Why do the DIR and LS functions in MATLAB not sort numerical filenames in numerical order?

The DIR and LS functions in MATLAB do not sort numerical filenames in numerical order.

For example, I have filenames of the form:

 1-1-1.tif

 1-1-2.tif

 1-1-3.tif

 . . .

 1-1-9.tif

 1-1-10.tif

 1-1-11.tif

However, when I use DIR or LS to list the files using:

files=dir('*.tif')

The results stored in "dir.name" are:

 1-1-1.tif

 1-1-10.tif

 1-1-11.tif

 1-1-2.tif

 1-1-3.tif

 1-1-9.tif

I would like to order these numerically.

Answer 1

[MathWorks Support Team]

The DIR or LS functions in MATLAB sort the strings in ASCII dictionary order. This can cause a problem if you want to sort numbered files which do not have leading zeros.

Currently, to work around this problem, you can use the text manipulation and sorting routines in MATLAB.

Here is an example:

files=dir('*.tif');

numfiles = size(files,1); % Find number of files

numdelim = 3; % Number of delimiters

delims = ['-' '.']; % Delimiters used

% Create a matrix of the file list index and the delimiters

filenums=[ [1:numfiles]' zeros(numfiles,3) ];

for i=1:numfiles % Cycles through list of files

      rem=files(i).name;

      for j=1:numdelim % Cycles through the filename delimiters

          [token,rem] = strtok(rem,delims);

          filenums(i,j+1) = str2num(token);

      end

end

% Sort the matrix by rows

filenums = sortrows(filenums,[2:numdelim+1]);

% Show the filenames in the new order

files(filenums(:,1)).name

% Create a cell array of the filenames in new order

for i=1:numfiles

      sortedfiles{i,1} = files(filenums(i,1)).name;

end

[Stephen23]

"The DIR or LS functions in MATLAB sort the strings in ASCII dictionary order."

Is this order documented anywhere? Neither the dir nor the ls documentation include this information.

Until August 2015 the dir documentation stated "Results appear in the order returned by the operating system":

https://web.archive.org/web/20150812013926/www.mathworks.com/help/matlab/ref/dir.html

This means when this answer was written in 2009 either it or the documentation was incorrect. Or both.

[Stephen23]

"When verifying with a few releases of MATLAB the result of "dir" function is indeed ASCII ordered."

That does not answer my question. I am very curious what possible method you could have used that relies on "verifying with a few releases" that would distinguish between these two cases:

• MATLAB sorting the filenames (which currently is not documented anywhere),
• MATLAB returning the filenames in the order provided by the OS (which was previously documented).

As far as I can tell, simple testing at the MATLAB user interface level will not distinguish between them.

Would you care to give a few details of this "verifying" method?

[Stephen23]

"The DIR or LS functions in MATLAB sort the strings in ASCII dictionary order."

Easily disproved by actually running LS and DIR on a few very basic filenames:

ls *.txt

10.txt	1.txt  2.txt

dir *.txt

1.txt   10.txt  2.txt   

Note that those are different orders.

Different orders means it is not possible for both of them to be "in ASCII dictionary order" as this answer (most likely incorrectly) states that MATLAB sorts them into. One of them does happen to be in ASCII order, but certainly not both of them (unless MST has invented a completely new, undocumented, variable ASCII order).

[Walter Roberson]

The R2021a documentation for dir() and ls() do not document any ordering.

ASCII dictionary order is not likely, as the supported operating systems use various forms of unicode.

I just did a test on my Mac;

!touch q R
ls

q  R

On my Mac, R was before q -- which is consistent with sorting by codepoint, but is not consistent with ascii dictionary order.

Answer 2

[Stephen23]

Simple solution: download my FEX submission natsortfiles, which sorts filenames into alphanumeric order:

S = dir('*.txt');
S.name

ans = '1.txt'

ans = '10.txt'

ans = '2.txt'

S = natsortfiles(S); % alphanumeric sort by filename
S.name

ans = '1.txt'

ans = '2.txt'

ans = '10.txt'

https://blogs.mathworks.com/pick/2014/12/05/natural-order-sorting/