Re: Sparse Array Question

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• Subject: [mg130595] Re: Sparse Array Question
• From: Szabolcs HorvÃt <szhorvat at gmail.com>
• Date: Thu, 25 Apr 2013 02:51:44 -0400 (EDT)
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```On 2013-04-24 01:39:04 +0000, christopher arthur said:

> 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

There is a system option for this.  Hopefully it works in 5.2 too, but
I cannot test.

In[1]:= Normal@SparseArray[{{1, 1} -> 1, {2, 1} -> 2, {1, 1} -> 3}]
Out[1]= {{1}, {2}}

In[2]:= SetSystemOptions["SparseArrayOptions" -> "TreatRepeatedEntries"
-> Total]

In[3]:= Normal@SparseArray[{{1, 1} -> 1, {2, 1} -> 2, {1, 1} -> 3}]
Out[3]= {{4}, {2}}

In[4]:=
(* set it back to the default *)
SetSystemOptions["SparseArrayOptions" -> "TreatRepeatedEntries" -> First]

Be very very careful with this!  It modified global behaviour, so
there's no telling what other indirect effects it will have.

```

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