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

• To: mathgroup at smc.vnet.net
• Subject: [mg61266] Re: How to reverse sign on the y's in a list of (x, y)'s?
• From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
• Date: Fri, 14 Oct 2005 05:54:05 -0400 (EDT)
• Organization: The Open University, Milton Keynes, UK
• References: <dikt77\$4tm\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```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!
>
Hi,

One way of doing this is as follows. First we create some pairs of
random machine-precision real numbers):

In[1]:=
data = Table[{Random[], Random[]}, {4}]

Out[1]=
{{0.519316,0.121246},{0.255378,0.733313},{
0.477478,0.170173},{0.673604,0.86816}}

Then we instruct Mathematica to take _all_ the lines and the second
column only of the matrix, change the sign and store the new values at
the same place

In[2]:=
data[[All,2]] = -data[[All,2]]

Out[2]=
{-0.121246,-0.733313,-0.170173,-0.86816}

We can check that data as the required form

In[3]:=
data

Out[3]=
{{0.519316,-0.121246},{0.255378,-0.733313},{
0.477478,-0.170173},{0.673604,-0.86816}}

Best regards,
/J.M.

```

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