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Re: Partition a list into sublists of unequal size

  • To: mathgroup at smc.vnet.net
  • Subject: [mg83964] Re: Partition a list into sublists of unequal size
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Wed, 5 Dec 2007 07:20:45 -0500 (EST)
  • References: <fj37cv$hq0$1@smc.vnet.net>

antf wrote:
> Hello,
> 
> I have a list that looks like the following:
> 
> list={1,0,0,1,0,0,1,1,0,1,0,0,0,0,1,0}
> 
> I then find the positions of the 1s in the list:
> 
> positions = Flatten@Position[list,1]
> 
> {1,4,7,8,10,15}
> 
> I then want to partition a third list into sublists so that the first
> sublist is from index 1;;1, the second sublist is from index 2;;4, the
> third sublist is from index 5;;7, the fourth sublist is just element
> 8, etc.
> 
> Currently, I use Partition on the positions sublist with overlap as
> in:
> 
> Partition[Join[{1},positions],2,1]
> 
> {{1,1},{1,4},{4,7},{7,8},{8,10},{10,15}}
> 
> I then use this list to partition the target list by Mapping Take to
> each element of this list and adjusting the first index appropriately.
> 
> I am sure there must be a simpler way to do this.  Can anyone tell me
> how to do this operation?


This isn't really simpler ... but here's another way:

In[1]:= l1 = RandomInteger[{0, 1}, {20}]
Out[1]= {1, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 0, 1, 1, 0, 1, 0, 1, 0, 1}

In[2]:= l2 = Accumulate[l1] - l1
Out[2]= {0, 1, 1, 1, 1, 1, 2, 3, 4, 5, 6, 7, 7, 8, 9, 9, 10, 10, 11, 11}

In[3]:= l3 = Range[20] (* the list to be partitioned *)
Out[3]= {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, \
18, 19, 20}

In[4]:= Split[Transpose[{l2, l3}],
           First[#1] == First[#2] &][[All, All, 2]]
Out[4]= {{1}, {2, 3, 4, 5, 6}, {7}, {8}, {9}, {10}, {11}, {12, 13}, 

          {14}, {15, 16}, {17, 18}, {19, 20}}

-- 
Szabolcs


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