Control Parameters
Various controls exist within the Optimizer to govern the solution procedure and the form of output. The majority of these take integer values and act as switches between various types of behavior. The tolerances on values are double precision, and there are a few controls which are character strings, setting names to structures. Any of these may be altered by the user to enhance performance of the Optimizer. However, it should be noted that the default values provided have been found to work well in practice over a range of problems and caution should be exercised if they are changed.
Retrieving and Changing Control Values
Console Xpress users may obtain control values by issuing the control name at the Optimizer prompt, >, and hitting the RETURN key. Controls may be set using the assignment syntax:
control_name = new_value
where new_value is an integer value, double or string as appropriate. For character strings, the name must be enclosed in single quotes and all eight characters must be given.
Users of the Xpress‑MP Libraries are provided with the following set of functions for setting and obtaining control values:
XPRSgetintcontrol XPRSgetdblcontrol XPRSgetstrcontrol XPRSsetintcontrol XPRSsetdblcontrol XPRSsetstrcontrol It is an important point that the controls as listed in this chapter must be prefixed with XPRS_ to be used with the Xpress‑MP Libraries and failure to do so will result in an error. An example of their usage is as follows:
XPRSgetintcontrol(prob, XPRS_PRESOLVE, &presolve); printf("The value of PRESOLVE is %d\n", presolve); XPRSsetintcontrol(prob, XPRS_PRESOLVE, 1-presolve); printf("The value of PRESOLVE is now %d\n", 1-presolve);
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