Crossover probability for SubtreeCrossover

[gkronber]

Some genetic programming variants allow a copy operation where neither crossover nor mutation are applied to selected parent trees.

This is helpful for certain problem instances using a multi-tree representation (via MultiEncoding) where we want to prevent that a crossover event causes crossover for all trees.

One way to add this is to add a crossover rate to the general genetic algorithm variations that we are using in HL for GP.

Another and easier way is to implement this in the crossover operator using a crossover probability.

[gkronber]

r17490: add crossover probability for subtree crossover and initialized it to 100%

This change breaks reproduceability of old experiments because we added a call to the random number generator.

[gkronber]

r17491: fix reproducability by calling the PRNG only when probability is < 1

[gkronber]

r17492: adapted unit test to compile with r17490

[mkommend]

Reviewed r17490, r17491, and r17492.

I have two comments that should be addressed:

• Why have you chosen to use a DoubleValue instead of a PercentValue? The latter is displayed more nicely, in line with InternalCrossoverPointProbability (which is also a PercentValue) and can be restricted to [0,1].
• What is the reason for randomly selecting a parent if no crossover is performed? I would have expected that the first parent (parent0) is always returned. Similarly to the fallback if no valid crossover points in the parents can be chosen.

[gkronber]

r17871:17872: changed CrossoverProbabilityParameter to PercentValue and return parent0 when no crossover is performed.

[gkronber]

For OSGPC and the Pagie-1 problem we have:

No big effect for symbolic regression problems with a single tree (higher execution time is caused by resampling in OS). The effect is expected to be more critical with problem encodings made up of several trees.