Re: Sum elements in a list with conditions

• To: mathgroup at smc.vnet.net
• Subject: [mg69840] Re: [mg69828] Sum elements in a list with conditions
• From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
• Date: Mon, 25 Sep 2006 03:52:58 -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
>

{{a1, b1}, {a2, b2}, {a2, b3}, {a2, b4}, {a3, b5},
{a3, b6}} //. {a___, {x_, y_}, b___, {x_, z_},
c___} :> {a, {x, y + z}, b, c}

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

Andrzej Kozlowski
Tokyo, Japan

```

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