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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: [mg61275] Re: [mg61231] How to reverse sign on the y's in a list of (x, y)'s?
  • From: ggroup at sarj.ca
  • Date: Fri, 14 Oct 2005 05:54:36 -0400 (EDT)
  • References: <200510130539.BAA04564@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Assuming:
dat = Table[{i,Random[]},{i,10}];

One way could be:
reversedat1 = {#[[1]],-#[[2]]}& /@ dat

Or:
reversedat2 = Transpose[ {dat[[All,1]], -dat[[All,2]]} ]

The second option is far faster...


On Thu, 13 Oct 2005 01:39:38 -0400 (EDT), sherifffruitfly 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!





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