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MathGroup Archive 2002

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Re: Bug in Sum causes index variable to remain set?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33897] Re: [mg33838] Bug in Sum causes index variable to remain set?
  • From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
  • Date: Sun, 21 Apr 2002 06:14:48 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

I forgot a couple of things. First, your second example seems to be 
wrong. At least with Mathematica 4.1 for MacOS X I get:

In[1]:=
Sum[1, {f[x, n], 1, 3}]

Out[1]=
3

In[2]:=
?f
Global`f

So only when you actually perform an evaluation within f that you get 
the behaviour you observed, e.g.

In[3]:=
Sum[1, {f[x*1, n], 1, 3}]

Out[3]=
3

In[4]:=
Information["f", LongForm -> False]

Global`f

f[x, n] = 3


Also, it seems to me that the simplest solution to your problem (which I 
am not prepared to call a bug) is to wrap Block around Sum as follows:

In[8]:=
Clear[f]

In[9]:=
Block[{f}, Sum[1, {f[x, n*1], 1, 3}]]

Out[9]=
3

In[10]:=

Information["f",LongForm -> False]

Global`f


On Sunday, April 21, 2002, at 03:46  PM, Andrzej Kozlowski wrote:

> I don't know whether this is a bug or not, and it seems to me that this 
> is just the sort of situation where we lack a clear definition of what 
> constitutes a bug (Behaviour undesirable to some users? Unintended 
> undesirable behaviour? Easily avoidable undesirable behaviour?). 
> However, I can point out that this behaviour is a universal feature of 
> Mathematica design in analogous cases. The same thing will happen if 
> you replace Sum by Product, Table or Do.  Integrate is not a good 
> example because it does not assign values to the variable of 
> integration (it just finds the antiderivative and evaluate limits). 
> NIntegrate would have been a better example, but it actually also does 
> not make assignments to an iterator. So bug or not it seems to be a 
> feature (feature!) of  Mathematica"s design, more precisely of the way 
> Matheamtica uses iterators.
>
>  Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/
>
>
>
>
>
> On Friday, April 19, 2002, at 03:28  PM, Martin Jenkins wrote:
>
>>
>> Hello, I have found what appears to be a bug in Sum in Mathematica 4.0.
>> I stumbled across is trying to sum an expression with subscripts.
>> Anyway:
>>
>> In[1]:= n = 1;
>>
>> In[2]:= Sum[1, {f[x, 1], 1, 3}]
>> Out[2]= 3
>> In[3]:= ?f
>> In[4]:= Sum[1, {f[x, n], 1, 3}]
>> Out[4]= 3
>> In[5]:= ?f
>>
>> In[6]:= Clear[f];
>> In[7]:= Sum[1, {f[x, 1*1], 1, 3}]
>> Out[7]= 3
>> In[8]:= ?f
>>
>> The way that f gets set by the Sum to the final value is a bug is it
>> not?  I would guess that Sum unsets the literal expression f[x, n] or
>> f[x, 1*1], rather than exactly what it set in the first place.  I can
>> get around this by f[x,n]=. whenever I use it... but I would rather
>> not have a line with a comment saying "Mathematica bug work-around".
>> Integrate works fine under similar circumstances.
>>
>> Thanks,
>> Martin
>>
>>
>>
>>
>



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