Re: Replacing Numbers in a List
- To: mathgroup at smc.vnet.net
- Subject: [mg68262] Re: Replacing Numbers in a List
- From: Peter Pein <petsie at dordos.net>
- Date: Mon, 31 Jul 2006 03:45:15 -0400 (EDT)
- References: <eahsgh$op1$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Gregory Lypny schrieb:
> 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?
>
> Regards,
>
> Gregory
>
ReplaceAll and Alternatives come to mind:
In[1]:=
x1 = x2 = (Fold[10*#1 + #2 & , 0, #1] & ) /@
Table[Random[Integer, {1, 3}], {10^6}, {3}];
y = {122, 211, 212, 221, 223, 232, 322};
Timing[Do[x1 = ReplacePart[x1, 222, Position[x1, y[[i]]]],
{i, Length[y]}]; ]
Out[3]=
{1.264079*Second, Null}
In[4]:=
Timing[x2 = x2 /. Alternatives @@ y -> 222; ]
Out[4]=
{0.4880301*Second, Null}
In[5]:=
x2 === x1
Out[5]=
True
Peter