Add ExprTk_BUILD_EXAMPLES to build examples

Also move examples into an examples folder

examples/exprtk_simple_example_17.cpp

@@ -0,0 +1,77 @@
+/*
+ **************************************************************
+ *         C++ Mathematical Expression Toolkit Library        *
+ *                                                            *
+ * Simple Example 17                                          *
+ * Author: Arash Partow (1999-2023)                           *
+ * URL: https://www.partow.net/programming/exprtk/index.html  *
+ *                                                            *
+ * Copyright notice:                                          *
+ * Free use of the Mathematical Expression Toolkit Library is *
+ * permitted under the guidelines and in accordance with the  *
+ * most current version of the MIT License.                   *
+ * https://www.opensource.org/licenses/MIT                    *
+ *                                                            *
+ **************************************************************
+*/
+
+
+#include <cstdio>
+#include <cstdlib>
+#include <ctime>
+#include <string>
+
+#include "exprtk.hpp"
+
+template <typename T>
+struct rnd_01 : public exprtk::ifunction<T>
+{
+   using exprtk::ifunction<T>::operator();
+
+   rnd_01() : exprtk::ifunction<T>(0)
+   { ::srand(static_cast<unsigned int>(time(NULL))); }
+
+   inline T operator()()
+   {
+      // Note: Do not use this in production
+      // Result is in the interval [0,1)
+      return T(::rand() / T(RAND_MAX + 1.0));
+   }
+};
+
+template <typename T>
+void monte_carlo_pi()
+{
+   typedef exprtk::symbol_table<T> symbol_table_t;
+   typedef exprtk::expression<T>   expression_t;
+   typedef exprtk::parser<T>       parser_t;
+
+   const std::string monte_carlo_pi_program =
+                  " var experiments[5 * 10^7] := [(rnd_01^2 + rnd_01^2) <= 1]; "
+                  " 4 * sum(experiments) / experiments[];                      ";
+
+   rnd_01<T> rnd01;
+
+   symbol_table_t symbol_table;
+   symbol_table.add_function("rnd_01",rnd01);
+
+   expression_t expression;
+   expression.register_symbol_table(symbol_table);
+
+   parser_t parser;
+   parser.compile(monte_carlo_pi_program,expression);
+
+   const T approximate_pi = expression.value();
+
+   const T real_pi = T(3.141592653589793238462643383279502); // or close enough...
+
+   printf("pi ~ %20.17f\terror: %20.17f\n",
+          approximate_pi,
+          std::abs(real_pi - approximate_pi));
+}
+
+int main()
+{
+   monte_carlo_pi<double>();
+   return 0;
+}