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MathGroup Archive 2006

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Re: how to sum lists of unequal length?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70393] Re: how to sum lists of unequal length?
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sun, 15 Oct 2006 00:19:09 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <egq2th$84h$1@smc.vnet.net>

Arkadiusz.Majka at gmail.com wrote:
> Hi all,
> 
> I have lists of unknown and unequal lengths. I want to sum them, e.g
> 
> {a1,a2,a3}+{b1,b2,b3,b4,b5} and obtain result
> {a1+b1,a2+b2,a3+b3,b4,b5}. Simply filling by zeros of the shorter list
> is impossible because I have no idea how many zeros I have to pad.

The following function takes as argument a list of all the lists of 
unequal length that one wants to sum, pads the shorter list with zeros, 
then computes the sum:

In[1]:=
mySum[expr_] := Module[{n},
		n = Max[Length /@ expr];
		Total[(PadRight[#1, n] & ) /@ expr]]

In[2]:=
lst1 = {a1, a2, a3};
lst2 = {b1, b2, b3, b4, b5};
expr = {lst1, lst2};
mySum[expr]

Out[5]=
{a1 + b1, a2 + b2, a3 + b3, b4, b5}

Regards,
Jean-Marc


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