Rather than using this slow, complex, and buggy way of storing your data, it would be simpler to store all of the data in one structure. Then accessing the data is simple using its fieldnames: no ugly eval is required.
Note that the MATLAB documentation specifically advises against what you are doing: "A frequent use of the eval function is to create sets of variables such as A1, A2, ..., An, but this approach does not use the array processing power of MATLAB and is not recommended. The preferred method is to store related data in a single array". Note that either creating or accessing variable names dynamically suffers from the same disadvantages described in the documentation.
The important question is: how did you get all of those variables into your workspace? Usually beginners do this by calling load without an output argument, and spamming lots of variables into the workspace, which are then very difficult to process (and so they resort to writing ugly, slow, complex, buggy code using eval, and getting the variable names using slow who or whos). There is most likely an easy way around what you are doing, e.g. by simply calling load with an output argument:
S = load(...);
You might also like to read this discussion of the topic:
