Mathematica 9 is now available
Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2006
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2006

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Sum elements in a list with conditions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69873] Re: [mg69828] Sum elements in a list with conditions
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Tue, 26 Sep 2006 00:59:39 -0400 (EDT)

On 24 Sep 2006, at 12:45, 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
>

Just for fun: here is a solution that is, I think, entirely different  
from all the others that have been posted.


List@@@(List@@(Simplify[Plus@@(Times@@@{{a1, b1},{a2,b2},{a2,b3}, 
{a2,b4},{a3,
                 b5}, {a3, b6}})]))

{{a1, b1}, {a2, b2 + b3 + b4}, {a3, b5 + b6}}

It will only work if the elements of the original lists are symbols  
but one can modify it to work with numbers. However, this way of  
programming is in general inefficient so the above is only meant as  
an example of the kind of programming that is unique to computer  
algebra systems.

Andrzej Kozlowski


  • Prev by Date: Re: Re: Differentiation problem/bug?
  • Next by Date: Re: Sum elements in a list with conditions
  • Previous by thread: Re: Sum elements in a list with conditions
  • Next by thread: Re: Sum elements in a list with conditions