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MathGroup Archive 2000

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Re: RE: Re: User problem

  • To: mathgroup at smc.vnet.net
  • Subject: [mg24079] Re: [mg24023] RE: [mg23997] Re: [mg23971] User problem
  • From: john.tanner at baesystems.com
  • Date: Fri, 23 Jun 2000 02:26:46 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

The "Magic Number" coincides with the point where Table starts to
produce PackedArrays: 

In[1]:= Developer`PackedArrayQ[Table[i, {i, 249}]]
Out[1]:= False

In[2]:= Developer`PackedArrayQ[Table[i, {i, 250}]]
Out[2]:= True

This break point of 250 seems to be for the total number of index
entries in the Table, thus a multi-dimensional Table has a break point
at (e.g.) Table[{i,j}, {i, 5},{j, 50}].  I cannot find a reference to
this in the documentation, but I appreciate there should be such a
break point even if I would like some control over it!

This sometimes has implications for expressions which are tested at a
small number of points and then used with a larger number: it is wise
to check with different Table sizes if you suspect the behaviour would
change.  PackedArrays are extremely useful, and their near-seamless
incorporation into Mathematica is a real joy, but they do behave
differently in some circumstances even in "legal" code.  It is
interesting, but not surprising given the output is in a different
form, to note this difference in "illegal" code.


hwolf (hwolf at debis.com@SMTP at MSMR02) writes:
>
>Notwithstanding the original code being illegal, 250 appears as a
>magic number. Compare:
>
>In[2]:= Table[i = i + 1, {i, 249}]
>
>In[3]:= Table[i = i + 1, {i, 250}]
>
>-- Hartmut
>
>> -----Original Message-----
>> From:	BobHanlon at aol.com [SMTP:BobHanlon at aol.com]
To: mathgroup at smc.vnet.net
>To: mathgroup at smc.vnet.net
>> Sent:	Monday, June 19, 2000 7:46 AM
>> To:	mathgroup at smc.vnet.net
>> Subject:	[mg23997] Re: [mg23971] User problem
>> 
>> 
>> In a message dated 6/18/2000 3:39:33 AM, joseph468 at yahoo.com
>> writes:
>> 
>> >I am trying to contstruct a table of random integers
>> >between 1 and 4. I'd like to vary the number of
>> >elements in the table from time to time. The
>> >contruction that I use in Mathematica is as follows:
>> >
>> >genome = Table[i = Random[Integer, {1, 4}], {i, 249}]
>> >
>> >The problem with this is as follows: the above
  >> >generates tables of varying length up to and including
>> >249 elements.  However, when I try to generate any
>> >table containing more than 249 elements, I get the
>> >following error message on my screen:
>> >
>> >This program has performed an illegal operation and
>> >will be shut down.  If the problem persists please
>> >contact the vendor.
>> >
>> >I have version 4 of Mathematica, and am running it on
>> >a Windows98 platform (Dell NoteBook)
>> >
>> 
>> genome = Table[Random[Integer, {1, 4}], {249}];
>> 
>> 
>> Bob Hanlon



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