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

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

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]];

{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}

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

Hi,

assume p and d are prior-vector and data:

p = {False, True, False, True, False, False, True, True, True, False,

True};

d = Table[i, {i, Length[p]}];

we then construct data items {dat,boolean} and put them in a

list:{{dat,boolean}. Next we apply repeatedly a rule that considers two

neighbours and appends the left neighbor to the list of the right one if

the right list consists of one item with boolean value of True. Last, we

pick out the data values:

t = t //. {x1 : ___, x2 : {__}, x3 : {{_, True}}, x4___} :> {x1, x2,

Append[x2, x3[]], x4} ;

t[[All, All, 1]]

hope this helps, Daniel