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MathGroup Archive 1999

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Re(2): Split on the Macintosh

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18412] Re(2): Split on the Macintosh
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Wed, 7 Jul 1999 00:11:08 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Carl

Unfortunately this no longer works in version 4, since the option of
having a new Head as the last argument in Partition has been removed!
Instead of it lots of new optional arguments have been added. The full
info on Partition now gives:

In[6]:=
?Partition
"Partition[list, n] partitions list into non-overlapping sublists of length \
n. Partition[list, n, d] generates sublists with offset d. Partition[list, \
{n1, n2, ... }] partitions a nested list into blocks of size n1 \[Cross] n2 \
\[Cross] \[Ellipsis] . Partition[list, {n1, n2, ... }, {d1, d2, ... }] uses \
offset di at level i in list. Partition[list, n, d, {kL, kR}] specifies that \
the first element of list should appear at position kL in the first sublist, \
and the last element of list should appear at or after position kR in the \
last sublist. If additional elements are needed, Partition fills them in by \
treating list as cyclic. Partition[list, n, d, {kL, kR}, x] pads if
necessary \
by repeating the element x. Partition[list, n, d, {kL, kR}, {x1, x2, ... }] \
pads if necessary by cyclically repeating the elements xi.
Partition[list, n, \
d, {kL, kR}, {}] uses no padding, and so can yield sublists of different \
lengths. Partition[list, nlist, dlist, {klistL, klistR}, padlist] specifies \
alignments and padding in a nested list."

The existence of the optional  4th argument in Partition  versions 2 and
3 was undocumented and I guess that is what can always happen to
undocumented features (Daniel Lichtblau and David Withoff are always
ready to remind us of this).

I am however still using our version of Split with Mathematica 2 and your
improvements is very welcome (although, as you have correctly observed,
it serves a merely academic purpose).

On Wed, Jun 30, 1999, Carl K.Woll <carlw at fermi.phys.washington.edu> wrote:

>Hi Andrzej,
>
>Apparently I never sent you a slight improvement (20% faster on my
machine) of
>the split function. Simply use the optional fourth argument of Partition:
>
>split2[li_,testQ_:SameQ]:=Module[{r1, r2},
>    r1 = Flatten[Position[Partition[li, 2, 1,testQ], False]];
>    r2 = Transpose[{Join[{1}, r1 + 1], Join[r1, {Length[li]}]}];
>    Take[li, #] & /@ r2]
>
>If Split is still quadratic on the Mac, then the above improvement may
be more
>than academic (although, of course, you are an academic, right?).
>
>By the way, I'm jealous. Since I'm a poor student, I will have to wait until
>August to get the new version.
>
>Carl Woll
>Physics Dept
>U of Washington
>
>Andrzej Kozlowski wrote:
>
>> Today at last I got my Mathematica 4.0 upgrade, much earlier than I ever
>> expected. I am very grateful to the mathgroup whose very existence has
>> undoubtedly contributed to this, from my point of view,  remarkable and
>> happy event.
>>
>> The very first thing I did after installing Mathematica 4 was to go over my own
>> personal list of bugs and problems that plagued the previous version to
>> check which have and which have not been fixed. I am very pleased to be
>> able to say that the first group is far larger (if one excludes functions
>> contained in various packages, which appear to have not been changed at
>> all. Particularly, given the improved performance of the built-in Limit
>> function, it seems to me that one should now generally avoid the
>> Calculus`Limit` package more then ever).
>>
>> However, the problem that I was most curious about still persists. It was
>> something discovered  about a year ago by Xah Lee, Carl Woll and myself
>> and concerns the Split function. At that time I wanted to emulate it in
>> mathematica 2.2 and we produced various candidates and compared their
>> efficiency. Then we discovered a curious thing. Our best attempt was the
>> following function:
>>
>> split2[li_, testQ_:SameQ] :=
>>   Module[{r1, r2},
>>     r1 = Flatten[Position[Apply[testQ, Partition[li, 2, 1], {1}], False]];
>>     r2 = Transpose[{Join[{1}, r1 + 1], Join[r1, {Length[li]}]}];
>>     Take[li, #] & /@ r2]
>>
>> We found that on the Macintosh alone (strictly speaking we only tested
>> PPC Macs) this function is more efficient than the built in Split for
>> very large lists. Not only that, its lead over Split grows with the size
>> of the list. This is not true on other platforms where the built-in Split
>> is much faster. Moreover, careful investigation revealed that on the Mac
>> the built in Split function does not scale linearly but seems to have a
>> non-negligible quadratic component, while split2 is linear. On other
>> platforms, however, Split scales linearly.
>>
>> This is strange. One reason is that the Kernel is supposed to work the
>> same way on all platforms so one would imagine that basic things as
>> asymptotic growth of functions would be roughly the same. Moreover, the
>> algorithms used certainly scale linearly: after all if we were able to
>> produce a linear function that one would certainly expect wri to be able
>> to do so.
>>
>> Xah Lee reported this problem to wri's techincal support. So I was
>> curious to see if there would be any change in this respect in version 4.
>>
>> I have just run my tests and found that while Split has become
>> considerably faster in version 4, it still shows non-linear asymptotic
>> behaviour and is still beaten by split2 for v. large lists. Those who
>> would like to see these results (valid only on the Mac) can download the
>> notebook:
>> <ftp://cleo.tuins.ac.jp//pub/Splitvsplit.nb>
>>
>> The most likely explanation seems to be that this might be something to
>> do with the memory management of the Mac. Yet it is strange that it seems
>> to affect only just one function, and that wri have not been able to do
>> anything about it. This is not necessarily a trivial matter because it is
>> precisely when dealing with very large lists that Split becomes very
>> useful. Another thing that it illustrates (in my opinion) is the
>> difficulties in applying purely theoretical efficiency arguments in the
>> case of a program like Mathematica in which built in function splay such
>> a big role.
>>
>> Andrzej Kozlowski
>>
>> Toyama International University
>> JAPAN
>> http://sigma.tuins.ac.jp/
>> http://eri2.tuins.ac.jp/
>
>
>
>
>


Andrzej Kozlowski
Toyama International University
JAPAN
http://sigma.tuins.ac.jp/
http://eri2.tuins.ac.jp/



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