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

*To*: mathgroup at smc.vnet.net*Subject*: [mg97192] Re: [mg97147] list/sequence problem: a 'keep' list to a list of*From*: Miles Rains <mileskrains at gmail.com>*Date*: Sat, 7 Mar 2009 02:41:51 -0500 (EST)*References*: <200903060926.EAA22779@smc.vnet.net>

Doctor (or is it Sir?), Very nice, thanks much. I was pretty sure there had to be a more concise, canonical way to accomplish the task. As you surmised, the first element would always be True. You have prompted me to gain a better understanding and appreciation of FoldList. I've only been using Mathematica for ~10 years, so I still have a lot to learn :) Miles On Fri, Mar 6, 2009 at 12:11, DrMajorBob <btreat1 at austin.rr.com> wrote: > Sorry, that fails (with "dummy" as second argument to FoldList) if the > first element of keepPrior is True. > > You haven't specified what you want to do in that case, but maybe this > would do: > > keepPrior = {True, False, True, False, True, False, False, True, True, > True, False, True}; > counter = 0; > Rest@FoldList[(counter++; > If[#2, Append[#1, counter], {counter}]) &, {}, keepPrior] > > {{1}, {2}, {2, 3}, {4}, {4, 5}, {6}, {7}, {7, 8}, {7, 8, 9}, {7, 8, 9, > 10}, {11}, {11, 12}} > > Bobby > > > On Fri, 06 Mar 2009 14:04:10 -0600, DrMajorBob <btreat1 at austin.rr.com> > wrote: > > 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 >

**References**:**list/sequence problem: a 'keep' list to a list of 'kept' lists***From:*mkr <MilesKRains@gmail.com>