C++17 std::cyl_bessel_i用法及代碼示例

// C++17 code for bessel function 
#include <bits/stdc++.h> 
using namespace std; 
  
// Compute the answer from the formulae for first 10 terms 
long double answer(long double x, long double v) 
{ 
  
    long double ans_by_expansion = 0; 
    long double fact = 1; 
  
    for (int k = 0; k < 10; fact = fact * (++k)) { 
        ans_by_expansion += pow((x / 2), (2 * k)) / pow(fact, 2); 
        cout << "ans_by_expansion till term k = "; 
        cout << k << " is " << ans_by_expansion << "\n"; 
    } 
  
  return ans_by_expansion; 
} 
  
// Driver code 
int main() 
{ 
    long double x = 2.798465; 
    long double v = 0; 
  
    // Compute the Regular Modified Bessel Function 
    // for v = 0, x = 2.798465 
    long double ans_by_function = cyl_bessel_i(v, x); 
  
    cout << setprecision(15) << fixed; 
    cout << "The answer by function for "
         << "Regular_Modified_Bessel_Function" << endl 
         << "(" << v << ", " << x << ") = "
         << ans_by_function << "\n"; 
  
    // calculate answer by expansion 
    long double ans_by_expansion = answer(x, v); 
  
    cout << "Absolute Error in answer by both the methods is = "; 
    cout << abs(ans_by_expansion - ans_by_function) << "\n"; 
  
    return 0; 
}