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An array is a data object
with the dimension attribute. Its rank—and possibly the extents—are
defined by an array specification. The array specification is enclosed
in parentheses and can be attached either to the DIMENSION attribute, as in: INTEGER, DIMENSION(17) :: a, b |
or to the array name, as in: If the array specification is attached both to the DIMENSION attribute and to the array name in the same declaration
statement, the specification attached to the name takes precedence.
In the following example: INTEGER, DIMENSION(4,7) :: a, b, c(15) |
a and b are declared as two-dimensional arrays, but c is declared as a one-dimensional array. An array specification can declare an array as one of the
following: The following sections describe these types and the form of
the array specification for each type. For information about initializing
arrays with the array constructor, see “Array
constructors”. Explicit-shape
arrays | |
An explicit-shape
array has explicitly declared bounds for each dimension;
the bounds are neither taken from an actual array argument (“assumed”)
nor otherwise specified prior to use (“deferred”).
Each dimension of an explicit-shape array has the following form: [lower-bound :] upper-bound where lower-bound and upper-bound are specification expressions and may be positive, negative,
or zero. The default for lower-bound is 1. For a given dimension, the values of lower-bound and upper-bound define the range of the array in that dimension.
Usually, lower-bound is less than upper-bound; if lower-bound is the same as upper-bound, then the dimension contains only one element; if
it is greater, then the dimension contains no elements, the extent
of the dimension is zero, and the array is zero-sized. The simplest form is represented by an array declaration in
which the name of the array is not a dummy argument and all bounds
are constant expressions, as in the following example: This form of array may have the SAVE attribute and may be declared in any program unit. Other forms of
the explicit-shape array include: An automatic
array: An array that is declared in a subprogram but
is not a dummy argument and has at least one nonconstant bound.
Automatic arrays may be declared in a subroutine or function, but
may not have the SAVE attribute nor be initialized. Character strings can also be declared as automatic data objects;
see “Character
strings as automatic data objects”. A dummy array:
An array that is identified by its appearance in a dummy argument
list; its bounds may be constants or expressions. Dummy arrays may
only be declared in a subroutine or function. An adjustable
array: A particular form of a dummy array. Its name
is specified in a dummy argument list, and at least one of its bounds
is a nonconstant specification expression.
Explicit-shape arrays may also be used as function results,
as described in “Array-valued
functions” and
in “Array
dummy argument”. The following code segment
illustrates different forms of explicit-shape arrays: SUBROUTINE sort(list1,list2,m,n)
! examples of arrays with explicit shape
INTEGER :: m,n
INTEGER :: cnt1(2:99)
! a rank-one array, having an explicit shape represented by
! the vector [98]
REAL :: list1(100), list2(0:m-1,-m:n)
! two dummy arrays with explicit shape: list1 is a rank-one
! array with an extent of 100; list2 is a rank-two array with an
! extent of m * (m+n+1). list2 is also an adjustable array.
REAL :: work(100,n)
! work is an automatic array; it does not appear in the dummy
! argument list and at least one of its bounds is not constant
INTEGER, PARAMETER :: buffsize = 0
REAL :: buffer (1: buffsize)
! buffer has explicit shape, but no elements and is zero-sized
.
.
.
END SUBROUTINE sort
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Assumed-shape
arrays | |
An assumed-shape
array is a dummy argument that assumes the shape of
the corresponding actual argument. It must not have the POINTER attribute. Each dimension of an assumed-shape
array has the form: [lower-bound] : where lower-bound is a specification expression. The default for lower-bound is 1. The actual argument and the corresponding dummy argument may
have different bounds for each dimension. An assumed-shape array
subscript may extend from the specified lower-bound to an upper bound that is equal to lower-bound plus the extent in that dimension of the actual argument
minus one. The following code segment
illustrates different declarations of assumed-shape arrays. SUBROUTINE initialize (a,b,c,n)
! examples of assumed-shape arrays
INTEGER :: n
INTEGER :: a(:)
! the array a is a rank-one assumed-shape array, it takes its
! shape and size from the corresponding actual argument; its
! lower bound is 1 regardless of the lower bound defined for
! the actual argument
COMPLEX :: b(ABS(n):)
! a rank-one assumed-shape array, the lower bound is ABS(n) and
! the upper bound will be the lower bound plus the extent of
! the corresponding actual argument minus one
REAL, DIMENSION(:,:,:,:,:) :: c
! an assumed-shape array with 5 dimensions; the lower bound for
! each dimension is 1
.
.
.
END SUBROUTINE initialize |
If a procedure
has an argument that is an assumed-shape array, its interface must
be explicit within the calling program unit. A procedure’s
interface is explicit if it is an internal procedure within the
caller procedure or if the interface is declared in an interface
block within the caller. For example, to call
the external subroutine initialize in the previous example, its interface must appear
in an interface block, as in the following: PROGRAM main
INTEGER :: parts(0:100)
COMPLEX :: coeffs(100)
REAL :: omega(-2:+3, -1:+3, 0:3, 1:3, 2:3)
INTERFACE
SUBROUTINE initialize (a,b,c,n)
INTEGER :: n
INTEGER :: a(:)
COMPLEX :: b(ABS(n):)
REAL, DIMENSION(:,:,:,:,:) :: c
END SUBROUTINE initialize
END INTERFACE
CALL initialize(parts,coeffs,omega,lbound(omega,1))
.
.
.
END PROGRAM main
SUBROUTINE initialize (a,b,c,n)
INTEGER :: n
INTEGER :: a(:)
COMPLEX :: b(ABS(n):)
REAL, DIMENSION(:,:,:,:,:) :: c
.
.
.
END SUBROUTINE initialize |
For more information about: Deferred-shape
arrays | |
A deferred-shape
array has either the POINTER attribute or the ALLOCATABLE attribute. Its shape is not specified until the
array is pointer assigned or allocated. Although a deferred-shape
array can have the same form as an assumed-shape array, the two
are different. The assumed-shape array is a dummy argument and must
not have the POINTER attribute. The array
specification for a deferred-shape array has the form: The specification for a deferred-shape array defines its rank
but not the bounds. The bounds are defined either when the array
is allocated or when an array pointer becomes associated with a
target. Array pointers and allocatable arrays are described in the
following sections. An array
pointer is a deferred-shape array with the POINTER attribute. Its bounds and shape are defined only
when the array is associated with a target in a pointer assignment statement
or in an ALLOCATE statement. An array pointer must not be referenced
until it is associated. Following are example
declarations of array pointers: ! p1 is declared as a pointer to a rank-one
! array of type real; p1 is not associated with any target
REAL, POINTER, DIMENSION(:) :: p1
! p2 is a pointer to an integer array of rank-two;
! it must be associated with a target before it can be referenced
INTEGER, POINTER :: p2(:,:)
! err is a pointer to a rank-3 array of type err_type
TYPE err_type
INTEGER :: class
REAL :: code
END TYPE err_type
TYPE(err_type), POINTER, DIMENSION(:,:,:) :: err
! The next statement is ILLEGAL: pointers cannot have an
! explicit shape.
INTEGER, POINTER :: p3(n) |
For information about associating an array pointer with a
target, see “Pointers”. For
information about the POINTER attribute and ALLOCATE statement, see Chapter 10, “HP Fortran
Statements.” An allocatable
array is a deferred-shape array with the ALLOCATABLE attribute. Its bounds and shape are defined when
it is allocated with the ALLOCATE statement. Once allocated, the allocatable array
may be used in any context in which any other array may appear.
An allocatable array can also be deallocated with the DEALLOCATE statement. An allocatable
array has an allocation status that can be tested with the ALLOCATED intrinsic inquiry function. Its status is unallocated when
the array is first declared and after it is deallocated in a DEALLOCATE statement. After the execution of the ALLOCATE statement, its status is allocated.
An allocatable array with the unallocated status
may not be referenced except as an argument to the ALLOCATED intrinsic or in an ALLOCATE statement. If it has the allocated status,
it may not be referenced in the ALLOCATE statement. It is an error to allocate an allocatable
array that is already allocated, or to deallocate an allocatable
array either before it is allocated or after it is deallocated. In HP Fortran,
an allocatable array that is unallocated, is
local to a procedure, and does not have the SAVE attribute. It is automatically deallocated when
the procedure exits. The following
example, alloc_array.f90, calls a subroutine that allocates and
deallocates an allocatable array and uses the ALLOCATED intrinsic function to test its allocation status: Example 3-1 alloc_array.f90 |
PROGRAM main
! driver program for calling a subroutine that allocates and
! deallocates an allocatable array
CALL test_alloc_array
END PROGRAM main
SUBROUTINE test_alloc_array
! demonstrate how to allocate and deallocate an allocatable array
! the array matrix is rank-2 allocatable array, with no
! shape or storage
REAL, ALLOCATABLE, DIMENSION(:,:) :: matrix
INTEGER :: n
LOGICAL :: sts
! sts is assigned the value .FALSE. as the array is not yet
! allocated
sts = ALLOCATED(matrix)
PRINT *, 'Initial status of matrix: ', sts
PRINT *, 'Enter an integer (rank of array to be allocated):'
READ *,n
! dynamically create the array matrix; after allocation, array
! will have the shape [ n, n ]
ALLOCATE(matrix(n,n))
! test allocation by assigning to array
matrix(n,n) = 9.1
PRINT *, 'matrix(',n,',',n,') = ', matrix(n,n)
! sts is assigned the value .TRUE. as the allocatable array
! does exist and its allocation status is therefore allocated
sts = ALLOCATED(matrix)
PRINT *, 'Status of matrix after ALLOCATE: ', sts
DEALLOCATE (matrix)
! sts is assigned the value .FALSE. as the
! allocation status of a deallocated array
sts = ALLOCATED (matrix)
PRINT *, 'Status of matrix after DEALLOCATE: ', sts
END SUBROUTINE test_alloc_array |
|
Here are the command lines to compile and execute the program,
along with the output from a sample run: $ f90 alloc_array.f90
$ a.out
Initial status of matrix: F
Enter an integer (rank of array to be allocated):
4
matrix( 4 , 4 ) = 9.1
Status of matrix after ALLOCATE: T
Status of matrix after DEALLOCATE: F
|
For information about the ALLOCATABLE, ALLOCATE, DEALLOCATE statements, see Chapter 10, “HP Fortran
Statements.” See also “ALLOCATED(ARRAY)”. Assumed-size
arrays | |
An assumed-size
array is a dummy argument whose size is taken from the
associated actual argument. Its declaration specifies the rank and
the extents for each dimension except the last. The extent of the
last dimension is represented by an asterisk (*), as in the following: All dummy array arguments and their corresponding actual arguments
share the same initial element and are storage associated. In the
case of explicit-shape and assumed-size arrays, the actual and dummy
array need not have the same shape or even the same rank. The size
of the dummy array, however, must not exceed the size of the actual
argument. Therefore, a subscript in the last dimension of an assumed-size
array may extend from the lower bound to a value that does not cause
the reference to go beyond the storage associated with the actual argument. Because the last dimension of an assumed-size array has no
upper bound, the dimension has no extent and the array consequently
has no shape. The name of an assumed-size array therefore cannot
appear in contexts in which a shape is required, such as a function
result or a whole array reference. The following
example, assumed_size.f90, illustrates two assumed-size arrays: x (declared in subr) and i_array (declared in func): Example 3-2 assumed_size.f90 PROGRAM main
REAL :: a(2,3) ! an explicit-shape array, represented by the
! vector [10, 10]
k = 0
DO i = 1, 3
DO j = 1, 2
k = k + 1
a(j, i) = k
END DO
END DO
PRINT *, 'main: a =', a
CALL subr (a)
END PROGRAM main
SUBROUTINE subr(x)
REAL :: x(2,*) ! an assumed-size array; the subscript for the
! last dimension may take any value 1 - 3
! PRINT *, x ! ILLEGAL, whole array reference not allowed
PRINT *, ‘main: x(2, 2) = ‘, x(2, 2)
PRINT *, 'returned by func: ', func(x), ', the value in x(2,3)'
END SUBROUTINE subr
REAL FUNCTION func(y)
REAL :: y(0:*) ! an assumed-size array; the subscript may
! take any value 0 - 5
func = y(5)
END FUNCTION func |
Here are the command lines to compile and execute the program,
along with the output from a sample run: $ f90 assumed_size.f90
$ a.out
main: a = 1.0 2.0 3.0 4.0 5.0 6.0
main: x(2, 2) = 4.0
returned by func: 6.0 , the value in x(2,3)
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An assumed-size array is a FORTRAN 77 feature that
has been superseded by the assumed-shape array; see “Assumed-shape
arrays”. |
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