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MathGroup Archive 2006

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Re: Problem with Thread over Plus?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65391] Re: [mg65370] Problem with Thread over Plus?
  • From: "W. Craig Carter" <ccarter at mit.edu>
  • Date: Thu, 30 Mar 2006 05:29:42 -0500 (EST)
  • References: <200603291134.GAA27493@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

>
> Why does this work:
>
> 	Thread[ Plus[ {1,1} , { {1,1} , {2,2} } ] ]

One way to think of it is that you are attempting to add a vector of 
length 2 to each column vector of a 2x2 matrix. (e.g., consider that
Thread[Plus[{a, b, c}, Transpose[{{1, 1, c}, {2, 2, d}}]]]//MatrixForm
does not give an error, but
Thread[Plus[{a, b, c}, {{1, 1, c}, {2, 2, d}}]]//MatrixForm
does give an error.)

Another way to think of it is that:
It adds:
a) the list {1,1} to the list of first elements of the second list 
{1,2}
producing {2,3}
b) the list {1,1} to the list of second elements of the second list 
{1,2} producing {2,3}


>
> but this doesn't?
>
> 	Thread[ Plus[ {1,1} , { {1,1} , {2,2} , {3,3} } ] ]

This attempts to add:

a) the list {1,1} to the list of first elements {1,2,3} and incurs 
an error.

It is not clear what you wanted---do you wish to add a 1 to each 
element of a list? Then 1 + { {a,b},{c,d}}
would do the trick.


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