error during selecting dynamically image from folder

Error using randi
First input must be a positive scalar integer value IMAX, or two integer values [IMIN IMAX] with IMIN less than or
equal to IMAX.

Error in tt (line 25)
thisfile  = files(randi(num_files)).name;

clc;
clear all;

tic;

%% Training images
  id_test = 0;
  id_train = 0;
  
 for folder_idx = 1 : 10                                        % no of classes  100               
     
 for i = 1 : 8                                                          % no of images per class  8
     
 %thisfile = fullfile('ROITrain', num2str(folder_idx), [num2str(i) '.bmp ']); 
  files = dir( fullfile('ROITrain', num2str(folder_idx), [num2str(i) '.bmp ']));
 num_files = numel(files);
thisfile  = files(randi(num_files)).name;
%image = imread(filename);
 
 B = imread(thisfile ); 
 X = double(B);
 
  X = imresize(X,[300 250],'bilinear');                             %300 250
     id_train = id_train+1;
    traindata{id_train}=ext_vein(X,1);
     traindata = traindata';
     
     % only four minutie is taken from one image
    reduced_traindata = cellfun(@(M) M(1:min(end,4), :), traindata, 'uniform', 0);
     
 end 
 
 end 

   save('db2.mat','reduced_traindata');    
    
    toc

[Stephen23]

files is empty, so num_files is zero, which is not valid for randi.

But in any case it makes no sense to use dir with the actual name of a file:

dir( fullfile(..., [num2str(i) '.bmp ']));

It looks like the code mixes up the two main ways of processing multiple files:

https://www.mathworks.com/help/matlab/import_export/process-a-sequence-of-files.html

Your code is very badly aligned. Badly aligned code is one way that beginners hide basic bugs in their code. I strongly recommend that you align your code consistently using the default MATLAB Editor settings. You can apply them to existing code by selecting all code text and pressing ctrl+i.

[Balaji M. Sontakke]

I want to randomaly select 8 images from specific folder how i can do that

Answer 1

[Stephen23]

"I want to randomaly select 8 images from specific folder how i can do that"

Something like this:

P = 'path to the folder where the files are saved';
S = dir(fullfile(P,'*.bmp'));
N = numel(S);
X = randperm(N);
F = {S(X(1:8)).name} % cell array of 8 randomly selected filenames from that folder

[Balaji M. Sontakke]

no sir not cell array, I need to pass one by one images to my ext_vein() function, sir repeat my question I have 10 folders named 1,2,....10 each folder contains 12 images, just i want to randmoly select 8 images from 12 images and pass those images one by one to ext_vein function. my program is.

clc;
clear all;

tic;

%% Training images
  id_test = 0;
  id_train = 0;
  
 for folder_idx = 1 : 10                                        % no of classes  100
     
     
 for i = 1 : 8                                                          % no of images per class  8
     
 thisfile = fullfile('ROITrain', num2str(folder_idx), [num2str(i) '.bmp ']); 
 B = imread(thisfile ); 
 X = double(B);
 
  X = imresize(X,[300 250],'bilinear');                             %300 250
     id_train = id_train+1;
    traindata{id_train}=ext_vein(X,1);
     traindata = traindata';
     
     % only four minutie is taken from one image
    reduced_traindata = cellfun(@(M) M(1:min(end,4), :), traindata, 'uniform', 0);
     
 end 
 
 end
 

     save('db2.mat','reduced_traindata');    
    
    toc   

[Stephen23]

"no sir not cell array,"

Why not? Cell arrays are trivial to loop over.

"I need to pass one by one images to my ext_vein() function,"

I don't see why using a cell array to store filenames prevents you from doing that.

"repeat my question.."

This is not a "repeat" of your question. Take another look at your original "question": does it contain anything like the explanation you just gave now? (hint: no, it contains an error message, buggy code, and no other text whatsoever).

"I have 10 folders named 1,2,....10 each folder contains 12 images, just i want to randmoly select 8 images from 12 images and pass those images one by one to ext_vein function."

Finally, an actual explanation!

I am sure that you can adapt my intentionally simple answer to suit your needs, e.g.:

P = 'ROITrain'; % i.e. relative/absolute path to where the subfolders are.
for ii = 1:10
    D = sprintf('%u',ii);
    S = dir(fullfile(P,D,'*.bmp')); % get all filenames in subfolder
    N = numel(S);
    X = randperm(N);
    F = {S(X(1:8)).name}; % randomly select 8 filenames.
    for jj = 1:numel(F)
        fn = fullfile(P,D,F{jj});
        im = imread(fn);
        ...
        ... ext_vein(...)
    end
end

[Balaji M. Sontakke]

Its simply wow!!! hats of you sir, Great and thanks