std:: expint, std:: expintf, std:: expintl

From cppreference.com
C++
Experimental
Mathematical special functions
double expint ( double arg ) ;

double expint ( float arg ) ;
double expint ( long double arg ) ;
float expintf ( float arg ) ;

long double expintl ( long double arg ) ;
(1)
double expint ( IntegralType arg ) ;
(2)
1) Computes the exponential integral of arg .
2) A set of overloads or a function template accepting an argument of any integral type . Equivalent to (1) after casting the argument to double .

As all special functions, expint is only guaranteed to be available in <cmath> if __STDCPP_MATH_SPEC_FUNCS__ is defined by the implementation to a value at least 201003L and if the user defines __STDCPP_WANT_MATH_SPEC_FUNCS__ before including any standard library headers.

Parameters

arg - value of a floating-point or Integral type

Return value

If no errors occur, value of the exponential integral of arg , that is - ∫ ∞-arg
e -t
t
d t , is returned.

Error handling

Errors may be reported as specified in math_errhandling .

  • If the argument is NaN, NaN is returned and domain error is not reported.
  • If the argument is ±0, -∞ is returned.

Notes

Implementations that do not support TR 29124 but support TR 19768, provide this function in the header tr1/cmath and namespace std::tr1 .

An implementation of this function is also available in boost.math .

Example

(works as shown with gcc 6.0)

Run this code 
#define __STDCPP_WANT_MATH_SPEC_FUNCS__ 1
#include <cmath>
#include <iostream>
 
int main()
{
    std::cout << "Ei(0) = " << std::expint(0) << '\n'
              << "Ei(1) = " << std::expint(1) << '\n'
              << "Gompetz constant = " << -std::exp(1) * std::expint(-1) << '\n';
}

Output: 

Ei(0) = -inf
Ei(1) = 1.89512
Gompetz constant = 0.596347

External links

Weisstein, Eric W. "Exponential Integral." From MathWorld--A Wolfram Web Resource.

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