In my opinion, the best course of action is to modify the boundary.m function or script. Since it takes no arguments, it probably is a script, which I'd change into a function. You can then add an optional argument to that function which would be the file name to write to, so you'd have:
function boundary(filename)
if nargin == 0 || ~isempty(filename)
filename = 'result.txt'; %default filename if not specified
end
%...
end
You'd then modify the main code to:
for step = 1:10
boundary(sprintf('result_%02d.txt', step));
end
to create files result_01.txt, result_02.txt, ..., result_10.txt.
This is really how your code should be designed. Hardcoded values (inputs, filenames, etc.) are never a good idea. There will always be a time when you don't want to use these hardcoded values. They should always be inputs. If necessary, make them optional.
If for some reason modifying boundary.m is not an option, you can always rename the created file. You only have to modify your main loop:
for step = 1:10
boundary
movefile('result.txt', sprintf('result_%02d.txt', step));
end
But again, the first approach is a much better design.
