Re: Complement replacement
- To: mathgroup at smc.vnet.net
- Subject: [mg58617] Re: [mg58608] Complement replacement
- From: Andrzej Kozlowski <akozlowski at gmail.com>
- Date: Sun, 10 Jul 2005 16:51:35 -0400 (EDT)
- References: <200507100912.FAA06500@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
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
- References:
- Complement replacement
- From: konstantpi@mail15.com
- Complement replacement