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MathGroup Archive 1998

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Re: Summing over like elements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13876] Re: Summing over like elements
  • From: buttgereit at netcologne.de (Peter Buttgereit)
  • Date: Wed, 2 Sep 1998 01:31:09 -0400
  • Organization: Dipl.-Sportl. Peter Buttgereit
  • References: <6sdb8i$peq@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

[This followup was posted to comp.soft-sys.math.mathematica and a copy 
was sent to the cited author.]

In article <6sdb8i$peq at smc.vnet.net>, qmak at pipeline.com says...
> 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}}
> 
> 
> I'm sure I can do this with a "Do" loop -- however it seems that in
> general "Do's" are pretty innefficient.
> 
> Does anyone have any suggestions?
> 
>                 Mitch
> 
> 
Hi Mitch,
how about

tmp=Split[ 
          Sort[ yourList, (#1[[1]]<#2[[1]])& ], 
          SameQ[ #1[[1]], #2[[1]] ]& 
    ]                                               (*1*)

sumP[a_List]:={#[[1,1]],Plus@@#[[2]]}&@Transpose[a] (*2*)

sumP/@tmp                                           (*3*)

(1) will produce sublists of identical elements in the first position of
your sublists - you have 
{{{i,pi1},...,{i,pin}},{{j,pj1},...,{j,pjm}},...}. (2) will take one of
these sublists, extract the first element, sum over  p and return {i,
sum[pin]}.
In (3) you map sumP on all the sublists of tmp, produced in (1), 
returning the desired output {{i,sum[pin]},{j,sum[pjm]}...} --- let's
see what the other guys come up with...(probably something faster;  at
least my version is without "Do" <g>). So long,
cheers,
Peter


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