how can i have random numbers with criteria?

i need to have a 50 numbers matrice, while the numbers are between 1 to 5, and i need the neighbouring numbers to be different from each other. for example: 1 - 3 - 4 - 2 - 1 - 2 - 5 - 3 - 4 - 2...

i use:

r = [mod(randperm(50),5)+1];

but it gives me something like this: 1 - 2 - 4 - 4 - 3 - 4 - 5 - 5 - 5 - 3 - 1 - 2 - 2 - ....

how can i make the numbers to be different from the numbers before and after them?

thank you.

Answer 1

[Guillaume]

Pick any of the 5 numbers for the first elements, for the subsequents, pick any of the four numbers making the set difference between 1:5 and the previous number:

r = [randi(5) randi(4, 1, 49)]; %number 2-50 are indices of which of the four numbers to pick in setdiff.
for idx = 2:numel(r)
   rr = setdiff(1:5, r(idx-1)); %exclude previous number
   r(idx) = rr(r(idx));
end

[Image Analyst]

I think they need the numbers on both sides not to match, not just the previous number.

[Guillaume]

If all numbers are different from their predecessor doesn't it follow they're also different from their successor?

[Image Analyst]

Uh yeah, dopey me (sound of hand slapping forehead).

Answer 2

[John D'Errico]

Easy peasy, and vectorized too. No setdiffs needed, no loops, no tests. So it will be quite fast. And essentially one line of code (if we ignore the fact that I defined n and k separately for clarity.)

For the numbers to be different from each other in sequence, this just means that each element must have a difference with its neighbors that is non-zero, modulo 5. I suppose you could also write it as a Markov process, where the state transition matrix is a simple one...

T = (ones(5) - eye(5))/4
T =
          0         0.25         0.25         0.25         0.25
       0.25            0         0.25         0.25         0.25
       0.25         0.25            0         0.25         0.25
       0.25         0.25         0.25            0         0.25
       0.25         0.25         0.25         0.25            0

Anyway, the simple solution is to pick the first element randomly, then pick a set of random differences, doing arithmetic mod 5. I'll write it for a general set of n numbers, each of which must lie in the set of integers [1:k], but with no consecutive elements that are the same. For your problem, n=50, k=5.

n = 50;
k = 5;
r = 1 + mod(cumsum([randi(k),randi(k-1,[1,n-1])]),k);

r
r =
  Columns 1 through 24
     4     2     1     3     1     5     2     3     5     4     1     4     2     4     5     2     5     4     1     5     4     3     2     3
  Columns 25 through 48
     1     2     3     4     5     2     4     3     1     4     1     5     3     4     5     1     2     4     1     2     3     4     5     4
  Columns 49 through 50
     3     2

As you can see by looking at the difference vector, there are no zero differences.

diff(r)
ans =
  Columns 1 through 24
    -2    -1     2    -2     4    -3     1     2    -1    -3     3    -2     2     1    -3     3    -1    -3     4    -1    -1    -1     1    -2
  Columns 25 through 48
     1     1     1     1    -3     2    -1    -2     3    -3     4    -2     1     1    -4     1     2    -3     1     1     1     1    -1    -1
  Column 49
    -1

And the min and max elements are 1 and 5 respectively.

max(r)
ans =
     5

min(r)
ans =
     1

[itay]

thanks :]

Answer 3

[Isabella Osetinsky-Tzidaki]

r=nan(50,1); rng('shuffle') r(1)=randi([1,5]); n=1; while n<50, rng('shuffle') p = randi([1,5]); if p~=r(n), n=n+1; r(n)=p; end end

[John D'Errico]

Please use the code formatting button, else your answer is unreadable, just a long strung out mess. Just select your text, and click on the little button that says "{} Code".