Another optional file format is BAS. Unlike the LP and MPS formats, this format is not used to store a description of the problem statement. Rather, it is used to store information about the solution to a problem, information known as a basis. Even after changes are made to the problem, using a prior basis to jump-start the optimization can speed solution time considerably. A basis can be written only after a problem has been solved. Try this now with the following command:

write example.bas

In response, ILOG CPLEX displays a confirmation message, like this:

Basis written to file 'example.bas'.

When a very large problem is being solved by the primal or dual simplex optimizer, a file with the format extension .xxx is automatically written after every 50,000 iterations (a frequency that can be adjusted by the set simplex basisinterval command). This periodically written basis can be useful as insurance against the possibility that a long optimization may be unexpectedly interrupted due to power failure or other causes, because the optimization can then be restarted using this advanced basis.