std::exponential_distribution::lambda
From cppreference.com
                    
                                        
                    < cpp | numeric | random | exponential distribution
                    
                                                            
                    |   RealType lambda() const;  | 
(since C++11) | |
Returns the λ distribution parameter the distribution was constructed with. The parameter defines the rate of events, per unit. The default value is 1.0.
[edit] Parameters
(none)
[edit] Return value
Floating point value identifying the rate of events per unit
[edit] See also
|    gets or sets the distribution parameter object   (public member function)  | |