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Re: Sparse Array Question

  • To: mathgroup at smc.vnet.net
  • 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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