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

.. _pack:

PACK --- Pack an array into an array of rank one
************************************************

.. index:: PACK

.. index:: array, packing

.. index:: array, reduce dimension

.. index:: array, gather elements

.. function:: PACK

  Stores the elements of :samp:`{ARRAY}` in an array of rank one.

  :param ARRAY:
    Shall be an array of any type.

  :param MASK:
    Shall be an array of type ``LOGICAL`` and 
    of the same size as :samp:`{ARRAY}`. Alternatively, it may be a ``LOGICAL`` 
    scalar.

  :param VECTOR:
    (Optional) shall be an array of the same type 
    as :samp:`{ARRAY}` and of rank one. If present, the number of elements in 
    :samp:`{VECTOR}` shall be equal to or greater than the number of true elements 
    in :samp:`{MASK}`. If :samp:`{MASK}` is scalar, the number of elements in 
    :samp:`{VECTOR}` shall be equal to or greater than the number of elements in
    :samp:`{ARRAY}`.

  :return:
    The result is an array of rank one and the same type as that of :samp:`{ARRAY}`.
    If :samp:`{VECTOR}` is present, the result size is that of :samp:`{VECTOR}`, the
    number of ``TRUE`` values in :samp:`{MASK}` otherwise.

  :samp:`{Standard}:`
    Fortran 90 and later

  :samp:`{Class}:`
    Transformational function

  :samp:`{Syntax}:`

  .. code-block:: fortran

    RESULT = PACK(ARRAY, MASK[,VECTOR])

  :samp:`{Example}:`
    Gathering nonzero elements from an array:

    .. code-block:: fortran

      PROGRAM test_pack_1
        INTEGER :: m(6)
        m = (/ 1, 0, 0, 0, 5, 0 /)
        WRITE(*, FMT="(6(I0, ' '))") pack(m, m /= 0)  ! "1 5"
      END PROGRAM

    Gathering nonzero elements from an array and appending elements from :samp:`{VECTOR}` :

    .. code-block:: fortran

      PROGRAM test_pack_2
        INTEGER :: m(4)
        m = (/ 1, 0, 0, 2 /)
        ! The following results in "1 2 3 4"
        WRITE(*, FMT="(4(I0, ' '))") pack(m, m /= 0, (/ 0, 0, 3, 4 /))
      END PROGRAM

  :samp:`{See also}:`
    UNPACK