Re: Position of Last Element in List
- To: mathgroup at smc.vnet.net
- Subject: [mg94158] Re: Position of Last Element in List
- From: SigmundV <sigmundv at gmail.com>
- Date: Sat, 6 Dec 2008 06:13:50 -0500 (EST)
- References: <ghb02f$oho$1@smc.vnet.net> <ghb5dv$rmb$1@smc.vnet.net>
Hello Raffy,
Excuse me, but I do not see what you want and what you would use it
for. The sentence "the last position of an
element at level 1 in a list" sounds a bit cryptic to me. Say that we
have the list {{1, 2, 4, 6, -1}, {4, 10, 8, -2, 10}}, which is a list
of lists. Here level 1 consists of the two lists {1, 2, 4, 6, -1} and
{4, 10, 8, -2, 10}, right? How should "the last position of an
element" be understood here?
Regards,
Sigmund
On Dec 5, 1:05 pm, Szabolcs Horv=E1t <szhor... at gmail.com> wrote:
> Raffy wrote:
> > I'm looking for the fastest way to find the last position of an
> > element at level 1 in a list.
>
> > The fastest way, which is still extremely bloated and extremely slow,
> > appears to be:
>
> > lastPos[v_List,p_:True,default_:$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).