..
  Copyright 1988-2021 Free Software Foundation, Inc.
  This is part of the GCC manual.
  For copying conditions, see the GPL license file

.. _acos:

ACOS --- Arccosine function 
****************************

.. index:: ACOS

.. index:: DACOS

.. index:: trigonometric function, cosine, inverse

.. index:: cosine, inverse

.. function:: ACOS(X)

  ``ACOS(X)`` computes the arccosine of :samp:`{X}` (inverse of ``COS(X)`` ).

  :param X:
    The type shall either be ``REAL`` with a magnitude that is
    less than or equal to one - or the type shall be ``COMPLEX``.

  :return:
    The return value is of the same type and kind as :samp:`{X}`.
    The real part of the result is in radians and lies in the range
    0 \leq \Re \acos(x) \leq \pi.

  :samp:`{Standard}:`
    Fortran 77 and later, for a complex argument Fortran 2008 or later

  :samp:`{Class}:`
    Elemental function

  :samp:`{Syntax}:`

  .. code-block:: fortran

    RESULT = ACOS(X)

  :samp:`{Example}:`

    .. code-block:: fortran

      program test_acos
        real(8) :: x = 0.866_8
        x = acos(x)
      end program test_acos

  :samp:`{Specific names}:`
    ============  =============  ===========  ====================
    Name          Argument       Return type  Standard
    ============  =============  ===========  ====================
    ``ACOS(X)``   ``REAL(4) X``  ``REAL(4)``  Fortran 77 and later
    ``DACOS(X)``  ``REAL(8) X``  ``REAL(8)``  Fortran 77 and later
    ============  =============  ===========  ====================

  :samp:`{See also}:`
    Inverse function: 
    COS 
    Degrees function: 
    ACOSD