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MathGroup Archive 2006

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Re: Sum elements in a list with conditions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69844] Re: [mg69828] Sum elements in a list with conditions
  • From: "Adriano Pascoletti" <pascolet at dimi.uniud.it>
  • Date: Mon, 25 Sep 2006 03:53:13 -0400 (EDT)

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}}
> 

A solution with Sow and Reap

In[1]:=
L = {{a1, b1}, {a2, b2}, {a2, b3}, {a2, b4}, {a3, b5}, {a3, b6}}; 

In[2]:=
Reap[(Sow[#1[[2]], #1[[1]]] & ) /@ L, _,  {#1, Total[#2]} & ][[2]]

Out[2]=
{{a1, b1}, {a2, b2 + b3 + b4}, {a3, b5 + b6}}


Adriano Pascoletti


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