Re: position lists
- To: mathgroup at smc.vnet.net
- Subject: [mg68717] Re: position lists
- From: dimmechan at yahoo.com
- Date: Thu, 17 Aug 2006 04:18:25 -0400 (EDT)
- References: <ebujvu$6lu$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Dear Igor,
You can achieve what you want with a little modification of what you
thought (copy and paste the inputs to your notebook)
In[1]:=
Clear["Global*"]
In[2]:=
a={1,1,2,2,1,3,3,3,4,4,7};
In[3]:=
Map[Position[a,#]&,Range[Max[a]]]
Out[3]=
{{{1},{2},{5}},{{3},{4}},{{6},{7},{8}},{{9},{10}},{},{},{{11}}}
In[4]:=
Map[Flatten,%]
Out[4]=
{{1,2,5},{3,4},{6,7,8},{9,10},{},{},{11}}
In[5]:=
%//.{x___,{},y___}\[Rule]{x,{0},y}
Out[5]=
{{1,2,5},{3,4},{6,7,8},{9,10},{0},{0},{11}}
As an one-liner function:
In[6]:=
f[lst_List] := Map[Flatten, Map[Position[lst, #] &, Range[Max[lst]]]]
//. \
{x___, {}, y___} -> {x, {0}, y}
In[7]:=
f[{1,1,2,2,1,3,3,3,4,4,7}]
Out[7]=
{{1,2,5},{3,4},{6,7,8},{9,10},{0},{0},{11}}
Another function that do what you want is the following:
In[8]:=
g[lst_List]:=Module[{c},c=
Map[Flatten,Map[Position[
lst,#]&, Range[Max[lst]]]];ReplacePart[c,{0},Position[c,{}]]]
In[9]:=
g[a]
Out[9]=
{{1,2,5},{3,4},{6,7,8},{9,10},{0},{0},{11}}
However, both functions need considerable time for bigger lists.
In[10]:=
list = Table[Random[Integer, {0, 10000}], {10000}];
In[20]:=
Timing[f[list]][[1]]
Out[20]=
24.718 Second
In[28]:=
Timing[g[list]][[1]]
Out[28]=
27.719 Second
Certainly, there are other function that achieve your goal in less
time.
Anyway, I hope you will find the above, helpful.
Regards,
Jim