Re: Find last NZ in list
- To: mathgroup at smc.vnet.net
- Subject: [mg46115] Re: Find last NZ in list
- From: "Bo Le" <bole79 at email.si>
- Date: Sat, 7 Feb 2004 04:01:58 -0500 (EST)
- References: <bvvnir$j5k$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Hi, Your words are not very clear on what you want, either the last nonzero element or its index, but from the examples you provide I guess it is the index. I came up with a nice solution for the element and somewhat the same (fancied up) construct to yours for the index: FindLastNonzeroElement[lst_]:= Module[{tmp=DeleteCases[lst,0]}, If[Length[tmp]!=0,Last[tmp],0] ] FindLastIndex[lst_]:= Catch[ For[i=Length[lst],i>=0,i--, If[lst[[i]]!=0,Throw[i]]; If[i==0,Throw[0]]; ] ] Bye! Borut Levart Slovenia "Carlos Felippa" <carlos at colorado.edu> wrote in message news:bvvnir$j5k$1 at smc.vnet.net... > This is kind of an "esthetics" question. In a program I am writing > I need to find the last nonzero entry of a 1D list millions of > times. For example FindLastNonzero[{1,2,3,4,0,a,b-b,0,0}] -> 6, > and FindLastNonzero[{0,0,0,0,0,0,0}] -> 0. > Here are two ugly C-style implementations: > > FindLastNonzero[a_]:=Module[{n=Length[a]}, > For [i=n,i>0,i--, If [a[[i]]==0,Continue[],Return[i],Return[i]]]; > Return[0]]; > > FindLastNonzero[a_]:=Module[{n=Length[a]}, > For [i=n,i>0,i--, If [a[[i]]!=0,Return[i],Continue[i],Return[i]]]; > Return[0]]; > > Is there a built-in function, or something more elegant, > available to do this job? Find only works for text streams. > > Note that the If case "cannot tell if entry is NZ" is important for > symbolic lists. That case should be evaluated as NZ.