std::fdim
From cppreference.com
                    
                                        
                    
                    
                                                            
                    |   Defined in header <cmath>
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|   float       fdim( float x, float y );  | 
(1) | (since C++11) | 
|   double      fdim( double x, double y );  | 
(2) | (since C++11) | 
|   long double fdim( long double x, long double y );  | 
(3) | (since C++11) | 
|   Promoted    fdim( Arithmetic x, Arithmetic y );  | 
(4) | (since C++11) | 
Returns the positive difference between x and y. This could be implemented as fmax (x - y, 0), so if x ≤ y, the result is always equals to 0, otherwise it is x - y.
4) If any argument has integral type, it is cast to double. If any other argument is long double, then the return type is long double, otherwise it is double.
[edit] Parameters
| x, y | - | floating point value | 
[edit] Return value
The positive difference value.
[edit] See also
|    (C++11)  | 
   computes absolute value of an integral value (|x|)  (function)  | 
|    (C++11)  | 
   larger of two floating point values   (function)  |