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MathGroup Archive 2007

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Re: check inside a loop?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg80767] Re: check inside a loop?
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Sat, 1 Sep 2007 00:26:37 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <fb82ov$7ka$1@smc.twtelecom.net>

Jeremy Price wrote:
> I have a large loop that is ndsolving/nintegrating a bunch of things, and a 
> lot of the results give me various errors due to the equations not being 
> very nice.  I'd like to have a way to check what values of my paramaters are 
> causing the errors, but I can't find a way to do that inside a loop.
> 
> For example, if I have something like,
> f[x_] = 1/Sin[2 pi x]
> 
> For[ i=1, i < 1000, i++,
> f[i]
> ]
> 
> I'm going to get a lot of "Power::infy: Infinite expression 1/0 
> encountered." errors.  I'd like to see what values of i these occur at.

It is possible that you are using a version prior 5.2, because none of 
your examples, even with the typo for Pi fixed, succeed in producing an 
error message.

In[1]:= f[x_] = 1/Sin[2 pi x]
For[i = 1, i < 1000, i++, f[i]]

Out[1]= Csc[2 pi x]

In[3]:= f[x_] = 1/Sin[2 Pi x]
For[i = 1, i < 1000, i++, f[i]]

Out[3]= Csc[2 \[Pi] x]

In[5]:= f[1]

Out[5]= ComplexInfinity

> I've tried something like
> 
> For[ i=1, i<1000, i++,
> Check[ f[i] , i]
> ]
> 
> But this just returns "Power::infy: Infinite expression 1/0 encountered." 
> errors without the i vaule, which is different than I get by evaluating 
> something like
> Check[ f[0],0]
> Which returns:
> Power::infy: Infinite expression 1/0 encountered.
> 0
> 
> Is there any way I can get it to return the index that the error occured at 
> for every error that occurs inside of the loop? 

You can use any function as second argument for Check. For instance, 
Print or Sow will do it:

In[1]:= g[n_] = 1/Mod[5, n]

Out[1]= 1/Mod[5, n]

In[2]:= For[i = 1, i < 10, i++,
  Check[g[i], Print["Division by zero for i = ", i], Power::infy]]

During evaluation of In[2]:= Power::infy: Infinite expression 1/0 \
encountered. >>

During evaluation of In[2]:= Division by zero for i = 1

During evaluation of In[2]:= Power::infy: Infinite expression 1/0 \
encountered. >>

During evaluation of In[2]:= Division by zero for i = 5

In[3]:= Reap[
   For[i = 1, i < 10, i++, Check[g[i], Sow[i], Power::infy]]][[2]]

During evaluation of In[3]:= Power::infy: Infinite expression 1/0 \
encountered. >>

During evaluation of In[3]:= Power::infy: Infinite expression 1/0 \
encountered. >>

Out[3]= {{1, 5}}

-- 
Jean-Marc


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