Re: Sum elements in a list with conditions
- To: mathgroup at smc.vnet.net
- Subject: [mg128434] Re: Sum elements in a list with conditions
- From: Bill Rowe <readnews at sbcglobal.net>
- Date: Fri, 19 Oct 2012 02:42:23 -0400 (EDT)
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- Delivered-to: l-mathgroup@wolfram.com
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On 10/18/12 at 2:36 AM, guillermo.sanchez at hotmail.com (Guillermo
Sanchez) wrote:
>I have a list of sublist of pairs. I wish sum the second elements of
>the sublists when the first elements are equals Example: Int[]:={{1,
>a}, {2, b}, {1, c}, {2, d}, {2, e}, {3, f}} I hope Out[]:= {{1, a +
>c}, {2, b + c + e}, {3, f}}
Here are a couple of methods
data = {{1, a}, {2, b}, {1, c}, {2, d}, {2, e}, {3, f}};
In[3]:= {#[[1, 1]], Last@Total@#} & /@ GatherBy[data, First]
Out[3]= {{1, a + c}, {2, b + d + e}, {3, f}}
In[4]:= Mean /@ GatherBy[data, First] /. Times[_, a_Plus] -> a
Out[4]= {{1, a + c}, {2, b + d + e}, {3, f}}
The first is likely faster for large data sets