Re: Value and position in list
- To: mathgroup at smc.vnet.net
- Subject: [mg33423] Re: Value and position in list
- From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
- Date: Thu, 21 Mar 2002 09:27:00 -0500 (EST)
- Organization: Universitaet Leipzig
- References: <a79ckp$7uk$1@smc.vnet.net>
- Reply-to: kuska at informatik.uni-leipzig.de
- Sender: owner-wri-mathgroup at wolfram.com
Hi,
minpos = Compile[{{lst, _Real, 2}},
Module[{i = 1},
MapIndexed[If[First[#] < lst[[i, 1]], i = First[#2], 0] &, lst];
{{i, 1}}
]
]
minpos[data] // Timing
should be faster.
Regards
Jens
Hugh Goyder wrote:
>
> I have a list where each element is a list of three values
> {{v11,v12,v13},{v21,v22,v23}...}.
> What is the fastest method for finding the position and value of the
> element which has the smallest first element.
>
> I have the following but is it possible to do this more quickly, perhaps
> without two sorts through the data?
>
> In[1]:=
> $Version
>
> Out[1]=
> 4.1 for Microsoft Windows (June 13, 2001)
>
> In[2]:=
> data=Table[{Random[],Random[]-0.5,10(Random[]-0.5)},{10000}];
>
> In[3]:=
> Timing[a=Min[Transpose[data][[1]]];p=Position[data,a];{a,p}]
>
> Out[3]=
> {0.33 Second,{0.0000316723,{{1706,1}}}}
>
> I must also find the element which has the smallest second element greater
> than zero. Here are my attempts so far. Is there a faster method?
>
> In[4]:=
> Timing[a=Infinity;i=0;
> While[i++;i<Length[data],If[0<data[[i,2]]<a,a=data[[i,2]];p=i]];{a,p}]
>
> Out[4]=
> {1.31 Second,{0.0000126855,1134}}
>
> In[5]:=
> Timing[a=Min[Transpose[data][[2]]/.v_/;v\[LessEqual] 0 \[Rule] Infinity];
> p=Position[data,a];{a,p}]
>
> Out[5]=
> {0.71 Second,{0.0000126855,{{1134,2}}}}
>
> In[6]:=
> Timing[a=Min[Select[Transpose[data][[2]],#>0&]];p=Position[data,a];{a,p}]
>
> Out[6]=
> {0.61 Second,{0.0000126855,{{1134,2}}}}
>
> Thank you for your ideas
>
> Hugh Goyder