for loop matrix dimensions for analysis with EEG Lab

Attempting to create a for-loop for analysis with EEG Lab. The error which keeps reoccurring up is in setting the directory from which to read the the files for analysis. Other parts of the script run fine in isolation from this. In creating the directory variable I've set up the dir(fullfile('S:', etc). When I begin the loop at for i = 1:1000 str - strcat(same file name from the directory, with the exception of (i) for subject number) I keep getting the error that the index exceeds the matrix dimensions. I've tried reformatting this to have the apostrophe indexes, or by combining the file directory with the backslash... Same error reoccurs each time.

Any help is appreciated. Thanks.