Hi Reen,
I'm not sure which method you want to use for calculating an accuracy value, but here's one approach we may try:
% output= evalfis(fis, input);
pred = round(output);
acc_count = nnz(pred==input);
acc = acc_count/length(input);
Essentially, I'm considering round values of the fuzzy system as the predictions obtained and then counting the number of correct predictions over the total number of inputs.
I think you could also use different functions, such as log or tanh, instead of rounding your predictions - that would be another kind of accuracy metric.
Hope it helps!