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MathGroup Archive 2005

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Re: Strange bug

  • To: mathgroup at smc.vnet.net
  • Subject: [mg53285] Re: Strange bug
  • From: Peter Pein <petsie at arcor.de>
  • Date: Tue, 4 Jan 2005 03:13:32 -0500 (EST)
  • References: <crb40r$akl$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Steve Gray wrote:
> I have procedural function "ordermat." It works ok if the next-to-last (useless) line is there but
> if I remove that one line, I get error messages due to the last line,  at execution time:
> 
> Set::write: Tag Times in Null\{0, 0, 0, 0, 0, 0, 0, 0, 0, 0, \[LeftSkeleton]1\
> \[RightSkeleton]} is Protected.
> 
> ("intersect" returns a small integer and computes "tafr.")
> 
> I've seen bugs like this before. I'm using 5.0.1.0 with Windows. Any clues? Thanks in advance.
> 
> Steve Gray
> 
> 
> 
> ordermat[gon_] := Module[{row, rx, cx},
>     gonl = Length[gon];
>     matx = Table[0, {i, 1, gonl}, {j, 1, gonl}];
>     gona = Join[gon, gon];
>     For [ rx = 1, rx <= gonl, rx++,
>       	row  = Table[0, {2*gonl}];
>       	row[[rx]] = 1;
>       	row[[rx + 1]] = 2;
>                ab = gona[[rx]];
>       	ae = gona[[rx + 1]];
>       	For [ cx = 1, cx <= gonl - 3, cx++,
>           	       bb = gona[[cx + rx + 1]];
>           	       be = gona[[cx + rx + 2]];
>           	       int = intersect[ab, ae, bb, be];
>           	       If [ int == 3, row[[cx + rx + 1]] = tafr + 1];
>           	   ]
>                rx;                  (*   Error unless some expression is here.?? *)
>                matx[[rx]] = Table[Max[row[[i]], row[[i + gonl]]], {i, gonl}];
>            ]
>     ] 
> 
Hi Steve,

removing "rx" while leaving the semicolon should do it. Without 
semicolon Mathematica tries to assign Table[Max...] to the product 
"For[..]matx[[rx]]" - or to Null*matx[[rx]], depending on the presence 
of a \n between For.. and matx.

Peter

-- 
Peter Pein
Berlin


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