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Re: When Wolfram's technical support cannot help

  • To: mathgroup at smc.vnet.net
  • Subject: [mg105283] Re: [mg105255] When Wolfram's technical support cannot help
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Wed, 25 Nov 2009 23:03:34 -0500 (EST)
  • References: <200911251114.GAA13425@smc.vnet.net>

First of all, I am very surprised that technical support replied to this 
at all. I have to considerably upgrade my estimation of them ;-)  Most 
people, certainly not me, would not think of sending what is essentially 
a programming question to technical support. The terms of Premier 
Service do include, I think, some kind of programming help (I am not 
sure about that - I have Premeir Service but have never thought of this 
possibility), but whatever it is, technical support is not the place one 
would turn to in such cases.

First of all, I think there is a small error in the function(which 
answers the first part of your question) you were sent by technical 
support. I think the right definition should have been:

findCycle1[ls_] :=
 Module[{p, a},
  p = Position[
      Table[BitXor[lis, RotateLeft[lis, i]], {i, 1, Length[lis]/2}],
      ConstantArray[0, Length[lis]]][[1]] /. {a_} -> a; ls[[1 ;; p]]]

This is a very good function and I do not think I can find a better one. 
There is an interesting and much shorter function that comes to my mind, 
but it is not going to be as reliable. Still, it comes as close to being 
a "built-in" function for this problem as one can find:

findCycle2[ls_] :=
 With[{f = FindSequenceFunction[ls]}, GatherBy[ls, f][[All, 1]]]

This will work in simple cases, like the one you sent:

 lis = Flatten[Join[Table[{1, 2, 3}, {20}]]];

findCycle2[lis]

{1,2,3}

However, one can construct more complicated cases where it fails (and 
the one sent by technical support succeeds). One such example will 
appear below. But first, let's turn to the second part of your question. 
This is much harder and one (but not the only) reason is that it is not 
mathematically well defined. Consider the following sequence:

 lis =
 Join[{8, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3}, {8, 1, 2, 3, 1, 2, 3,
   1, 2, 3, 1, 2, 3}]

{8,1,2,3,1,2,3,1,2,3,1,2,3,8,1,2,3,1,2,3,1,2,3,1,2,3}


What should be the "repeating pattern here"? If we interpret this 
question in the first sense the answer is {8, 1, 2, 3, 1, 2, 3, 1, 2, 3, 
1, 2, 3}. But your second definition of "cycle" is allowed then one the 
answer is ambiguous, it is either {8, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 
3} or simply {1,2,3}. This example confuses my findCycle2

findCycle2[lis]

{8,1,3}

but the technical support function works fine:

findCycle1[lis]

{8,1,2,3,1,2,3,1,2,3,1,2,3}

In order for your second function to be defined you have to give an 
unambiguous criterion to use in such cases. One possible answer would be 
first be that one should look for cycles by starting with the whole 
sequence and then dropping terms from the beginning. It is quite easy to 
implement an inefficient version of this just starting with the function 
you were sent. I start by modifying findCycle1 to make it return $Fail 
whenever there is no cycle. I am really using a hack here (Quiet) and it 
should really be done more nicely but I do not want to spend too much 
time on this (perhaps someone else will). So, here is a function that 
works exactly like findCycle1 but returns $Fail when there is no cycle:

findCycle3[ls_] :=
 Quiet[Module[{u = findCycle1[ls]},
   If[Head[u] === List && u == Take[ls, Length[u]], u, 
$Fail]]]

You can check that this works:

 ls = Join @@ Table[{1, 2, 3}, {1}];

In[141]:= findCycle3[ls]

 $Fail

 ls = Join @@ Table[{1, 2, 3}, {2}];

findCycle3[ls]

 {1,2,3}

Now once we have got it, it is very easy to construct a function that 
will just go through the list looking for cycles, if it doesn't fine one 
it will drop the first element and then go through the list again etc. I 
programmed it in the procedural way paying no regard to performance. It 
can be done better using Sow and Reap, but again I do not want to spend 
time doing that; its only meant as an example of how one can do this. 
So:

findCycle4[ls_] :=
 Module[{p = ls, q = findCycle3[ls]},
  While[Length[p] > 0,
   If[q =!= $Fail, Return[q], q = findCycle3[p = Rest[p]]]]; q]

Now, this will work in all cases (I think). Here are your two examples:

 data1 = {1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3};

 data2 = {5, 9, 11, 8, 1, 2, 3, 1, 2, 3, 1, 2, 3, 1, 2, 3};

findCycle4[data1]

{1,2,3}

findCycle4[data2]

{1,2,3}


Andrzej Kozlowski





On 25 Nov 2009, at 20:14, Ant King wrote:

> Hi
>
> I sent this email to technical support (as I hold a premier licence)
>
> I am looking for a single function that will extract the cyclic part of (a)
> a completely cyclic sequence and (b) an eventually cyclic sequence. So if
>
> data1={1,2,3,1,2,3,1,2,3,1,2,3} then cyclicpart[data1] should return {1,2,3}
>
> And if
>
> data2={5,9,11,8,1,2,3,1,2,3,1,2,3,1,2,3} then cyclicpart[data2] should also
> return {1,2,3}
>
> And this was the reply that I got
>
> Here is one way to extract out the cyclic part:
>
> lis = Flatten[Join[Table[{1, 2, 3}, {8}]]]
>
> p = Position[Table[BitXor[lis, RotateLeft[lis, i]], {i, 1, 10}],
>    ConstantArray[0, Length[lis]]][[1]] /. {a_} -> a
> lis[[1 ;; p]]
>
> The above however will only work for case where the list always contains
> the repeated pattern.
>
> There is no built-in function as such that will extract out the pattern
> automatically. I have filed a suggestion with our developers and you will
> be notified when this suggestion gets implemented. Again, my apologies for
> the delay and my thanks for your patience.
>
> Now I don't believe that. I think that it should be quite achievable. Anyone
> got any ideas
>
> Thanks a lot
>
> Ant
>



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