FAQ: How can I process a sequence of files?

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Jan on 26 Dec 2012
Edited: Jan on 8 Apr 2021
How can I process a sequence of files?
Walter Roberson
Walter Roberson on 16 Feb 2020
Pre-allocate an output array that is at least 3 dimensions, with one dimension being the number of times the first loop will execute, a second dimension being the number of times the second loop will execute, and the third dimension being the number of times the third loop will execute. Then assign to the output according to how many loop iterations you have done.
For example,
Lvals = [-83, 149, -5, 0, -2, 101];
numL = length(Lvals);
output = zeros(7, 3, numL);
for J = 9:17:11
Jidx = round((J-9)/17) + 1;
for K = 1000:500:2000
Kidx = round((K-500)/500);
for Lidx = 1 : numL
L = Lvals(Lidx);
output(Jidx, Kidx, L) = J.^2 + K/7 + sin(L.^3);

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Accepted Answer

Jan on 26 Dec 2012
Edited: Jan on 8 Apr 2021
If the files that you want to process are sequentially numbered, like "file1.txt", "file2.txt", "file3.txt", etc. then you can use SPRINTF or NUM2STR to create the filename and LOAD, IMREAD, FOPEN, etc. to retrieve the data from the file. (Also note the three different ways of building the filename - you can use your favorite way.)
% Read files file1.txt through file20.txt, mat1.mat through mat20.mat
% and image1.jpg through image20.jpg. Files are in the current directory.
folder = cd;
for k = 1:20
matFilename = sprintf('mat%d.mat', k);
matData = load(fullfile(cd, matFilename));
jpgFilename = sprintf('image%d.jpg', k);
imageData = imread(jpgFilename);
textFilename = sprintf('file%d.txt', k);
fid = fopen(fullfile(folder, textFilename), 'rt');
textData = fread(fid);
In the above code, matData, imageData, and textData will get overwritten each time. You should save them to an array or cell array if you need to use them outside the loop, otherwise use them immediately inside the loop.
If instead you want to process all the files whose name matches a pattern in a directory, you can use DIR to select the files. Note that while this example uses *.jpg as the pattern and IMREAD to read in the data, as with the previous example you could use whatever pattern and file reading function suits your application's needs:
Folder = 'C:\Documents and Settings\yourUserName\My Documents\My Pictures';
if exist(Folder, 'dir') ~= 7 % isfolder(Folder) in modern Matlab versions
Message = sprintf('Error: The following folder does not exist:\n%s', Folder);
filePattern = fullfile(Folder, '*.jpg');
jpegFiles = dir(filePattern);
for k = 1:length(jpegFiles)
baseFileName = jpegFiles(k).name;
fullFileName = fullfile(Folder, baseFileName);
fprintf('Now reading %s\n', fullFileName);
imageArray = imread(fullFileName);
imshow(imageArray); % Display image.
drawnow; % Force display to update immediately.
[EDITED] With modern Matlab versions (I assume since at least R2016b):
You can use '**' for a recursive search, if the files are in multiple subfolders:
Folder = 'C:\Your\Folder';
FileList = dir(fullfile(Folder, '**', '*.jpg'));
for iFile = 1:numel(FileList)
thisFolder = FileList(iFile).folder;
thisFile = FileList(iFile).name;
File = fullfile(thisFolder, thisFile);
... Import the File now as you need...
Or you can try a "File Exchange Pick of the Week": FileExchange: FileFun.
This answer is a modified version of:
joshua Abam
joshua Abam on 24 Jun 2019
Hi Walter
I seem not to have follow your suggestion can you help by adding some more comments to ease my grap or better still use this example to illustrate it and where best can I possible attached that to a GA code.
function f = gaintobj(x)
f = rastriginsfcn([x(1)-6 x(2)-13]);
f = f + rastriginsfcn([x(3)-3*pi x(4)-5*pi]);
lb = [1 1 -30 -30];
ub = [20 20 70 70];
% Set the integer variables and number of variables.
IntCon = [1 2];
nvar = 4;
% Set options to call the custom output function, and to initially have
% little crossover.
options = optimoptions('ga','OutputFcn',@gaoutputfunround,'CrossoverFraction',0.2);
% For reproducibility, set the random number generator.
% Set the objective function and call the solver.
fun = @gaintobj;
[x,fval] = ga(fun,nvar,[],[],[],[],lb,ub,[],IntCon,options)

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More Answers (2)

Sean de Wolski
Sean de Wolski on 31 May 2013

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Sean de Wolski
Sean de Wolski on 29 Jul 2016
Edited: Sean de Wolski on 29 Jul 2016
In MATLAB R2014b and newer, you can use a datastore to read a sequence of files in a load-analyze-discard manner or in one shot with the readall() command.
  1 Comment
Ismail Qeshta
Ismail Qeshta on 11 Nov 2017
Hi all. Thanks for sharing it. How about if my files are in permutations sequence such as, Data11, Data12, Data 13, etc. Also, what if I would like to plot two different files names within a folder? Regards.

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