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MathGroup Archive 2006

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Re: Problem with Thread over Plus?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65405] Re: [mg65370] Problem with Thread over Plus?
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Thu, 30 Mar 2006 05:30:04 -0500 (EST)
  • References: <200603291134.GAA27493@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Mar 29, 2006, at 6:34 AM, Virgilio, Vincent - SSD wrote:

>
>
> Hello,
>
> Why does this work:
>
> 	Thread[ Plus[ {1,1} , { {1,1} , {2,2} } ] ]
>
> but this doesn't?
>
> 	Thread[ Plus[ {1,1} , { {1,1} , {2,2} , {3,3} } ] ]

In the first case the argument lists are the same length

Thread[Plus[{1,1},{{1,1},{2,2}}]] =
Thread[{{2,2},{3,3}}] (since the argument to thread is evaluated  
first) =
Thread[List[{2,2},{3,3}] (I'm just explicitly listing the head here  
to make what happens clear) =
{List[2,3],List[2,3]} (This is the result of threading List over its  
arguments) =
{{2,3},{2,3}}

I suspect this is not what you intended even though the result is  
equivalent, I believe you wanted to thread Plus over its arguments,  
but remember it  got evaluated before Thread was applied.  There was  
a recent mailing list discussion of the interaction of Thread and the  
evaluator starting here http://forums.wolfram.com/mathgroup/archive/ 
2006/Feb/msg00323.html.

At any rate In the second case the argument lists are not the same  
length.  Thread[Plus[{1,1},{{1,1},{2,2},{3,3}}]] gives two warning  
messages the first of which is due to the expression Plus[{1,1}, 
{{1,1},{2,2},{3,3}}] which issues a warning message upon evaluation  
because you cannot add {1,1} to {{1,1},{2,2},{3,3}}.  The second  
warning message is due to the attempt to evaluate Thread[Plus[{1,1}, 
{{1,1},{2,2},{3,3}}]].

Regards,

Ssezi


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