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Re: Howto sum up a list?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg106964] Re: Howto sum up a list?
  • From: JH <jlucio at ubu.es>
  • Date: Fri, 29 Jan 2010 07:49:34 -0500 (EST)
  • References: <hjrf6g$mur$1@smc.vnet.net>

On 28 ene, 08:43, Jonas Stein <n... at jonasstein.de> wrote:
> How can i sum up the list:
>
> MyList = {{eggs, 4}, {milk, 1}, {eggs, 1}, {milk, 1}}
>
> I want to get something like:
>
> {{eggs, 5},{milk, 2}}
>
> I tyied Tally[], but that ignored the factor 4 in {eggs, 4}
>
> Thank you.
>
> --
> Jonas Stein <n... at jonasstein.de>

Hello, Jonas:

You can try this:

MyList = {{eggs,4},{milk,1},{eggs,1},{milk,1}};
listgather = Gather[MyList, #1[[1]] == #2[[1]] &];
Table[{listgather[[i,1,1]], Total[listgather[[i,All,2]]]},{i,Length
[listgather]}]

The output is what you expected (grouping by first element and summing
second element in each group):
{{eggs, 5}, {milk, 2}}

I hope it helps you.

   JH


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