< cpp‎ | numeric
Defined in header <cmath>
constexpr float       lerp( float a, float b, float t ) noexcept;
(1) (since C++20)
constexpr double      lerp( double a, double b, double t ) noexcept;
(2) (since C++20)
constexpr long double lerp( long double a, long double b, long double t ) noexcept;
(3) (since C++20)
constexpr Promoted    lerp( Arithmetic1 a, Arithmetic2 b, Arithmetic3 t ) noexcept;
(4) (since C++20)
1-3) Computes a+t(b−a), i.e. the linear interpolation between a and b for the parameter t (or extrapolation, when t is outside the range [0,1]).
4) A set of overloads or a function template for all combinations of arguments of arithmetic type not covered by 1-3). 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

a, b, t - values of floating-point or integral types

[edit] Return value


When isfinite(a) && isfinite(b), the following properties are guaranteed:

  • If t == 0, the result is equal to a.
  • If t == 1, the result is equal to b.
  • If t >= 0 && t <= 1, the result is finite.
  • If isfinite(t) && a == b, the result is equal to a.
  • If isfinite(t) || (!isnan(t) && b-a != 0), the result is not NaN.

Let CMP(x,y) be 1 if x > y, -1 if x < y, and 0 otherwise. For any t1 and t2, the product of CMP(lerp(a, b, t2), lerp(a, b, t1)), CMP(t2, t1), and CMP(b, a) is non-negative. (That is, lerp is monotonic.)

[edit] Notes

lerp is available in the global namespace when <math.h> is included, even if it is not a part of C.

[edit] Example

#include <iostream>
#include <cmath>
int main()
    float a=10.0f, b=20.0f;
    std::cout << "a=" << a << ", " << "b=" << b << '\n'
              << "mid point=" << std::lerp(a,b,0.5f) << '\n'
              << std::boolalpha << (a == std::lerp(a,b,0.0f)) << ' '
              << std::boolalpha << (b == std::lerp(a,b,1.0f)) << '\n';


a=10, b=20
mid point=15
true true

[edit] See also

midpoint between two numbers or pointers
(function template) [edit]