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.