Re: Problem with Thread over Plus?

• To: mathgroup at smc.vnet.net
• Subject: [mg65389] Re: [mg65370] Problem with Thread over Plus?
• From: "Virgilio, Vincent - SSD" <Vincent.Virgilio at itt.com>
• Date: Thu, 30 Mar 2006 05:29:40 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

```Thank you, I see now.

I wasn't thinking correctly about the original expressions.  In fact,
Mathematica handles both of them properly.

Vince Virgilio

-----Original Message-----
From: Andrzej Kozlowski [mailto:akoz at mimuw.edu.pl]
To: mathgroup at smc.vnet.net
Subject: [mg65389] Re: [mg65370] Problem with Thread over Plus?

> On 29 Mar 2006, at 13:34, 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
>
>

Thread evaluates its argument(s) so in the first case you get the
equivalent of this

Plus[{1, 1}, {{1, 1}, {2, 2}}]

{{2, 2}, {3, 3}}

{{2, 3}, {2, 3}}

while in the second

Plus[ {1, 1} , { {1, 1} , {2, 2} , {3, 3} } ]

tries to add lists of different lengths and fails.

In fact, are you sure that you really meant to get the answer you did
get in the case you claim "works"? Did you not rather mean this:

Thread[Unevaluated[Plus[{1, 1} , {{1, 1}, {2, 2}}]]]

{{2, 2}, {3, 3}}

?

It is a little harder to make the second case work in the same way but
of course it can be done:

Thread[ Unevaluated[Plus[ {1,1} , { {1,1} , {2,2} , {3,3} } ] ],List,-1]

{{2, 2}, {3, 3}, {4, 4}}

Andrzej Kozlowski

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