[Date Index]
[Thread Index]
[Author Index]
Re: Need a nice way to do this
*To*: mathgroup at smc.vnet.net
*Subject*: [mg40288] Re: [mg40279] Need a nice way to do this
*From*: Andrzej Kozlowski <akoz at mimuw.edu.pl>
*Date*: Sun, 30 Mar 2003 04:07:59 -0500 (EST)
*Sender*: owner-wri-mathgroup at wolfram.com
Here is one (nice?) way of doing this that does not depend on how many
different values s contains:
f1[s_List] := MapIndexed[Length[DeleteCases[Take[s, First[#2]], #1]] &,
s]
and here is another one that works only (as it stands) with just two
distinct values:
f2[l_List] := Module[{mult =
Length /@ Split[l], list1,
list2, values}, list1 = Rest[FoldList[Plus, 0, Table[mult[[i]],
{i, 1,
Length[mult],
2}]]]; list2 =
FoldList[Plus,
0, Table[mult[[i]], {i, 2, 2(
Floor[(Length[mult] + 1)/2]) - 1, 2}]];
values = Take[Flatten[Transpose[{list2,
list1}]], Length[mult]]; Flatten[Table[Table[values[[i]],
{
mult[[i]]}], {i, 1, Length[mult]}]]]
The second function is a lot more complex and may not satisfy your
criteria of "niceness" but it is also a lot more efficient.
Let's first make sure they both work correctly with your original s:
In[3]:=
s={a,b,b,a,a,a,b,a,b,a,a};
In[4]:=
f1[s]
Out[4]=
{0,1,1,2,2,2,4,3,5,4,4}
In[5]:=
f2[s]
Out[5]=
{0,1,1,2,2,2,4,3,5,4,4}
Now let's try something bigger:
In[6]:=
s=Table[If[Random[Integer]==1,a,b],{10^3}];
In[7]:=
a=f1[s];//Timing
Out[7]=
{1.37 Second,Null}
In[8]:=
b=f2[s];//Timing
Out[8]=
{0.04 Second,Null}
In[9]:=
a==b
Out[9]=
True
So "niceness" doesn't always pays, it seems.
Andrzej Kozlowski
Yokohama, Japan
http://www.mimuw.edu.pl/~akoz/
http://platon.c.u-tokyo.ac.jp/andrzej/
On Saturday, March 29, 2003, at 07:19 pm, Steve Gray wrote:
> Given a list consisting of only two distinct values, such as
> s={a,b,b,a,a,a,b,a,b,a,a}, I want to derive a list of equal length
> g={0,1,1,2,2,2,4,3,5,4,4}. The rule is: for each position
> 1<=p<=Length[s], look at list s and set g[[p]] to the number of
> elements in s to the left of p which are not equal to s[[p]].
> In a more general version, which I do not need now, s would
> not be restricted to only two distinct values.
>
> Thank you for any ideas, including other applications where
> this particular calculation is used. The current application is an
> unusual conjecture in geometry.
>
>
>
>
Prev by Date:
**Re: Need a nice way to do this**
Next by Date:
**Re: A difficult problem**
Previous by thread:
**Re: Need a nice way to do this**
Next by thread:
**Re: Need a nice way to do this**
| |