Using Variable to Assess a Struct

I'm trying to have this statement pass me the length of the '..._matlabout(1,%i)' Structure Array by utilizing a variable so I can have the script perform the task on multiple documents. The structure array and individual value are mapped to variables 'b' and 'c' The problem I have is that the "length(b)" calculation returns the length of characters in "test_%i_out_matlabout(1,%i).data" instead of the actual length of the numerical array.

Does anyone know how to solve this issue? The same somewhat applies to the use of the variable 'c' as well in that I am trying to compare the array value instead of the string value of 'c'.

Thanks!

for z=1:2
b = sprintf('test_%i_out_matlabout(1,%i).data',i1,z);
    for i=1:length(b)
        c = sprintf('test_%i_out_matlabout(1,%i).data(%d)',i1,z,i);
        if (c >= 3550)
            c = 5550;
        end