Re : removing of - sign from a list
- To: mathgroup at smc.vnet.net
- Subject: [mg20283] Re [mg20256]: removing of - sign from a list
- From: Ranko Bojanic <bojanic at math.ohio-state.edu>
- Date: Mon, 11 Oct 1999 02:19:55 -0400
- Organization: Ohio State University
- Sender: owner-wri-mathgroup at wolfram.com
In [mg20256] dummy index list Arturas Acus wrote:
> I want the fastest way to select dummy symbols
> from some expression. Suppose we have a list of
> dummy indices {a,-b,c, -d}. What is the fastest way to
> get rid of the minus sign?
Let
testList={a,-b,c, -d}
One solution that comes immediately to mind is
testList /. -x_-> x
{a,-b,c, -d}
The following solution is just as simple, but more difficult to
explain. It seems that in Mathematica we can define f by writing
f[ -x_ ] := x
f[ x_ ] := x
Then
Map[f,testList]
{a,b,c,d}
However, neither method works with numeric lists:
If testList ={1,-2,3,-4},we get
testList / . -x_-> x
{1,-2,3,-4}
Map[f,testList]
{1,-2,3,-4}
We have clearly to use the Abs function in that case:
Map[Abs,testList]
{1,2,3,4}