How to form 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

I am trying to make a function (with a for-loop in it) that finds the onsets of (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:

function [start_of_trial, trial_value] = find_onsets(E)
    start_of_trial = find([1;diff(E(:))~= 0]);
    trial_value = E(start_of_trial);
end

Now, although it is not necessary, I have to make a for-loop to find the same output, instead of the function above.

How can I make a for-loop with this function? I tried several things, but I am stuck with how to use the for-loop properly in this case

[TADA]

What did you try? Where did you get stuck?

[Walter Roberson]

I am not clear as to when trials start? Does a new trial start every time E changes value, including zeros? Or are zeros excluded?

[Sterre]

I tried the following function with a counter:

function [start_of_trial, trial_value] = find_onsets_loop(E)
	counter = 1;

	a = 5;
	start_of_trial = zeros(a);
	trial_value = zeros(a);

	scan_number = 1;

	for i = 2:length(E)
		if E(i) ~= E(i-1)
  		start_of_trial(counter) = scan_number;
			trial_value(counter) = E(i-1);

			scan_number = 0;
			counter = counter + 1;
		else
			scan_number = scan_number + 1;
        end
 end
 

but I keep getting the wrong outputs..

[Sterre]

Zeros are included. So the start of zeros is also a new trial (5 trials in the example above)

[Walter Roberson]

Your start of trial should be the same as i. Your scan_number is not doing anything useful for you.

[Sterre]

Thank you Walter.

Now, both outputs are working:

start_of_trial =

    11     0     0     0     0
    16     0     0     0     0
    24     0     0     0     0
    28     0     0     0     0
     0     0     0     0     0

trial_value =
     0     0     0     0     0
     1     0     0     0     0
     0     0     0     0     0
     2     0     0     0     0
     0     0     0     0     0

I get the outputs like this, do you know how I can change this?

[Walter Roberson]

What is your revised code?

[Sterre]

Here, my revised code:

function [start_of_trial, trial_value] = find_onsets_loop(E)

    counter = 1;

	a = 5;
	start_of_trial = zeros(a);
	trial_value = zeros(a);
    
	for i = 2:length(E)
		if E(i) ~= E(i-1)
  		start_of_trial(counter) = i;
			trial_value(counter) = E(i-1);

			i = 0;
			counter = counter + 1;
		else
			i = i + 1;
        end
    end

Answer 1

[Andrei Bobrov]

n = numel(E);
count1 = 1;
start_of_trial = [1;zeros(n-1,1)];
trial_value = zeros(n,1);
for ii = 2:n
    if E(ii) - E(ii-1) ~= 0
        count1 = count1 + 1;
        start_of_trial(count1) = ii;
        trial_value(count1) = E(ii);
    end
end
start_of_trial = start_of_trial(1:count1);
trial_value = trial_value(1:count1);