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MathGroup Archive 2005

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Re: Complement replacement

  • To: mathgroup at smc.vnet.net
  • Subject: [mg58633] Re: Complement replacement
  • From: konstantpi at mail15.com
  • Date: Sun, 10 Jul 2005 16:52:01 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

thanks very much for the answers,
my previous question was submitted, because at first i do not know 
about the Complement function, i know that a function like this must 
be in the Union, Intersection family, but somehow i have not noticed 
the Complement in the documentation so i wrote my old way program:

pp = Table[Random[Integer, {1, 1000}], {i, 1000}];
r = Table[i,{i,1000}];
(lst = {}; Do[If[MemberQ[pp, r[[i]]] == False,
      lst = Join[lst, {r[[i]]}];
                           ]
      , {i, 1, Length[pp]}];
lst = Union[lst, lst];) // Timing


{0.22 Second, Null}
on P4 2 GHz 128k cache 256M ram


after i have written my letter i have discovered the Complement 
function, and noticed its speed, so i keep my original message to 
compare different strategies and methods which may presented by this 
group marvelous contributors
Thanks for all


From:  Andrzej Kozlowski <akozlowski at gmail.com> ( Add to address 
To: mathgroup at smc.vnet.net
book)  
Subject: [mg58633]  Re: [mg58608] Complement replacement 
 
 
 
On 10 Jul 2005, at 18:12, konstantpi at mail15.com wrote:

> hi
> in the list:
> pp=Table[Random[Integer, {1, 1000}], {i, 1000}];
> how could i know which numbers from 1 to 1000 does not exist in the
> pp List.
> but without using:
> Complement[Table[i,{i,1000}],pp]
> regards
>
>


Perhaps you should explain why you do not want to use Complement as  
it will be very hard if not impossible to match its performance.  
Indeed, on ny machine I get:

In[1]:=
pp=Table[Random[Integer, {1, 1000}], {i, 1000}];

In[2]:=
a=Complement[Range[1000],pp];//Timing

Out[2]=
{0.001657 Second,Null}

while using the most obvious alternative:

In[3]:=
b=Select[Range[1000],Not[MemberQ[pp,#]]&];//Timing

Out[3]=
{0.408538 Second,Null}

In[4]:=
Union[b]==a

Out[4]=
True

The difference in performance is huge. There are a number of ways  
that give a better performance but I can't find any that beets  
Complement. Here s one that is at least better than the most obvious  
way above. First define a function f:

SetAttributes[f, Listable]

Scan[(f[#]=Sequence[])&,pp];//Timing

{0.020849 Second,Null}

f[x_] := x

(note that evaluating this defintion itself takes more time than  
running Complement)

now:

In[8]:=
c=f[Range[1000]];//Timing

Out[8]=
{0.008274 Second,Null}

In[9]:=
Union[c]==a

Out[9]=
True

Andrzej Kozlowski

Chiba, Japan
 

> Peter Pein <petsie at dordos.net>:

> Please tell us why you want to do so.
> 
> Cleaning up and generating testing data (I took a million values 
instead
> of thousand for distinguishable timings):
> 
> In[1]:= Clear[n, pp, missing];
>   n = 10^6;
>   pp = Table[Random[Integer, {1, n}], {n}];
> 
> just as reference timing:
> 
>   First[Timing[missing[1] = Complement[Range[n], pp]; ]]
> Out[4]  0.844*Second
> 
> Sort the random list joined with the whole range, sort and split 
this
> list and get the unique elements:
> 
> In[5]:  First[Timing[missing[2]     Cases[Split[Sort[Join[pp, Range
[n]]]],
> {x_Integer} :> x];
> ]]
> Out[5]  1.875*Second
> 
> Method 'mind the gap' ;-) :
>  Build ranges out of the gaps in the random list:
> 
> In[6]:First[Timing[missing[3]   Inner[Range, Drop[#1, -1] + 1, Drop
[#1, 1]
> - 1, Join]&
>     [Union[pp, {0, n + 1}]];
> ]]
> Out[6]  1.609*Second
> 
> Verification:
> 
> In[7]:  SameQ @@ missing /@ {1, 2, 3}
> Out[7]  True
> 
> 
> 
> --
> Peter Pein
> Berlin
> http://people.freenet.de/Peter_Berlin/
> 


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