Re: Re: Replacing Numbers in a List
- To: mathgroup at smc.vnet.net
- Subject: [mg68335] Re: [mg68294] Re: Replacing Numbers in a List
- From: "Carl K. Woll" <carlw at wolfram.com>
- Date: Tue, 1 Aug 2006 07:00:09 -0400 (EDT)
- References: <eahsgh$op1$1@smc.vnet.net> <200607311006.GAA02358@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Paul Abbott wrote:
> In article <eahsgh$op1$1 at smc.vnet.net>,
> Gregory Lypny <gregory.lypny at videotron.ca> wrote:
>
>> Hello everyone,
>>
>> I have a long list of integers called x, and I want to replace those
>> that are members of list y with the number 222.
>>
>> I did it this way with a Do loop.
>>
>> y = {122, 211, 212, 221, 223, 232, 322};
>> Do[x = ReplacePart[x,222, Position[x, y[[i]]]],{i, Length@y} ]
>>
>> Is this OK, or is there a more direct way?
>
> No one has suggested the obvious
>
> x /. Thread[y -> 222]
>
> However, this is about twice as slow as Bob Hanlon's suggestion,
>
> If[MemberQ[y,#],222,#]& /@ x
>
> Test:
>
> y={122,211,212,221,223,232,322};
>
> x=Table[Random[Integer,{100,325}],{10^5}];
>
> Timing[x1 = x /. Thread[y -> 222];] // First
> 0.051946 Second
>
> Timing[x2 = If[MemberQ[y,#],222,#]& /@ x;] // First
> 0.037817 Second
>
> x1 == x2
> True
>
> Cheers,
> Paul
>
> _______________________________________________________________________
> Paul Abbott Phone: 61 8 6488 2734
> School of Physics, M013 Fax: +61 8 6488 1014
> The University of Western Australia (CRICOS Provider No 00126G)
> AUSTRALIA http://physics.uwa.edu.au/~paul
Paul,
We can use Dispatch to speed up the Thread version:
Timing[x1=x/.Dispatch[Thread[y->222]];]//First
0.063 Second
Timing[x2=If[MemberQ[y,#],222,#]&/@x;]//First
0.078 Second
Carl Woll
Wolfram Research