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.