How to obtain repetition patterns (start, end indices) in an array?

Hi,

My question is as follows:

Given an array N (see below) and a key value, I would like to identify the start and end indices of the pattern associated with the key.

   N = [1 1 2 2 2 2 3 3 3 4 4 4 4 1 1 1 1 2 2 2 3 3 3 3 3];
   key = 2;

For example, in the above, the pattern associated with '2' can be captured by a sequence of start indices and end indices. Thus, I would like the output:

   start_indices = [3 18]
   end_indices = [6 20]

How can I do this in MATLAB? Any help appreciated. Thanks in advance!

PS. The broader pattern in this problem is always cyclical. For example, a sequence of 1's is always followed by sequence of 2's, followed by sequence of 3's, followed by sequence of 4's and then back to a sequence of 1's.

Answer 1

[Chaitanya P]

I found a simple way out, thanks for the response @Philipp - since this is a part of a big code I tried to keep away from looping.

key_inds = find(N==key);

start_index = key_inds(1);
end_index = key_inds(end);

start_indices = [start_index key_inds(find(diff(key_inds)>1)+1)];
end_indices = [key_inds(find(diff(key_inds)>1)) end_index];

Answer 2

[Philipp Maier]

Please feel free to simplify the following:

flag = 1;
start_indices = [];
end_indices = [];
for i=1:numel(N)   
    if ((N(i)==key) && flag == 1)
        start_indices = [start_indices i];
        flag = 0;
    elseif (flag == 0 && (N(i)~=key) )
        end_indices = [end_indices i-1];
        flag = 1;
    end
end