Randomize a vector with letters making a N back test

Hi,

I wish to create a random letter vector that has 150 element with only the letters from A to J, which i have done here:

s='A':'J'; % Generate the Letters from A to J

str=s(randi(numel(s),150,1))';

But i wish that some of the every 3rd element after 1 element is identical. For eksample

[A G J B G H D I H C]

Thank u

Answer 1

[Jan]

s = 'A':'J';
str = s(randi(numel(s), 1, 150));
idx = 5:4:numel(str);
str(idx) = str(idx - 3)

str = 'FBJGBBHEBCICCJEFJFJDFDECDHAGHBAIBJDBJCEDCHCAHFDIFCHDCFIFFDFHDBFHBFEIFEJFEFACFHIAHICIIHEHHABHADJCDFCAFBIGBEIJEDIIDFEDFDCGDFJHFJHFJFCHFDBDDBDCBDJADAHAAE'