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MathGroup Archive 2006

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Re: Problem with Thread over Plus?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65390] Re: Problem with Thread over Plus?
  • From: dh <dh at metrohm.ch>
  • Date: Thu, 30 Mar 2006 05:29:41 -0500 (EST)
  • References: <e0dsg9$r67$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Vincent,
The argument of Thread will be evaluated before beeing fed to Thread. 
You are actually feeding the result of Plus[ {1,1} , { {1,1} , {2,2} } 
], namely:

List[{1 + a, 2 + b}, {1 + a, 2 + b}]

to thread. The function "List" is then threaded.

On the other hand,
Plus[ {1,1} , { {1,1} , {2,2} , {3,3} } ]
will produce the error you saw.

However, your first attempt does not actually give the correct answer. 
If you want to thread Plus over {1,1} and  { {1,1} , {2,2} } you would 
use MapThread:
MapThread[Plus, {{1, 1}, {{1, 1}, {2, 2}}}]
and get:
{{2, 2}, {3, 3}}

Daniel

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} } ] ]
> 
> Thanks,
> 
> Vince Virgilio
> 
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