How to find repeat element of vector?

Suppose, i have a matrix b=[1 1 1 0 0 0 1 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1] and i want to find the repeated element of this matrix by 3 times using for loop . as the new matrix will be a=[1 0 1 1 0 0 1 1] . How easily can solve using for loop ??

[Matt J]

Why using a for loop?

[Image Analyst]

what is the repeated element? 1, which is repeated many times. 0, which is repeated many times? 11, which is repeated many times? 00 which is repeated many times. 1 1 1 1 1 1 which is repeated twice? And so on. Or is it just the first elements before a zero occurs, 111, which will occur 9 times? Please explain. Also explain what "by 3 times" means because I have no idea. Do you want to find the repeated element location three times instead of just once for some unexplained reason?

Answer 1

[Matt J]

 for i=1  
  a=b(1:3:end);
 end