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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```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

```

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