Error when writing file names into a txt file ("nonscalar strings are unsupported")

I have a number of pictures in a folder and I would like to simply get their file names and write them into a txt file:

% input: pictures contained in "myfolder"
% 0013_DSC8315.jpg
% 0020_SOF8150.jpg
% 0068_DSC8440.jpg
% 0077_SOF8212.jpg
% 0089_DSC8481.jpg

% my attempt
a = dir('/.../myfolder');
b = {a(3:end).name}';
b = string(strrep(b, '.jpg', ''))
fid =fopen('/.../pics_names.txt', 'w' );
fwrite(fid, b);
fclose(fid);

However, I get the following output and error message

b = 

  5×1 string array

    "0013_DSC8315"
    "0020_SOF8150"
    "0068_DSC8440"
    "0077_SOF8212"
    "0089_DSC8481"

Error using fwrite
Cannot write value: nonscalar strings are unsupported.

Error in untitled (line 5)
fwrite(fid, b); 

How to solve this error? :-)

[Stephen23]

b = {a(3:end).name}';

https://www.mathworks.com/matlabcentral/answers/13978-list-of-files-sorting#answer_19278

https://www.mathworks.com/matlabcentral/answers/40949-omitting-pointers-when-listing-folder-contents#answer_50568

https://www.mathworks.com/matlabcentral/answers/166629-is-there-any-way-to-list-all-folders-only-in-the-level-directly-below-a-selected-directory#comment_510206

https://www.mathworks.com/matlabcentral/answers/484430-using-struct-dir-selpath-what-do-and-mean-as-struct-name#answer_395754

https://www.mathworks.com/matlabcentral/answers/1699230-folder-listing-with-dir-on-mac#answer_945260

As Walter Roberson wrote: "That code is wrong on every operating system that MATLAB has ever run on".

The recommended approach is to specify a non-empty DIR search name, or use e.g. ISMEMBER, SETDIFF, or similar:

https://www.mathworks.com/matlabcentral/answers/166629-is-there-any-way-to-list-all-folders-only-in-the-level-directly-below-a-selected-directory#comment_624696

[Dyuman Joshi]

In addition to what Stephen has mentioned, since you know that the extension of all the files is same and you want to read all the files with the same given extension, why not search accordingly?

Here's a simplfied version of what you want to do -

%Generating and saving some plots for example
for k=1:5
    plot(rand(1,10))
    ax = gca;
    exportgraphics(ax,"Plot_" + randi(1000) + ".jpg")
end

%Check the contest of the current folder
ls

Plot_173.jpg  Plot_28.jpg  Plot_38.jpg	Plot_552.jpg  Plot_965.jpg

%Search and get files with .jpg extension
jpgFiles = dir('*.jpg');
%Extract the names
names = {jpgFiles.name}

names = 1×5 cell array
    {'Plot_173.jpg'}    {'Plot_28.jpg'}    {'Plot_38.jpg'}    {'Plot_552.jpg'}    {'Plot_965.jpg'}

%Get the names without the extension
y = extractBefore(names, '.jpg').'

y = 5×1 cell array
    {'Plot_173'}
    {'Plot_28' }
    {'Plot_38' }
    {'Plot_552'}
    {'Plot_965'}

%Write to a text file via writecell() - Assuming you are working with R2019a
%or a later version
writecell(y, 'pics_names.txt')

%Display the contents of the saved text file
type pics_names.txt

Plot_173
Plot_28
Plot_38
Plot_552
Plot_965

[Sim]

Thanks a lot @Stephen23 and @Dyuman Joshifor your insightful comments and solutions!

I will try to integrate both of your solutions to my stuff, many many thanks! :-) :-) :-)

Answer 1

[Anjaneyulu Bairi]

Hi,

I understand that you are trying to write the filenames into a text file using fwrite but getting error on "fwrite". You can visit the article on the "fwrite" error at the end of the answer and instead of using "fwrite", you can use "fprintf" to write the filenames into a text file.

Here is the full code for reference:

a = dir('/.../myfolder');
b = {a(3:end).name}';
b = string(strrep(b, '.jpg', ''))
fid =fopen('/.../pics_names.txt', 'w' );
for i = 1:length(b)
    fprintf(fid, '%s\n', b(i));
end
fclose(fid);

Atricle on fwrite error : https://in.mathworks.com/matlabcentral/answers/92726-why-do-i-receive-cannot-write-value-unsupported-class-struct-error-when-writing-a-structure-using

• You can also visit the below MathWorks documentaion link for more information on "fwrite" and "fprintf" : https://in.mathworks.com/help/matlab/ref/fwrite.html
• https://in.mathworks.com/help/matlab/ref/fprintf.html

I hope it helps to resolve your query.

[Sim]

Thanks a lot @Anjaneyulu Bairi, very kind! :-)

[Stephen23]

The code has superfluous type changes and some latent bugs (e.g. assumes but does not check the folder contains only the required image files, assumes but does not check file extensions, assumes but does not check if dot directories are the first two names returned by DIR).

More robust by using the DIR format. Simpler with WRITECELL:

S = dir('/.../myfolder/*.jpg');
C = regexprep({S.name}, '\.jpg$', '', 'ignorecase');
writecell(C(:),'/.../pics_names.txt')

or using FPRINTF:

fid = fopen('/.../pics_names.txt', 'wt');
fprintf(fid, '%s\n', C{:});
fclose(fid);