Re: Position of Last Element in List
- To: mathgroup at smc.vnet.net
- Subject: [mg94148] Re: Position of Last Element in List
- From: Szabolcs Horvát <szhorvat at gmail.com>
- Date: Fri, 5 Dec 2008 07:05:27 -0500 (EST)
- Organization: University of Bergen
- References: <ghb02f$oho$1@smc.vnet.net>
Raffy wrote:
> I'm looking for the fastest way to find the last position of an
> element at level 1 in a list.
>
> The fastest way, which is still extremely bloated and extremely slow,
> appears to be:
>
> lastPos[v_List,p_:True,default_:$Failed]:=(
> $Temp=Position[Reverse[v],p,{1},1,Heads->False];
> If[Length[$Temp]>0,$Temp[[1,1]],default]
> );
>
> Any suggestions?
>
> Use the following code to compare against lastPos:
>
> v = Range[1000000];
> Do[lastPos[v,900000],{100}]//Timing
>
This is about 4-5 times faster on my machine (possibly because it avoids
unpacking the array):
lastPos =
Compile[{{v, _Integer, 1}, {e, _Integer, 0}},
Module[{i = Length[v]}, While[i > 0 && e != v[[i]], i--]; i]]
But it only works with lists of integers (it's possible to write a
similar function for reals).