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Re: Re: Set::setraw error

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90382] Re: [mg90352] Re: [mg90326] Set::setraw error
  • From: DrMajorBob <drmajorbob at att.net>
  • Date: Tue, 8 Jul 2008 02:24:21 -0400 (EDT)
  • References: <32330841.1215353835393.JavaMail.root@m08>
  • Reply-to: drmajorbob at longhorns.com

Sorry again, the infinite loop was cause by my leaving out initializations  
for x, y, and r.

Clear[a, b, xgcd]
xgcd[0, 0] = {0, 0, 1};
xgcd[0, b_] = {Abs@b, 0, Sign@b};
xgcd[a_, 0] = {Abs@a, Sign@a, 0};
xgcd[aa_, bb_] :=
  Module[{a = Abs@aa, b = Abs@bb, x = 1, y = 0, r = 0, s = 1, c, q},
   While[b != 0, {c, q} = {Mod[a, b], Quotient[a, b]};
    {a, b, r, s, x, y} = {b, c, x - q*r, y - q*s, r, s}];
   {a, x*Sign@aa, y*Sign@bb}]
xgcd[2, 3]

{1, -1, 1}

Bobby

On Mon, 07 Jul 2008 11:46:12 -0500, DrMajorBob <drmajorbob at att.net> wrote:

> I had a copying error or something that duplicated the line
>
> xgcd[aa_,bb_]:=Module[{a=Abs@aa,b=Abs@bb},
>
> Take one of them out.
>
> Bobby
>
> On Mon, 07 Jul 2008 04:05:31 -0500, DrMajorBob <drmajorbob at att.net>  
> wrote:
>
>> The error message tells you that you can't Set variables used as
>> arguments, in this case a=-a, for instance.
>>
>> There are other problems, including the fact that x, y, and r are
>> undefined the first time you use them in
>>
>> {a, b, r, s, x, y} = {b, c, x - q*r, y - q*s, r, s}
>>
>> which means (if you get rid of the "setraw" error), that statement will  
>> be
>> an infinite loop all by itself.
>>
>> If we knew what you were trying to accomplish, we might go farther, but
>> for now I'll show you a partial solution WITH the infinite loop still
>> present:
>>
>> Clear[a,b,xgcd]
>> xgcd[0,0]={0,0,1};
>> xgcd[0,b_]={Abs@b,0,Sign@b};
>> xgcd[a_,0]={Abs@a,Sign@a,0};
>> xgcd[aa_,bb_]:=Module[{a=Abs@aa,b=Abs@bb},
>> xgcd[aa_, bb_] := Module[{a = Abs@aa, b = Abs@bb},
>>    While[b != 0,
>>     {c, q} = {Mod[a, b], Quotient[a, b]};
>>     Print@{c, q};
>>     {a, b, r, s, x, y} = {b, c, x - q*r, y - q*s, r, s}];
>>    {a, x*Sign@aa, y*Sign@bb}
>>    ]
>> xgcd[2,3]
>>
>> {2,0}
>> $IterationLimit::itlim: Iteration limit of 4096 exceeded. >>
>> $IterationLimit::itlim: Iteration limit of 4096 exceeded. >>
>> $IterationLimit::itlim: Iteration limit of 4096 exceeded. >>
>> General::stop: Further output of $IterationLimit::itlim will be  
>> suppressed
>> during this calculation. >>
>> {1,1}
>> {0,2}
>> {1,-Hold[r]+Hold[x],-Hold[s]+Hold[y]}
>>
>> Notice I've changed the structure of the function to one that's much
>> clearer (IMHO).
>>
>> To become proficient in Mathematica you need to, among other things,  
>> erase
>> Return from your memory.
>>
>> Never use it again. EVER.
>>
>> Bobby
>>
>> On Sun, 06 Jul 2008 06:17:46 -0500, Steven Siew <stevensiew2 at gmail.com>
>> wrote:
>>
>>> I have encountered this Set:setraw error for the code below.
>>>
>>> I'm stumped. Does anyone have any idea what went wrong?
>>>
>>> xgcd[a_,b_]:=Module[{xsign=1,ysign=1,x=1,y=0,r=0,s=1,c,q},
>>>     If[a == 0 && b == 0,Return[{0,0,1}]];
>>>     If[a == 0,Return[{Abs[b],0,b/Abs[b]}] ];
>>>     If[b == 0,Return[{Abs[a],a/Abs[a],0}]];
>>>     If[a<0,a=-a; xsign=-1];
>>>     If[b<0,b=-b; ysign=-1];
>>>     While[b\[NotEqual]0,
>>>       {c,q}={Mod[a,b],Quotient[a,b]};
>>>       {a,b,r,s,x,y}={b,c,x-q*r,y-q*s,r,s}
>>>       ];
>>>     Return[{a,x*xsign,y*ysign}]
>>>     ]
>>>
>>> xgcd[2, 3]
>>>
>>> \!\(\*
>>>   RowBox[{\(Set::"setraw"\), \(\(:\)\(\ \)\), "\<\"Cannot assign to
>>> raw \
>>> object \\!\\(2\\). \\!\\(\\*ButtonBox[\\\"More\[Ellipsis]\\\", \
>>> ButtonStyle->\\\"RefGuideLinkText\\\", ButtonFrame->None, \
>>> ButtonData:>\\\"Set::setraw\\\"]\\)\"\>"}]\)
>>>
>>>
>>
>>
>>
>
>
>



-- 
DrMajorBob at longhorns.com


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