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Calculating an Integral

Similar to the generic derivative example, we can calculate integrals in a similar manner:

template<typename value_type, typename function_type>
inline value_type integral(const value_type a,
                           const value_type b,
                           const value_type tol,
                           function_type func)
{
   unsigned n = 1U;

   value_type h = (b - a);
   value_type I = (func(a) + func(b)) * (h / 2);

   for(unsigned k = 0U; k < 8U; k++)
   {
      h /= 2;

      value_type sum(0);
      for(unsigned j = 1U; j <= n; j++)
      {
         sum += func(a + (value_type((j * 2) - 1) * h));
      }

      const value_type I0 = I;
      I = (I / 2) + (h * sum);

      const value_type ratio     = I0 / I;
      const value_type delta     = ratio - 1;
      const value_type delta_abs = ((delta < 0) ? -delta : delta);

      if((k > 1U) && (delta_abs < tol))
      {
         break;
      }

      n *= 2U;
   }

   return I;
}

The following sample program shows how the function can be called, we begin by defining a function object, which when integrated should yield the Bessel J function:

template<typename value_type>
class cyl_bessel_j_integral_rep
{
public:
   cyl_bessel_j_integral_rep(const unsigned N,
      const value_type& X) : n(N), x(X) { }

   value_type operator()(const value_type& t) const
   {
      // pi * Jn(x) = Int_0^pi [cos(x * sin(t) - n*t) dt]
      return cos(x * sin(t) - (n * t));
   }

private:
   const unsigned n;
   const value_type x;
};

   /* The function can now be called as follows: */
int main(int, char**)
{
   using boost::math::constants::pi;
   typedef boost::multiprecision::cpp_dec_float_50 mp_type;

   const float j2_f =
      integral(0.0F,
      pi<float>(),
      0.01F,
      cyl_bessel_j_integral_rep<float>(2U, 1.23F)) / pi<float>();

   const double j2_d =
      integral(0.0,
      pi<double>(),
      0.0001,
      cyl_bessel_j_integral_rep<double>(2U, 1.23)) / pi<double>();

   const mp_type j2_mp =
      integral(mp_type(0),
      pi<mp_type>(),
      mp_type(1.0E-20),
      cyl_bessel_j_integral_rep<mp_type>(2U, mp_type(123) / 100)) / pi<mp_type>();

   // 0.166369
   std::cout
      << std::setprecision(std::numeric_limits<float>::digits10)
      << j2_f
      << std::endl;

   // 0.166369383786814
   std::cout
      << std::setprecision(std::numeric_limits<double>::digits10)
      << j2_d
      << std::endl;

   // 0.16636938378681407351267852431513159437103348245333
   std::cout
      << std::setprecision(std::numeric_limits<mp_type>::digits10)
      << j2_mp
      << std::endl;

   //
   // Print true value for comparison:
   // 0.166369383786814073512678524315131594371033482453329
   std::cout << boost::math::cyl_bessel_j(2, mp_type(123) / 100) << std::endl;
}


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