if conditions MATLAB

Using MATLAB, I wish to make a specific calculation when a .mat file is loaded. This is the problem:

If file a is loaded, calculate speed = 160 * str . If file b is loaded, calculate speed = 200 * str where str is a common data in the two files but vary because they are simulation results of a model in two different conditions.

Since I wish to make the calculations in one program, I think there is a need to use the "if" statement but I do not know how.