Arrays

An array is an indexed group of variables of the same type that are referred to by a single name. Arrays are useful for storing lists of related data. The individual elements of an array are accessed by using a subscript. The following Fortran example illustrates how to declare a 1D array with 3 entries and set each of its elements to zero:

integer :: i
real :: a(3) ! A 1D array with 3 entries
do i = 1,3 ! Loop over each entry and set to zero
a(i) = 0.0
enddo 

To define a 2D array with 3 rows and 3 columns and set all of its elements to zero:

a(i,j) = 0.0

! A 2D array with 3 rows and 3 columns
real :: a(3,3)

! Loop over each entry and set to zero
do i = 1,3 ! Loop over rows
    do j = 1,3 ! Loop over each column within the row
    enddo
enddo 

Arrays in Fortran can have a maximum of 7 dimensions. In most practical applications, the dimensions of an array will not be known before the program is run. If the array is used for storing a matrix for example, the dimensions of the matrix will probably vary from problem to problem. For this reason, it is possible to define dynamic arrays whose dimensions can be set or change while the program is running. This accomplished in Fortran as follows:

write (*,*) 'Error in memory allocation'
exit
matrix(j,i) = 0.0<
do j=1,rows
enddo
deallocate(matrix)

program dynamic
implicit none
real, allocatable :: matrix(:,:)

! Matrix is dynamic - will specify size below
integer :: components
integer :: rows, cols
integer :: info
integer :: i,j
rows = 3
cols = 3

! Allocate the required memory for this matrix
allocate(matrix(rows,cols),stat=info)
<

! Check status of allocation: Error if info not
! equal to 0
if (info .NE. 0) then
endif

! Initialise all matrix elements to zero
do i=1,cols
enddo

! Perform other computations here...
! Deallocate matrix - return memory to system
! First check if memory has been allocated to
! matrix
if (allocated(matrix)) then
endif
end program dynamic

The status of allocate() should always be checked to ensure that the memory has been successfully allocated. Memory allocation can fail if for example an attempt is made to allocate more memory than is available on the system. In addition, allocated memory should always be deallocated so that the memory is returned to the system.