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MathGroup Archive 2004

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Re: More on Delete Problems

  • To: mathgroup at smc.vnet.net
  • Subject: [mg51732] Re: More on Delete Problems
  • From: p-valko at tamu.edu (Peter Valko)
  • Date: Sat, 30 Oct 2004 03:49:00 -0400 (EDT)
  • References: <cl9t75$7nl$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Once again, Andzej was right (and mature in his judgement). 

He convinced me that the problem is caused by the "Packed Array
Technology".
It seems, that the Dot[] function (or in short-hand the "." itself)
brings in the packed array. Then the Delete may (or may not) cause a
memory violation.

Therefore I suggest a temporary workaround:

Unprotect[Dot]; Dot[x_, y_] := Inner[Times, x, y, Plus]; Protect[Dot];
a = {{1, 2}, {3, 4}};
b = {{1, 0}, {0, 1}};
c = a.b
Delete[c, {1, 1}]

Without the first line, the above code (run from scratch) crashes
Mathematica (Version 5, Windows XP)

With the first line added, packed arrays will not be used at all and
there will be no crash. Of course, when packed arrays will not be
used, Mathematica will be slower and more memory hungry on typical
number crunching applications.

"Dot" seems to be not the only function invoking packed array
technology, but far the most dangerous, because it applies the
technology even to a four - element matrix. "Table" also brings in
packed arrays, but only at heavier usage.

In fact the shortest "program" I could crash my Mathematica with is
the following "oneliner" (run from scratch):
 
Delete[Table[{1, 1}, {250}], {1, 1}]

Evaluating it first works OK, but the second evaluation of the same
line causes crash (Version 5, Windows XP, starting from scratch).
Also, it does not matter if you put a semicolon after it or not.

Using the ByteCount I can see what is the difference between a 249-
and a 250- row Table:
 ByteCount[Table[{1,1},{249}]]  is 16952
 and   
 ByteCount[Table[{1,1},{250}]]  is 2060, 

that is in the second case approximately 500*4 byte is used, the same
as would be used by a Fortran (C, VB)-like integer matrix of 500
elements.

I think before a next version comes out a temporary solution might be
a way to switch off PackedArray usage for the Session. I wonder if
anybody knows how to do this (something like Off[PackedArrays] ? ...)


Regards and Thanks
Peter




Gove Effinger <effinger at skidmore.edu> wrote in message news:<cl9t75$7nl$1 at smc.vnet.net>...
> Here's simple code which bombs for me:
> 
> a={{1.,2.,3.},{4.,5.,6.}}
> 
> b = {{1,0,1},{0,-2,3}, {0,1,0}}
> 
> c = a . b
> 
> Delete[c,{1,1}]
> 
> Why?
> 
> G. Effinger


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