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MathGroup Archive 2005

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Re: How to reverse sign on the y's in a list of (x, y)'s?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg61267] Re: [mg61231] How to reverse sign on the y's in a list of (x, y)'s?
  • From: János <janos.lobb at yale.edu>
  • Date: Fri, 14 Oct 2005 05:54:09 -0400 (EDT)
  • References: <200510130539.BAA04564@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Oct 13, 2005, at 1:39 AM, sherifffruitfly at gmail.com wrote:

> Hi all,
>
> This is really dumb, but I just don't know how to do it...
>
> Import from excel gives me a list of ordered pairs, and I want the 2nd
> element of each pair to be of opposite sign. Sticking the negative  
> sign
> in front of the entire Import statement reverses both the 1st and the
> 2nd elements (duh). How do I get just the second item to reverse sign?
>
> thanks!

Here is a newbie approach:

In[1]:=
lst = Table[{x, (-1)^i*y},
    {i, 1, 5}]
Out[1]=
{{x, -y}, {x, y}, {x, -y},
   {x, y}, {x, -y}}

In[2]:=
newlst =
   ({#1[[1]], -#1[[2]]} & ) /@
    lst
Out[2]=
{{x, y}, {x, -y}, {x, y},
   {x, -y}, {x, y}}

János


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