I would expect this to work:
names = dir([file_path '/*x' num2str(i) 'y' num2str(j) '.txt']);
z = {names.name}
Let's do a quick check and make sure there are some files that match your pattern in the directory.
ls(fullfile(file_path, '*.txt'))
If this says something like "*.txt not found" then the directory doesn't contain any files with the .txt extension, or MATLAB can't find the directory whose path is stored in the variable file_path. [You've expressed it as a UNIX path; are you on Mac OS X or Linux? If you're on Windows, you'll need to change file_path to be a Windows path.]
If that does show some files, let's check for those whose names contain x then y, separated by some characters.
ls(fullfile(file_path, '*x*y*.txt'))
Does that list any files? If it does, let's go the next step and write a little tweak of the command in the first code section.
filepattern = sprintf('*x%dy%d.txt', i, j) % Check that this looks like what you want
names = dir(fullfile(file_path, filepattern))
If there are files in the directory that match the pattern from the third code section above (x*y.txt) that also match the pattern stored in filepattern (*x#y#.txt where # is one- or potentially two-digit number) they should show up in the names struct array.
