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)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com*Delivered-to*: l-mathgroup@wolfram.com*Delivered-to*: mathgroup-outx@smc.vnet.net*Delivered-to*: mathgroup-newsendx@smc.vnet.net

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