Add ExprTk_BUILD_EXAMPLES to build examples

Also move examples into an examples folder

examples/exprtk_simple_example_02.cpp

@@ -0,0 +1,72 @@
+/*
+ **************************************************************
+ *         C++ Mathematical Expression Toolkit Library        *
+ *                                                            *
+ * Simple Example 2                                           *
+ * 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 <string>
+
+#include "exprtk.hpp"
+
+template <typename T>
+void square_wave()
+{
+   typedef exprtk::symbol_table<T> symbol_table_t;
+   typedef exprtk::expression<T>   expression_t;
+   typedef exprtk::parser<T>       parser_t;
+
+   const std::string expr_string =
+                  "a*(4/pi)*"
+                  "((1 /1)*sin( 2*pi*f*t)+(1 /3)*sin( 6*pi*f*t)+"
+                  " (1 /5)*sin(10*pi*f*t)+(1 /7)*sin(14*pi*f*t)+"
+                  " (1 /9)*sin(18*pi*f*t)+(1/11)*sin(22*pi*f*t)+"
+                  " (1/13)*sin(26*pi*f*t)+(1/15)*sin(30*pi*f*t)+"
+                  " (1/17)*sin(34*pi*f*t)+(1/19)*sin(38*pi*f*t)+"
+                  " (1/21)*sin(42*pi*f*t)+(1/23)*sin(46*pi*f*t)+"
+                  " (1/25)*sin(50*pi*f*t)+(1/27)*sin(54*pi*f*t))";
+
+   static const T pi = T(3.141592653589793238462643383279502);
+
+   const T f = pi / T(10);
+   const T a = T(10);
+         T t = T(0);
+
+   symbol_table_t symbol_table;
+   symbol_table.add_variable("t",t);
+   symbol_table.add_constant("f",f);
+   symbol_table.add_constant("a",a);
+   symbol_table.add_constants();
+
+   expression_t expression;
+   expression.register_symbol_table(symbol_table);
+
+   parser_t parser;
+   parser.compile(expr_string,expression);
+
+   const T delta = (T(4) * pi) / T(1000);
+
+   for (t = (T(-2) * pi); t <= (T(+2) * pi); t += delta)
+   {
+      const T result = expression.value();
+      printf("%19.15f\t%19.15f\n", t, result);
+   }
+}
+
+int main()
+{
+   square_wave<double>();
+   return 0;
+}