## Produce a random number between between 0 and 1. If the random number
## is smaller than 0.5, then return -1. Else return 1.
## Usage : r=random_disc(N,num)


function r=rand_disc(N,num)
  
  rn=rand(N,1);
  r=(rn<num);
  li=find(r == 0);
  r(li)=-1;

endfunction
