This section of code, as originally written, assumes you're on Linux or Mac. We can make this a bit more platform independent.
SourcePath = fullfile(rootDir, 'subf1', 'subf2', 'subf3', 'subf4', 'subf5', 'subf6');
Now let's assemble the participant part.
for i = 1 : length(index)
As you saw when you tried, you can't put 'sub-s' (5 characters) into 1 element of newParticipant. I'm going to use a string array here instead of a char array because it makes the code to define newParticipant really easy.
newParticipant = "sub-s" + index(i);
Now let fullfile do all the work for you; don't mix manually concatenating the source path, the participant directory name, and more sub-paths together.
SourceFolder = fullfile(SourcePath,newParticipant,'subf8','subf9')
end
SourceFolder = "/subf1/subf2/subf3/subf4/subf5/subf6/sub-s1001/subf8/subf9"
SourceFolder = "/subf1/subf2/subf3/subf4/subf5/subf6/sub-s1002/subf8/subf9"
Note that because newParticipant was a string array, so are both the SourceFolder variables. Most functions that accept char arrays should also accept string arrays, but if you have to use a function that doesn't call char on it to convert it into a char vector. Note that even though the code above displayed two SourceFolder values, that's because it was in a for loop. The variable contains only the value from the last iteration, so SourceFolderChar will contain only one char vector.
SourceFolderChar = char(SourceFolder)
SourceFolderChar = '/subf1/subf2/subf3/subf4/subf5/subf6/sub-s1002/subf8/subf9'
