add crossover/mutation examples

examples/7_genetic_operators.cpp

@@ -0,0 +1,76 @@
+/* Example showing the usage of the genetic operators in the GAs. */
+
+#include "gapp.hpp"
+#include <algorithm>
+#include <iostream>
+#include <cassert>
+
+using namespace gapp;
+
+class MyCrossover : public crossover::Crossover<PermutationGene>
+{
+public:
+    using Crossover::Crossover;
+
+    CandidatePair<GeneType> crossover(const GA<GeneType>&, const Candidate<GeneType>& parent1, const Candidate<GeneType>& parent2) const override
+    {
+        auto child1 = parent1;
+        auto child2 = parent2;
+
+        // perform the crossover ...
+
+        return { std::move(child1), std::move(child2) };
+    }
+};
+
+class MyMutation : public mutation::Mutation<PermutationGene>
+{
+public:
+    using Mutation::Mutation;
+
+    void mutate(const GA<GeneType>&, const Candidate<GeneType>&, Chromosome<GeneType>& chromosome) const override
+    {
+        if (rng::randomReal() < mutation_rate())
+        {
+            std::reverse(chromosome.begin(), chromosome.end());
+        }
+    }
+};
+
+int main()
+{
+    PermutationGA ga;
+    ga.solve(problems::TSP52{}); // run using the default crossover and mutation methods
+
+    // using other crossover, mutation operators
+
+    ga.crossover_method(crossover::perm::Edge{}); // using the default crossover probability
+    ga.mutation_method(mutation::perm::Inversion{ /* mutation_rate = */ 0.3 });
+
+    std::cout << "The default crossover probability is "<< ga.crossover_rate() << ".\n";
+
+    // changing the crossover and mutation probabilities
+
+    ga.crossover_method(crossover::perm::Edge{ /* crossover_rate = */ 0.92 });
+    std::cout << "The crossover probability is " << ga.crossover_rate() << ".\n";
+
+    ga.crossover_rate(0.71);
+    ga.mutation_rate(0.1);
+
+    std::cout << "The crossover probability is " << ga.crossover_rate() << ".\n";
+    std::cout << "The mutation probability is " << ga.mutation_rate() << ".\n";
+
+    // user defined crossover and mutation methods
+
+    ga.crossover_method(MyCrossover{ /* crossover_rate = */ 0.123 });
+    ga.mutation_method(MyMutation{ /* mutation_rate = */ 0.456 });
+
+    // using a repair function
+
+    ga.repair_function([](const GA<PermutationGene>&, const Chromosome<PermutationGene>& chrom)
+    {
+        auto new_chrom = chrom;
+        std::swap(new_chrom.front(), new_chrom.back());
+        return new_chrom;
+    });
+}