creating a random vector of the values 0,1 with specific gaps

hey, I'm trying to creating a random vector of the values 0,1, in which 20% of the values are 1. I don't want to get two 1s in a row (e.x 0 0 1 1 0 0 0 0 0 0 ). does any one has an idea? thank u!!!!

Answer 1

[Amos]

Hi Shani, you could first generate a vector of 0 and 1 with the desired fraction of 1 and then check for each 1 if one of the neighbours is 1, too. If yes, give this one a new random position in the vector. Then you can proceed until no 1 has another 1 as neighbour.

Answer 2

[Roger Stafford]

There are two possible meanings you might have for the phrase "20% of the values are 1" :

 1) The percentage of ones has a statistically expected (average) value of 20%.
 2) The number of elements in the vector is a multiple of five and exactly 20% of these are ones.

Which of these meanings do you have in mind, Shani?

If the meaning is 1), (statistical) let n be the desired length of the vector, and do this:

   V = zeros(n,1);
   V(1) = ceil(rand-0.8);
   for k = 2:n
     if V(k-1) == 0
       V(k) = ceil(rand-0.75);  % The .75 is necessary to maintain 20% chance of ones
     end
   end

If the meaning is 2) (strictly 20%), let the desired length of V be n, a multiple of five, and do this:

   k = n/5; % n must be an integral multiple of 5
   V = zeros(n,1);
   V((0:k-1)+sort(randperm(4*k+1,k))) = 1;

[shani peles]

thanks!!!! i will use the second one. another problem i have is that i cant have the first cell=1. do u have any ideas (since i have to use a multiple of 5)?

[Roger Stafford]

To avoid a 1 in the first element of V, change the code for meaning 2) to:

   k = n/5; % n must be an integral multiple of 5
   V = zeros(n,1);
   V((1:k)+sort(randperm(4*k,k))) = 1;

[shani peles]

thank you!!! you're awesome ;)