Provides storage space for allocatable arrays and pointer
targets.
Syntax
ALLOCATE (allocation-list[, STAT= scalar-integer-variable]) |
 |
- allocation-list
is a comma-separated list of allocation.
- allocation
is allocate-object [(allocate-shape-spec-list)].
- allocate-object
is variable-name or derived-type-component.
Each allocate-object must be an allocatable
array or a pointer.
- allocate-shape-spec-list
is a comma-separated list of allocate-shape-spec.
- allocate-shape-spec
is [lower-bound:]upper-bound.
The bounds in an allocate-shape-spec
must be scalar integer expressions.
- STAT=scalar-integer-variable
returns the error status after the statement executes.
If given, it is set to zero if the statement successfully executed,
and to one of the following nonzero values if an error occurred:
- 1
Error occurred after the array was allocated; for
example, an attempt to allocate a previously allocated array.
- 2
Dynamic memory allocation failure (memory not available)
or invalid size (array too large).
- 3
Errors of both types 1 and 2 have occurred. This
kind of an error can only occur if the same ALLOCATE statement is
used to allocate more than one array, and both kinds of errors occur.
If there is no scalar-integer-variable,
the occurrence of an error causes the program to terminate.
Description
The ALLOCATE
statement creates space for allocatable arrays and targets for variables
(scalars or arrays) with the POINTER
attribute. The ALLOCATE
and DEALLOCATE
statements give the user the ability to manage space dynamically
at execution time.
For allocatable arrays, an error occurs when an attempt is
made to allocate an already allocated array or to deallocate an
array that is not allocated. The ALLOCATED
intrinsic function may be used to determine whether an allocatable
array is allocated.
A pointer can be associated with a target, either with the
pointer assignment statement or by use of the ALLOCATE
statement. It is not an error to allocate an already associated
pointer; its old target connection is replaced by a connection to
the newly allocated space. However, if the previous target was allocated
and no other pointer became associated with it, the space is no
longer accessible.
Examples
In the following example, a complex array with the POINTER
attribute is declared. Target space is allocated to it at run-time,
the amount being determined by two integer values read in. Later
in the program, the space is recovered by use of the DEALLOCATE
statement.
COMPLEX, POINTER :: hermitian (:, :)
READ *, m, n
ALLOCATE (hermitian (m, n))
DEALLOCATE (hermitian, STAT = ierr) |
In the next example, a real allocatable array is declared.
The amount of space allocated to it depends on how much is available.
! Rank-2 allocatable array
REAL, ALLOCATABLE :: intense(:,:)
CALL init_i_j(i, j)
DO
ALLOCATE (intense(i, j), STAT = ierr4)
! ierr4 will be positive if there is not enough space to
! allocate this array
IF (ierr4 == 0) EXIT
i = i/2; j = j/2
END DO |
The derived type node
in the next example is the basis of a binary tree structure. It
consists of a real value component (val)
and two pointer components, left
and right, both
of type node.
The variable top
(of type node)
is declared, and space is allocated for targets for the pointers
top%left and
top%right.
The ALLOCATE
and DEALLOCATE
statements and pointer variables of type node
make it possible to allocate space for nodes in such a tree structure,
traverse it as required, and then recover the space when it is no
longer needed.
TYPE node
REAL val
TYPE(node), POINTER :: left, right ! Pointer components
END TYPE node
TYPE(node) top
ALLOCATE (top % left, top % right) |
In the final example, two CHARACTER
arrays, para
and key, are
declared with the POINTER
attribute. para
is allocated space; key
is made to point at a section of para.
! Pointers to char arrays
CHARACTER, POINTER :: para(:), key(:)
CALL init_k_m(k, m)
ALLOCATE (para(1000))
key => para (k : k + m) |
Related statements
ALLOCATABLE
(statement and attribute), DEALLOCATE,
NULLIFY, and
POINTER (statement
and attribute)
Related concepts
For related information, see the following:
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