Solution Options
We describe here some of the basic options in solving linear programming problems. Although the tutorial example does not make use of these options, you will find them useful when handling larger, more realistic problems. The topics are:
For detailed information on performance options, refer to the ILOG CPLEX User's Manual.
Every time ILOG CPLEX solves a problem, much of the information appearing on the screen is also directed into a log file. This file is automatically created by ILOG CPLEX with the name cplex.log. If there is an existing cplex.log file in the directory where ILOG CPLEX is launched, ILOG CPLEX will append the current session data to the existing file. If you want to keep a unique log file of a problem session, you can change the default name with the set logfile command. (See the ILOG CPLEX User's Manual.) The log file is written in standard ASCII format and can be edited with any text editor.
You may re-solve the problem by reissuing the optimize command. ILOG CPLEX restarts the solution process from the previous optimal basis, and thus requires zero iterations. If you do not wish to restart the problem from an advanced basis, use the set advance command to turn off the advanced start indicator.
Remember that a problem must be present in memory (entered via the enter command or read from a file) before you issue the optimize command.
In addition to the optimize command, ILOG CPLEX can use the primal simplex optimizer (primopt command), the dual simplex optimizer (tranopt command), the barrier optimizer (baropt command) and the network optimizer (netopt command). Many problems can be solved faster using these alternative optimizers, which are described in more detail in the ILOG CPLEX User's Manual. If you want to solve a mixed integer programming problem, the optimize command is equivalent to the mipopt command.
Our short example was solved very quickly. However, larger problems, particularly mixed integer problems, can take much longer. Occasionally it may be useful to interrupt the optimization process. ILOG CPLEX allows such interruptions if you use control-c. (The control and c keys must be pressed simultaneously.) Optimization is interrupted, and ILOG CPLEX issues a message indicating that the process was stopped and displays progress information. If you issue another optimization command in the same session, ILOG CPLEX will resume optimization from where it was interrupted.
