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Re: list/sequence problem: a 'keep' list to a list of

  • To: mathgroup at smc.vnet.net
  • Subject: [mg97185] Re: [mg97147] list/sequence problem: a 'keep' list to a list of
  • From: DrMajorBob <btreat1 at austin.rr.com>
  • Date: Sat, 7 Mar 2009 02:40:35 -0500 (EST)
  • References: <200903060926.EAA22779@smc.vnet.net>
  • Reply-to: drmajorbob at bigfoot.com

keepPrior = {False, True, False, True, False, False, True, True, True,
     False, True};
counter = 0;
Rest@FoldList[(counter++; If[#2, Flatten@{#1, counter}, {counter}]) &,
    dummy, keepPrior]

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

or

counter = 0;
Rest@FoldList[(counter++; If[#2, Append[#1, counter], {counter}]) &,
   dummy, keepPrior]

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

Bobby

On Fri, 06 Mar 2009 03:26:41 -0600, mkr <MilesKRains at gmail.com> wrote:

> 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}
>
>



-- 
DrMajorBob at bigfoot.com


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