Re: Matrix Element Extraction
- To: mathgroup at smc.vnet.net
- Subject: [mg46643] Re: [mg46622] Matrix Element Extraction
- From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
- Date: Thu, 26 Feb 2004 17:53:35 -0500 (EST)
- References: <200402251807.NAA05947@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
On 25 Feb 2004, at 19:07, Curt Fischer wrote:
> In a matrix, I want to find the positions of all the integer elements
> in the
> rightmost column or the bottom row.
>
> In[1]:=
> mat={{1,2,1},{4,5,6},{7,8,-Infinity}}
>
> Out[1]=
> {{1,2,1},{4,5,6},{7,8,-Infinity}}
>
> In[2]:=
> Position[Map[((MemberQ[Union[Flatten[{mat[[Length[mat]]],
> mat[[All,Dimensions[mat][[
> 2]] ]]}]],#])&&(#!=-Infinity))&,mat,{2}],True]
>
> Out[2]=
> {{1,1},{1,3},{2,3},{3,1},{3,2}}
>
> This _mostly_ does what I want, but took me forever to think of. I'm
> convinced there must be an easier way...
>
> Especially, is there an easy way to keep the point {1,1} from
> appearing on
> this
> list, other than using a replacement rule and an If[] command? ({1,1}
> is
> not a position in the bottom row or right most column, so I don't want
> this
> point returned.)
>
> Thanks for any advice.
>
> --
> Curt Fischer
>
>
>
How about this:
f[mat_?MatrixQ] := Block[{d = Dimensions[mat], dog, cat},
Join @@ Last[Reap[MapIndexed[If[IntegerQ[#1] && (First[#2] ==
First[
d] || Last[#2] == Last[d]), Sow[#2, dog], Sow[#2, cat]] &, mat,
{2}],
dog]]]
Now with:
mat = {{1, 2, 1}, {4, 5, 6}, {7, 8, -Infinity}};
f[mat]
{{1,3},{2,3},{3,1},{3,2}}
Andrzej Kozlowski
Chiba, Japan
http://www.mimuw.edu.pl/~akoz/
- References:
- Matrix Element Extraction
- From: "Curt Fischer" <crf3@po.cwru.edu>
- Matrix Element Extraction