# How to use a for-loop to find the onset (first element) of trials of zeros, ones and twos, in an array of sequences of zeros, ones and twos

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Sterre on 31 Mar 2019
Answered: Catalytic on 31 Mar 2019
I am trying to make a function that finds the onsets (three different) trials (in this case, the trial starts when a new sequence of zeros, ones and twos starts in an array of numbers, this array of numbers always consists of sequences of ones, zeros and twos).
See the example of such an array (E) for a clarification:
E = [zeros(1,10) ones(1,5) zeros(1,8) 2*ones(1,4) zeros(1,12)]
This array is always a rowvector. The example consists of 5 trials, and 39 elements (10+5+8+4+12).
With the funtion, I want to find the elements (1:39) of the starts of every trial: [start_of_trial], with the corresponding value 0, 1 or 2: [trial_value]. This way, the function will be as follows:
[start_of_trial, trial_value] = find_onsets(E)
How can I use a for-loop to make this function work? I tried several things, but nothing worked properly

Andrei Bobrov on 31 Mar 2019
function [start_of_trial, trial_value] = find_onsets(E)
start_of_trial = find([1;diff(E(:))~= 0]);
trial_value = E(start_of_trial);
end
Sterre on 31 Mar 2019
The function works, thank you.
Although it is not necessary here, I need to have a for-loop in the function.. Can you help me with that?

Catalytic on 31 Mar 2019
No, we're not going to do your homework for you. Go away and leave us alone.