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MathGroup Archive 1998

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Re: Summing over like elements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13865] Re: [mg13855] Summing over like elements
  • From: wself at viking.emcmt.edu (Will Self)
  • Date: Wed, 2 Sep 1998 01:30:59 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Mitch asks for the following:

>I have a 2 column array.  Column 1 has some values, many of which are
>repeated.  Column 2 has probabilities of these values.
>
>I would like to sum the probabilities for each unique element in column
>1. For example:
>
>{{1, .5} , {2, .2} , {1, .1}}  should give me:
>
>{{1, .6} , {2, .2}}


Mitch, this will do it:

In[1]:= mm={{1, .5} , {2, .2} , {1, .1}}

Out[1]= {{1, 0.5}, {2, 0.2}, {1, 0.1}}

In[2]:= Function[x,{x,Plus@@(#[[2]]&/@Select[mm,#[[1]]==x&])}]/@
          Union[#[[1]]&/@mm]

Out[2]= {{1, 0.6}, {2, 0.2}}


Will


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