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MathGroup Archive 2009

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list/sequence problem: a 'keep' list to a list of 'kept' lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg97147] list/sequence problem: a 'keep' list to a list of 'kept' lists
  • From: mkr <MilesKRains at gmail.com>
  • Date: Fri, 6 Mar 2009 04:26:41 -0500 (EST)

I have a list of Booleans, indicating whether prior values in sequence
are to be 'kept'.  I want to generate a list of accumulated 'kept'
positions for each position in the sequence.  The following code
works, but I would love to see a cleaner, more obivous approach.
Anyone?

In[]=

keepPrior=
{False,True,False,True,False,False,True,True,True,False,True};

keptPriors=Function[aclist,If[Split[aclist][[-1,1]]==True,Range[Length
[aclist]-Length[Split[aclist][[-1]]],Length[aclist]],{Length
[aclist]}]]/@Rest[FoldList[Append,{},keepPrior]];

TableForm[Transpose[{keepPrior,ToString/@keptPriors}],TableHeadings->
{Automatic,None}]


Out[]=

1	False	{1}
2	True	{1, 2}
3	False	{3}
4	True	{3, 4}
5	False	{5}
6	False	{6}
7	True	{6, 7}
8	True	{6, 7, 8}
9	True	{6, 7, 8, 9}
10	False	{10}
11	True	{10, 11}



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