Mutating Programs

After the best programs are chosen, they are replicated and modified for the next generation. As mentioned earlier, mutation takes a single program and alters it slightly. The tree programs can be altered in a number of ways—by changing the function on a node or by altering its branches. A function that changes the number of required child nodes either deletes or adds new branches, as shown in Figure 11-3.

The other way to mutate is by replacing a subtree with an entirely new one, as shown in Figure 11-4.

Mutation is not something that should be done too much. You would not, for instance, mutate the majority of nodes in a tree. Instead, you can assign a relatively small probability that any node will be modified. Beginning at the top of the tree, if a random number is lower than that probability, the node is mutated in one of the ways described above; otherwise, the test is performed again on its child nodes.