Question

I have a vector like:

x = [0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 1 2 1 2 1 2 1 2 1 2 1 2 1 2 0 0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1]

How can I grab the start and end index of the alternating sequence? (i.e. the 1’s and 2’s)

Answer 1

I think this is a cool method, though numerical/array methods would be faster 😛 You could use regex!

% convert into a space-separated string
% (surely there's a better way than converting to cell array first?)
str = strcat({num2str(x)})
% for some reason all elements are separated by more than one space, convert
%  so they're separated by a single space
str = regexprep(str,' +',' ')

% find start & end indices (into str) alternating sequences
[s e]=regexp(str,'\<([^ ]+) ((?!\1)[^ ]+)(?: \1 \2)+(?: \1)?\>'),'start','end')
% convert these indices into indices into `x`
% (at the moment s & e include spaces)
s = (cell2mat(s)+1)/2
e = (cell2mat(e)+1)/2

% run i is at x( s(i):e(i) )