Re: Sparse Array Question

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• Subject: [mg130581] Re: Sparse Array Question
• From: Ray Koopman <koopman at sfu.ca>
• Date: Wed, 24 Apr 2013 06:58:27 -0400 (EDT)
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In[1]:= \$Version
Out[1]= 5.2 for Mac OS X (June 20, 2005)

In[2]:= myList = {{1,1}->1,{2,1}->2,{1,1}->3};

In[3]:= Rule[#[[1,1]],Tr@#[[All,2]]]& /@
Split[Sort@myList,#1[[1]]==#2[[1]]&]

Out[3]= {{1,1}->4,{2,1}->2}

On Mon, Apr 22, 2013 @ 09:04 PM, christopher arthur <chris.arthur1 at gmail.com> wrote:

> Hello,
>
> I have a list of rules for defining a SparseArray.  The list has some
> coordinates repeated with different values.  If a coordinate appears in
> the list more than once, I want to sum up the values, and then have a
> SparseArray
>
> i.e., if my list were myList={{1,1}->1,{2,1}->2,{1,1}->3} then my
> SparseArray should have {1,1}->4.  I've tried using ReplacePart on an
> array to say myArray=ReplacePart[myArray,...] but this seems horribly
> inefficient as a way to change values in an array.
>
> I'm using Mathematica 5.2
>
> Cheers,
>
> Chris

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