Re: List Operations

• To: mathgroup at smc.vnet.net
• Subject: [mg65700] Re: List Operations
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Sun, 16 Apr 2006 01:45:02 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <e1nn6s\$lmm\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```LectorZ wrote:
> Hi guys,
>
> My question:
>
> mylist={{1,a, b},{1,a, b},{1,a,
> b},{2,b,z},{2,b,z},{2,b,z},{2,b,z},....,{n,x,y},{n,x,y},{n,x,y},...{n,x,y}}
>
> The sublists are of different length.
>
> I need to calculate the product between the 2nd and 3rd element of
> every sublist (e.g. a*b) and then add them up according to the 1st
> element: sum of all products where the 1st element is 1, 2, ...n.
>
> The result should be a list like that:
> {{1,a*b+a*b+a*b},{2,b*z+b*z+b*z+b*z}, ...,{n, x*y+x*y+x*y+...+x*y}}
>
> Thank you for your help.
>
> LZ
>
Hi Lector,

I have come up with the following expression:

In[1]:=
mylist = {{1, a, b}, {1, a, b}, {1, a, b}, {2, b, z},
{2, b, z}, {2, b, z}, {2, b, z}, {3, x, y},
{3, x, y}};

In[2]:=
({#1[[1,1]], Plus @@ #1[[2]]} & ) /@
Transpose /@ Split[mylist /. {n_, x_, y_} ->
{n, x*y}, #1[[1]] == #2[[1]] & ]

Out[2]=
{{1, 3*a*b}, {2, 4*b*z}, {3, 2*x*y}}

Hope this helps,
Jean-Marc

```

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