ATANPI (The GNU Fortran Compiler)

### 8.31 `ATANPI` — Circular arc tangent function ¶

Description:

  `ATANPI(X)` computes \atan(x) / \pi. `ATANPI(Y, X)` computes {\rm atan2}(y, x) / \pi. These provide a measure of an angle in half-revolutions.

Standard:

  Fortran 2023

Class:

  Elemental function

Syntax:

  `RESULT = ATANPI(X)`
  `RESULT = ATANPI(Y, X)`

Arguments:

  Y | The type shall be `REAL`.
  X | If Y appears, X shall have the same type and kind as Y. If Y is zero, then X shall not be zero. If Y does not appear in a function reference, then X shall be `REAL`.

Return value:

  The return value has the same type and kind as X. It is expressed in half-revolutions and satisfies -0.5 \leq \atanpi (x) \leq 0.5.

Example:

      program test_atanpi
        implicit none
        real, parameter :: x = 0.123, y(3) = [0.123, 0.45, 0.8]
        real, parameter :: a = atanpi(x), b(3) = atanpi(y)
        call foo(x, y)
      contains
        subroutine foo(u, v)
          real, intent(in) :: u, v(:)
          real :: f, g(size(v))
          f = atanpi(u)
          g = atanpi(v)
          if (abs(a - f) > 8 * epsilon(f)) stop 1
          if (any(abs(g - b) > 8 * epsilon(f))) stop 2
        end subroutine foo
      end program test_atanpi

See also:

  `ACOSPI` — Circular arc cosine function
  `ASINPI` — Circular arc sine function
  `ATAN2PI` — Circular arc tangent function