Thread Subject:

Random -1's and 1's

Hi Everyone,

What is the most efficient way of creating a vector filled with -1's and 1's, with probability(-1) = probability(1)?

Regards,

Ulrik

On 9/3/2011 5:54 AM, Ulrik Nash wrote:
> Hi Everyone,
>
> What is the most efficient way of creating a vector filled with -1's and 1's,
> with probability(-1) = probability(1)?
>
> Regards,
>
> Ulrik

I am sure there is a better way, but why not use randperm(2)
and just look at the first value returned, and if it is say
1, call this your '1', and if it is '2' call this your '-1'

Since randperm calls rand, and you have only 2 elements, then
the prob is 1/2 for each to come out?

---------------------
EDU>> N=20;
A=zeros(N,2);
for i=1:N
     A(i,:)=randperm(2);
end
A(A(:,1)==2)=-1;
A(:,1)
----------------

ans =

     -1
     -1
      1
     -1
      1
      1
     -1
      1
      1
      1
      1
      1
      1
     -1
     -1
      1
     -1
     -1
      1
     -1


--Nasser

"Ulrik Nash" <uwn@sam.sdu.dk> wrote in message <j3t81j$rd$1@newscl01ah.mathworks.com>...
> Hi Everyone,
>
> What is the most efficient way of creating a vector filled with -1's and 1's, with probability(-1) = probability(1)?
>

 v = 2*(rand(1,100) < 0.5) - 1

Bruno

"Bruno Luong" <b.luong@fogale.findmycountry> wrote in message <j3tacj$75k$1@newscl01ah.mathworks.com>...
> "Ulrik Nash" <uwn@sam.sdu.dk> wrote in message <j3t81j$rd$1@newscl01ah.mathworks.com>...
> > Hi Everyone,
> >
> > What is the most efficient way of creating a vector filled with -1's and 1's, with probability(-1) = probability(1)?
> >
>
> v = 2*(rand(1,100) < 0.5) - 1
>
> Bruno

Thank you both. I think I will go with Bruno's idea. Very neat!

"Ulrik Nash" <uwn@sam.sdu.dk> wrote in message <j3t81j$rd$1@newscl01ah.mathworks.com>...
> Hi Everyone,
>
> What is the most efficient way of creating a vector filled with -1's and 1's, with probability(-1) = probability(1)?
>
> Regards,
>
> Ulrik

This generates a Bernoulli Process Xi = 1 with prob. p, Xi = -1 with prob. (1-p), i = 1,2,...,n.
function bern = BernProc(n,p)
     bern = -1+2*(rand(1,n) <= p);