Re: Sum elements in a list with conditions
- To: mathgroup at smc.vnet.net
- Subject: [mg69847] Re: Sum elements in a list with conditions
- From: jmhigg at gmail.com
- Date: Mon, 25 Sep 2006 03:53:19 -0400 (EDT)
- References: <ef5066$mm5$1@smc.vnet.net>
Guillermo,
You can try this:
(i) First use Split with your own test function to split up the pairs
that have the same first element:
ls = {{a1, b1}, {a2, b2}, {a2, b3}, {a2, b4}, {a3, b5}, {a3, b6}} ;
Split[ls,(First[#1]==First[#2])&]
{{{a1,b1}},{{a2,b2},{a2,b3},{a2,b4}},{{a3,b5},{a3,b6}}}
(ii) Then map Fold onto each sublist above to add the second element of
that sublist
Here is the final result as single line of code:
Map[{First[Flatten[#]],Fold[(#1+Last[#2])&,0,#]}&,Split[ls,(First[#1]==First[#2])&]]
{{a1,b1},{a2,b2+b3+b4},{a3,b5+b6}}
Cheers,
Brian
Guillermo Sanchez wrote:
> Dear group
> I have a pair of elements list. The second elements of the list should
> be summed when the first element of the pairs are equals. Example
>
> Given
>
> {{a1, b1},{a2,b2},{a2,b3},{a2,b4},{a3, b5}, {a3, b6}}
>
> the output should be
>
> {{a1, b1},{a2, b2+b3+b4},{a3, b5+b6}}
>
> Thanks
>
> Guillermo