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Re: Stop iteration iconstructing Table

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90603] Re: Stop iteration iconstructing Table
  • From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
  • Date: Tue, 15 Jul 2008 06:18:39 -0400 (EDT)
  • Organization: The Open University, Milton Keynes, UK
  • References: <g59tn8$omh$1@smc.vnet.net> <g5f5ug$rbu$1@smc.vnet.net>

dh wrote:
> Hi Angel,
> 
> if you stop a Table command, it does not return the result. You may do 
> 
> this using a breakable loop (Do,While,For) and Append. However, if you 
> 
> use an ordinary list to append to, this is very inefficient, as the 
> 
> whole list is reallocated each time. Therefore, the it is much faster to 
> 
> create a nested list, that  does not need reallocation. E.g. to create a 
> 
> list from 1..5 with a loop that runs to 10
> 
> list={};
> 
> Do[
> 
>     AppendTo[list,{i}];
> 
>     If[i>=5,Break[]];
> 
> ,{i,10}];
> 
> list= Flatten[list]
> 
> hope this helps, Daniel

Just to add to Daniel's excellent advice, you could investigate how to 
use the Sow[] and Reap[] functions. For instance,

     In[1]:= Reap[Do[If[i >= 5, Break[], Sow[i]], {i, 10}]][[2, 1]]

     Out[1]= {1, 2, 3, 4}

Regards,
-- Jean-Marc


> malawk at gmail.com wrote:
> 
>> Hi
> 
> 
>> Sorry I'm not very familiar with Mathematica. I want to create a
> 
>> simple Table of this sort:
> 
> 
>> Table[{{$1, $2, $3, ...}, {...}, ..., {...}}, {i,1,imax}],
> 
> 
>> where $2 decreases as the iteration proceeds, and imax should be such
> 
>> that only those elements with $2 >= 100 are returned. When $2 goes
> 
>> below 100 then the iteration should  stop and the final Table given. I
> 
>> want to do this because I'm not interested in computing those elements
> 
>> whose $2 <100.
> 
> 
>> Any ideas how can I do this simple thing?
> 
> 
>> Thank you so much!
> 
>> Angel
> 
> 
> 
> 
> 
> 
> 
> 



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