The bounds (and shape) of an allocatable array are determined when it is allocated. Subsequent redefinition or undefinition of any entities in the bound expressions does not affect the array specification.
If the lower bound is greater than the upper bound, that dimension has an extent of zero, and the array has a size of zero. If the lower bound is omitted, it is assumed to be 1.
When an array is allocated, it is definable. If you try to allocate a currently allocated allocatable array, an error occurs.
The intrinsic function ALLOCATED can be used to determine whether an allocatable array is currently allocated; for example:
REAL, ALLOCATABLE :: E(:,:) ... IF (.NOT. ALLOCATED(E)) ALLOCATE(E(2:4,7))
During program execution, the allocation status of an allocatable array is one of the following:
The array was never allocated or the last operation on it was a deallocation. Such an array must not be referenced or defined.
The array was allocated by an ALLOCATE statement. Such an array can be referenced, defined, or deallocated.
If an allocatable array has the SAVE attribute, it has an initial status of "not currently allocated". If the array is then allocated, its status changes to "currently allocated". It keeps that status until the array is deallocated.
If an allocatable array does not have the SAVE attribute, it has the status of "not currently allocated" at the beginning of each invocation of the procedure. If the array's status changes to "currently allocated", it is deallocated if the procedure is terminated by execution of a RETURN or END statement.
Example 6-1 shows a program that performs virtual memory allocation. This program uses Fortran 95/90 standard-conforming statements instead of calling an operating system memory allocation routine.
Example 6-1 Allocating Virtual Memory
! Program accepts an integer and displays square root values INTEGER(4) :: N READ (5,*) N ! Reads an integer value CALL MAT(N) END ! Subroutine MAT uses the typed integer value to display the square ! root values of numbers from 1 to N (the number read) SUBROUTINE MAT(N) REAL(4), ALLOCATABLE :: SQR(:) ! Declares SQR as a one-dimensional ! allocatable array ALLOCATE (SQR(N)) ! Allocates array SQR DO J=1,N SQR(J) = SQRT(FLOATJ(J)) ! FLOATJ converts integer to REAL ENDDO WRITE (6,*) SQR ! Displays calculated values DEALLOCATE (SQR) ! Deallocates array SQR END SUBROUTINE MAT
For More Information:
For details on the ALLOCATED intrinsic function, see Section 9.3.10.