Sampling images without replacement

Hi everyone,

I have 10 trials and 2 conditions (neutral and condition). I have to random draw 10 images from a folder of 100 images without replacement for each trial and for each condition and to display it with psychtoolbox. I created a function, but images are sometimes repeating.

function print_random_images(w, type, deviceIndex, path, img,folder, N, text, ...
    backgroundColor, textColor, dispW, dispH, slack)
    images = dir(fullfile(path, '*.jpg'));
    files = fullfile({images.folder}, {images.name});
    index = 1:numel(files);

            if type == 0 % neutral condition
                displayTextCentered(w, text, backgroundColor,textColor, 1.5,50, 'center', 'center');
                waitTTL(deviceIndex)
                
                
                for i = 1:numel(files) / 10
                    selected = randperm(numel(index), N); % N = 10
                    file = img{selected};
                    image = imread(fullfile(folder,file));
                    imageDisplay = Screen('MakeTexture',w,image);
                    showImagesAndFixationCross(w,backgroundColor,dispW,dispH,slack, imageDisplay)
                    index(selected) = [];
                end


            elseif type == 1 % regulation condition
                displayTextCentered(w, text, backgroundColor,textColor, 1.5,50, 'center', 'center');
                waitForSpaceKeyPress()
                

                for i= 1:numel(files)/ 10
                    selected = randperm(numel(index), N); % N = 10
                    file = img{selected};
                    image = imread(fullfile(folder, file));
                    imageDisplay = Screen('MakeTexture', w, image);
                    showImagesAndFixationCross(w,backgroundColor,dispW,dispH,slack, imageDisplay)
                end
            end
end

Can someone help me, please ??

Thanks a lot

Answer 1

[Dave B]

Hi Pamela:

It looks like you're calling randperm for each iteration of your loop, but I think you just want to call randperm once right? I'm not sure I followed your code exactly, partly because I was confused about img (the input argument) vs. images (where you have the file name).

Here's some code that shows a generic loop which operates on 10 unique (no replacement) random files:

all_filenames=... % Here's where you get a list of all filenames, one per cell, presumably 100 long

ind = randperm(100,10); % list of 10 unique values selected from 1:100
for i = 1:10
   this_filename = all_filenames{ind(i)}; % get the filename from one of the randomly selected values
   % here's where you'd do something with the filename
end