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

• To: mathgroup at smc.vnet.net
• Subject: [mg61352] Re: [mg61240] Re: [mg61231] How to reverse sign on the y's in a list of (x, y)'s?
• From: Murray Eisenberg <murray at math.umass.edu>
• Date: Sun, 16 Oct 2005 00:18:05 -0400 (EDT)
• Organization: Mathematics & Statistics, Univ. of Mass./Amherst
• References: <200510130539.BAA04564@smc.vnet.net> <200510140953.FAA28364@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Method 1 fails when "data" is a list of just two pairs of numbers!

data = {{1, 2}, {3, 4}};
data /. {a_, b_} -> {a, -b}
{{1, 2}, {-3, -4}}

curtis wrote:
> Hey,
>
> (**** Method 1: pattern matching ****)
>
> data = {{0, 1}, {0.1, 1.1}, {0.2, 1.2}, {0.3, 1.3}, {0.4, 1.4}, {0.5, 1.5}, {0.6,
>
>     1.6}, {0.7, 1.7}, {0.8, 1.8}, {0.9, 1.9}, {1., 2.}}
>
> data /. {a_, b_} -> {a, -b}
>
> (**** Method 2: ripping apart, negating, putting back together ****)
>
> Needs["Statistics`DataManipulation`"]
>
> Transpose[{Column[data, 1], -Column[data, 2]}]
>
>
> Of course, there are a lot of ways to do the same thing. I'm not sure
> what's fastest, but either of these methods works fine for me on
> moderately large arrays.
>
>            Cheers,
>                 C. O.
>
>
>
> 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!
>>
>>
>>
>>
>
>

--
Murray Eisenberg                     murray at math.umass.edu
Mathematics & Statistics Dept.
Lederle Graduate Research Tower      phone 413 549-1020 (H)
University of Massachusetts                413 545-2859 (W)
710 North Pleasant Street            fax   413 545-1801
Amherst, MA 01003-9305

```

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