As the error message tells you, ismember operates on cells only, if they are cell strings. In your care the data are not cell strings. You can find out, what they are instead:
dbstop if all error
Then runs the code again. When Matlab stops at the error, type this in the command window:
class(looking_up)
class(looking_up{1})
class(ff)
class(ff{1})
Due to the absence of comments, we cannot guess what you want to achieve.
