UNPACK - The GNU Fortran Compiler
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### 8.255 `UNPACK` — Unpack an array of rank one into an array

Description:
  Store the elements of VECTOR in an array of higher rank.
Standard:
  Fortran 95 and later
Class:
  Transformational function
Syntax:
  `RESULT = UNPACK(VECTOR, MASK, FIELD)`
Arguments:

  VECTOR | Shall be an array of any type and rank one. It shall have at least as many elements as MASK has `TRUE` values.
  MASK | Shall be an array of type `LOGICAL`.
  FIELD | Shall be of the same type as VECTOR and have the same shape as MASK.

Return value:
  The resulting array corresponds to FIELD with `TRUE` elements of MASK replaced by values from VECTOR in array element order.
Example:

                PROGRAM test_unpack
                  integer :: vector(2)  = (/1,1/)
                  logical :: mask(4)  = (/ .TRUE., .FALSE., .FALSE., .TRUE. /)
                  integer :: field(2,2) = 0, unity(2,2)

                  ! result: unity matrix
                  unity = unpack(vector, reshape(mask, (/2,2/)), field)
                END PROGRAM

See also:
  PACK, SPREAD