can do it individually by simple assignment:
> m{1,2]=1
> That won't work for me because there are thousands of elements...the
> techniques I have tried so far
> have not worked...any help is greatly appreciated.

Hi, Richard!

If I understood your problema correctly, then a possible solution is as

Suppose your matrix is


and also

v= {{1,2},{1,3},{2,3},{3,1}};
w= {1,5,-2,1};

Define a function foo which changes individual elements using ReplacePart:


and then use Fold to make the changes throughout:


Hope this helps,

Tomas Garza
Mexico City

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