Democode for https://psychtoolbox.discourse.group/t/request-for-priority-support-psychtoolbox-crashing-unpredictably/4241

samplefoo.m

trial_number = 90;

% Input set of images:
test_images = [1,2,3,4,5]
nrImages = length(test_images);

% Present each 3 times:
max_image_presentations = trial_number / nrImages
tic
if 1
    % More efficient procedure:
    
    % Provide initial set of test_indices, randomized, nrImages repetitions for
    % each image index:
    test_indices = BalanceFactors(max_image_presentations, 1, 1:nrImages);
    test_index = [];
    test_index_sequence = nan(1, trial_number);

    for i=1:trial_number
        % Try to find an index out of the remaining indices that is not the same as
        % in last iteration:
        while (test_index == test_indices(1)) && length(unique(test_indices)) > 1
            % Reshuffle if no success:
            test_indices = Shuffle(test_indices);
        end

        % Got one, take it and take it out of set of remaining test_indices for
        % next trial:
        test_index = test_indices(1);
        test_indices = test_indices(2:end);
        test_index_sequence(i) = test_index;
    end

    % Done. test_index_sequence contains whole sequence of test_index values:
    test_index_sequence
else
    % Less efficient procedure, prone to hangs or infinite loops:
    tracker = zeros(1,length(test_images));

    for i=1:trial_number
        image_chosen = false;
        while image_chosen == false
            test_index = randi(length(test_images));
            if i == 1
                image_chosen = true;
            elseif test_index ~= previous_test_index &&...
                    tracker(test_index) < max_image_presentations
                image_chosen = true;
            end
        end
        cnt = i
        previous_test_index = test_index
        tracker(test_index) = tracker(test_index) + 1;

        test_index_sequence(i) = test_index;
    end

    % Done. test_index_sequence contains whole sequence of test_index values:
    test_index_sequence
end
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