MathGroup Archive 1997

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Xah's partition question

  • To: mathgroup at
  • Subject: [mg8443] Xah's partition question
  • From: wself at (Will Self)
  • Date: Sat, 30 Aug 1997 00:42:54 -0400
  • Sender: owner-wri-mathgroup at

Xah asks for the following:

>I want to partition a list, using special values in the list as boundary
>indicators. For example, myList={3,7,10,6,1,1,6,1,7,10,3,4,6} and my special
>value for boundary is 10, then my desired result would be

Here is one way.  The idea is to start with the fact that
Position[myList, 10, 1] gives the list{{3}, {10}}.  Use this list to generate 
the list  takeLocations = {{1,2}, {3,9}, {10,13}}; then

Take[myList,#]& /@ takeLocations

will give the desired result.

The following function does it:

xahPartition[someList_, separator_]:=
v=Union[{1},Flatten[Position[someList, separator, 1]]]},

This works correctly whether or not separator occurs as the first
element of someList, and gives the desired result if separator does not
occur in the top level of someList.

Will Self

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