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MathGroup Archive 2006

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Re: generalized foldlist problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg69093] Re: generalized foldlist problem
  • From: "sashap" <pavlyk at gmail.com>
  • Date: Wed, 30 Aug 2006 06:32:30 -0400 (EDT)
  • References: <ed0rja$sll$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Arkadiusz,

I think the following does what you wanted:

In[5]:=
GeneralizedFoldList[e1_List,e2_List]/;ArrayQ[e2,1,(IntegerQ[#]&&Positive[#])&]\
&&Length[e1]\[Equal]Length[e2]:=Module[{cntrs,fun},
    cntrs=e2;
    fun[k_Integer]:=If[Positive[Extract[cntrs,k]],

cntrs=ReplacePart[cntrs,(Extract[cntrs,k]-1),k];Extract[e1,k],0];
    Rest[FoldList[Plus,0,Hold[fun]/@Range[Length[e1]]]]//ReleaseHold
    ]

In[6]:=
GeneralizedFoldList[{a,b,c,d,e},{3,2,5,1,6}]

Out[6]=
{a,a+b,a+b+c,c+d,c+e}

Oleksandr Pavlyk
Wolfram Research

Arkadiusz Majka wrote:
> DearAll,
>
> Please, help!
>
> I have two list
>
> list1={a,b,c,d,e}
> list2={3,2,5,1,6}
>
> and I want to apply a modified version of FoldList to list1 in the
> following way: list2 indicates that element a appears only 3 times (if
> space enough) beginning from the beginning of the list , element b
> appears 2 times, c - 5 times , etc.
>
> So the output should be
>
> GeneralizedFoldList[list1,list2]={a,a+b,a+b+c,c+d,c+e}
> 
> Thanks for any hints,
> 
> arek


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