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MathGroup Archive 1998

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Re: Re: Split in Mathematica 2.2/ $MaxRecursion crash

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13032] Re: [mg12979] Re: Split in Mathematica 2.2/ $MaxRecursion crash
  • From: Daniel Lichtblau <danl>
  • Date: Tue, 30 Jun 1998 00:26:29 -0400
  • References: <6msqgp$i7j$3@dragonfly.wolfram.com> <199806280652.CAA24350@smc.vnet.net.>
  • Sender: owner-wri-mathgroup at wolfram.com

Matthias Weber wrote:
> 
> Hello
> 
> I can confirm the slow behaviour of Split on a PowerMac 6100/66 (which
> has a 601 PPC, vm turned on) reported recently here, and the correct
> fast behaviour on an Irix.
> 
> One of the proposed
> user-implemented alternatives however is even crashing my Mac:
> 
> split2[li_,fQ_:SameQ]:=Module[{liLength,index,i},liLength=Length at li;
> index={};For[i=1,i<liLength,i++,
>       If[Not@(fQ@@Part[li,{i,i+1}]),index={index,i}]];
> (Head at li)@@((Take[li,#]&)/@(
> 
> Transpose[{Flatten[{1,#+1}],Flatten[{#,liLength}]}]&)@index)];
> 
> This being highly recursive, to execute (say)
> 
> w = Table[Random[Integer], {10000}]; split2[w];//Timing
> 
> I need to increase
> 
> $RecursionLimit=10000;
> 
> This works on our Irix but crashes my poor little Mac (Bus error...,
> restart required) after one error message  $RecursionLimit::"reclim":
> with a lower $RecursionLimit, I either get the usual bunch of error
> messages or just one of them, and the Mac crashes. Enjoy.
> 
> Matthias Weber


In these circumstances it is useful to rewrite your code so that
$RecursionLimit is not an issue. This is because otherwise you run the
risk of segmentation fault by blowing up the recursion stack.

After some experimentation I used the code below. I changed the For loop
to a Do to get a slight speed improvement. Note that I do NOT change
$RecursionLimit to a non-default setting.

split2[li_, fQ_:SameQ] := Module[{liLength, index, i},
	liLength = Length[li];
	index = {};
	Do [If[!fQ @@ li[[{i,i+1}]], index = {index,i}], {i,liLength-1}];
	index = Flatten[index];
	index = (Transpose[{Flatten[{1, index + 1}],
	  Flatten[{index, liLength}]}] & )[index];
	Head[li] @@ (Take[li, #1] & ) /@ index
	]
Timing[s2 = split2[w];]

Unfortunatwly there are still recursion pitfalls lurking. For example,
try

w = Table[Random[Integer,10], {100000}];

which will give rise to a deeply nested index set in the course of
running split2. This will seg fault in versions 3.0.x and prior of
Mathematica due to a recursive implementation of Flatten. We have fixed
this problem in our development version.


Daniel Lichtblau
Wolfram Research


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