 Conditional algorithm parameter

A conditional algorithm parameter matters only when a stated condition on other settings holds. In an algorithm parameter space, the condition determines whether the child parameter is active. The ConfigSpace guide samples conditional settings only when their activation conditions are met.^[ref]

For an original example, an algorithm has modes plain and tuned. A depth setting $d\in \{1,2,3,4\}$ affects only tuned mode. The naive rectangular encoding has eight pairs, but the distinct effective settings areThe four plain-mode encodings describe the same effective choice, so there are five settings, not eight. The star denotes inactivity; it is not an additional depth value used by the algorithm.

ParamILS excludes local moves that change only an inactive parameter, since such a move does not alter effective behavior.^[ref] Switching from plain to tuned activates a choice of depth, while changing an unused plain-mode depth alone supplies no new measured behavior.

A forbidden parameter combination instead rules out a joint assignment. Inactive does not mean forbidden, missing data or a numeric zero. Recording the mode and activity conditions makes a returned configuration interpretable, especially when comparing costs or reusing settings in another implementation.