Re: Fast List-Selection
- To: mathgroup at smc.vnet.net
- Subject: [mg19925] Re: [mg19880] Fast List-Selection
- From: "Andrzej Kozlowski" <andrzej at tuins.ac.jp>
- Date: Tue, 21 Sep 1999 02:22:54 -0400
- Sender: owner-wri-mathgroup at wolfram.com
I have managed to produce four additional examples, three of which seem to
be faster than yours. I will change the search to 6 rather than 7
consecutive identical elements because I want to use for my testing a famous
example:
In[1]:=
l = RealDigits[N[Pi, 10000]][[1]];
Hear are the functions I will test, beginning with yours:
In[2]:=
findsequence[1][l_] := Do[If[Count[t = Take[l, {i, i + 5}], t[[1]]] == 6,
Print[i]], {i, 1, Length[l] - 5}]
In[3]:=
findsequence[2][l_] :=
Position[l /. {a___, y_, y_, y_, y_, y_, y_, b___} -> {a, mark, b}, mark,
1]
In[4]:=
findsequence[3][l_] :=
Module[{m = Split[l], mark},
Position[Flatten[m /. Cases[m, _?(Length[#] == 6 &)][[1]] -> mark],
mark,
1]]
In[5]:=
findsequence[4][l_] :=
Module[{m = Split[l]},
Length[Flatten[
Take[m, Position[m, Select[m, Length[#] == 6 &][[1]]][[1, 1]] -
1]]] + 1]
In[6]:=
f[x_, x_, x_, x_, x_, x_] := 0;
f[y__] := 1;
g[l_, i_] := f[Apply[Sequence, Take[l, {i, i + 5}]]];
findsequence[5][l_] := Scan[If[g[l, #] == 0, Return[#]] &,
Range[Length[l]]];
Now the test:
In[9]:=
Table[findsequence[i][l] // Timing, {i, 1, 5}]
763
Out[9]=
{{1.38333 Second,
Null}, {2.18333 Second, {{763}}}, {0.783333 Second, {{763}}}, {0.683333
Second, 763}, {0.183333 Second, 763}}
The last one wins by a big margin The programs using Split may do better on
other machines (I am using Mac PowerBook G3 ,233 mghz) because there seems
to$B!!(Jbe something wrong with Split on the Mac where it doesn't scale linearly
with the size of the input.
Finally: In[10]:=
Take[l, {763, 763 + 5}]
Out[10]=
{9, 9, 9, 9, 9, 9}
--
Andrzej Kozlowski
Toyama International University
JAPAN
http://sigma.tuins.ac.jp
http://eri2.tuins.ac.jp
----------
>From: Hans Havermann <haver at total.net>
To: mathgroup at smc.vnet.net
>To: mathgroup at smc.vnet.net
>Subject: [mg19925] [mg19880] Fast List-Selection
>Date: Mon, Sep 20, 1999, 7:47 AM
>
> I have a list 's' composed of a large number of (small) integers. I wish to
> search this list for instances of 7 consecutive, identical elements.
>
> My approach is:
>
> Do[If[Count[t = Take[s, {i, i + 6}], t[[1]]] == 7,
> Print[i]], {i, 1, Length[s] - 6}]
>
> Can anyone think of a *faster* way of doing this?
>
>
>
--
Andrzej Kozlowski
Toyama International University
JAPAN
http://sigma.tuins.ac.jp
http://eri2.tuins.ac.jp
----------
>From: Hans Havermann <haver at total.net>
To: mathgroup at smc.vnet.net
>To: mathgroup at smc.vnet.net
>Subject: [mg19925] [mg19880] Fast List-Selection
>Date: Sun, 19 Sep 1999 18:47:32 -0400
>
> I have a list 's' composed of a large number of (small) integers. I wish to
> search this list for instances of 7 consecutive, identical elements.
>
> My approach is:
>
> Do[If[Count[t = Take[s, {i, i + 6}], t[[1]]] == 7,
> Print[i]], {i, 1, Length[s] - 6}]
>
> Can anyone think of a *faster* way of doing this?
>
>
>