If I understand correctly, you want to find where in a large vector (event_codes, size 1x19277) a small vector (e.g., att_event_codes{1,1}, size 1x11) can be found. The smaller vector must be in the same order and contiguous in the larger vector. If that's the case, you can use strfind. Here's an example:
% construct example large vector and cell array containing small vector:
att_event_codes = {[9 10 12 11 20 12 33 21 42 22 18]};
event_codes = randi(50,1,19277);
% make elements 3647 to 3657 of event_codes match att_event_codes{1}:
event_codes(3647:3657) = att_event_codes{1};
% use strfind to find the start index of all matches of att_event_codes{1} in event_codes:
idx = strfind(event_codes,att_event_codes{1});
if isempty(idx)
% no match
test = [];
else
% at least one match;
% take the first match and add index offsets to the length of att_event_codes{1}
test = idx(1) + (0:numel(att_event_codes{1})-1);
end
% check the result
disp(test)
