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

*To*: mathgroup at smc.vnet.net*Subject*: [mg61278] Re: How to reverse sign on the y's in a list of (x, y)'s?*From*: "Bill White" <minutiae at gmail.com>*Date*: Fri, 14 Oct 2005 05:54:39 -0400 (EDT)*References*: <dikt77$4tm$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Here's a simple-minded approach (to help me learn about lists :-). First, a simple list: list = {{a, 1}, {b, 2}, {c, 3}} Note that we can grab the elements in question with this: list[[All, 2]] We can make them negative by Map'ing Minus over them: Minus /@ list[[All, 2]] And the Help Browser says, "You can always reset one or more pieces of a list by doing an assignment like m[[ \[Ellipsis] ]] = value." So we should be able to change the sign of the 2nd elements with this: list[[All, 2]] = Minus /@ list[[All, 2]] Indeed, here is the new list: In[18]:= list Out[18]= {{a,-1},{b,-2},{c,-3}} -------------------------------------- Summary: In[19]:= list={{a,1},{b,2},{c,3}} Out[19]= {{a,1},{b,2},{c,3}} In[20]:= list[[All,2]]=Minus/@list[[All,2]] Out[20]= {-1,-2,-3} In[21]:= list Out[21]= {{a,-1},{b,-2},{c,-3}} Cheers