Array handling is included in Fortran 90 for two main reasons:
DO i = 1,n x(i) = b(i) / a(i, i) b(i+1:n) = b(i+1:n) - a(i+1:n, i) * x(i) END DOno special code is required for the final iteration where i = n. We note that a zero-sized array is regarded as being defined; however, an array of shape (0,2) is not conformable with one of shape (0,3), whereas
x(1:0) = 3is a valid 'do nothing' statement.
REAL, DIMENSION(0:10, 0:20) :: a : CALL sub(a)the corresponding dummy argument specification defines only the type and rank of the array, not its size. This information has to be made available by an explicit interface, often using an interface block (see Part 5). Thus we write just
SUBROUTINE sub(da) REAL, DIMENSION(:, :) :: daand this is as if da were dimensioned (11,21). However, we can specify any lower bound and the array maps accordingly. The shape, not bounds, is passed, where the default lower bound is 1 and the default upper bound is the corresponding extent.
SUBROUTINE swap(a, b) REAL, DIMENSION(:) :: a, b REAL, DIMENSION(SIZE(a)) :: work work = a a = b b = work END SUBROUTINE swapThe actual storage is maintained on a stack.
MODULE work_array INTEGER n REAL, DIMENSION(:,:,:), ALLOCATABLE :: work END MODULE PROGRAM main USE work_array READ (input, *) n ALLOCATE(work(n, 2*n, 3*n), STAT=status) : DEALLOCATE (work)The work array can be propagated through the whole program via a USE statement in each program unit. We may specify an explicit lower bound and allocate several entities in one statement. To free dead storage we write, for instance,
DEALLOCATE(a, b)We will meet this later, in the context of pointers.
REAL, DIMENSION(10) :: a, b a = 0. ! scalar broadcast; elemental assignment b = sqrt(a) ! intrinsic function result as array objectIn the second assignment, an intrinsic function returns an array-valued result for an array-valued argument. We can write array-valued functions ourselves (they require an explicit interface):
PROGRAM test REAL, DIMENSION(3) :: a = (/ 1., 2., 3./), & b = (/ 2., 2., 2. /), r r = f(a, b) PRINT *, r CONTAINS FUNCTION f(c, d) REAL, DIMENSION(:) :: c, d REAL, DIMENSION(SIZE(c)) :: f f = c*d ! (or some more useful functio of c and d) END FUNCTION f END PROGRAM test
WHERE (a /= 0.0) a = 1.0/a ! avoid division by 0(note: test is element-by-element, not on whole array), or as a construct:
WHERE (a /= 0.0) a = 1.0/a b = a ! all arrays same shape END WHEREor
WHERE (a /= 0.0) a = 1.0/a ELSEWHERE a = HUGE(a) END WHERE
REAL, DIMENSION(100, 100) :: awe can reference a single element as, for instance, a(1, 1). For a derived-data type like
TYPE triplet REAL u REAL, DIMENSION(3) :: du END TYPE tripletwe can declare an array of that type:
TYPE(triplet), DIMENSION(10, 20) :: tarand a reference like
tar(n, 2)is an element (a scalar!) of type triplet, but
tar(n, 2)%duis an array of type real, and
tar(n, 2)%du(2)is an element of it. The basic rule to remember is that an array element always has a subscript or subscripts qualifying at least the last name.
lower] : [ upper] [: stride]
REAL a(10, 10) a(i, 1:n) ! part of one row a(1:m, j) ! part of one column a(i, : ) ! whole row a(i, 1:n:3) ! every third element of row a(i, 10:1:-1) ! row in reverse order a( (/ 1, 7, 3, 2 /), 1) ! vector subscript a(1, 2:11:2) ! 11 is legal as not referenced a(:, 1:7) ! rank two sectionNote that a vector subscript with duplicate values cannot appear on the left-hand side of an assignment as it would be ambiguous. Thus,
b( (/ 1, 7, 3, 7 /) ) = (/ 1, 2, 3, 4 /)is illegal. Also, a section with a vector subscript must not be supplied as an actual argument to an OUT or INOUT dummy argument. Arrays of arrays are not allowed:
tar%du ! illegalWe note that a given value in an array can be referenced both as an element and as a section:
a(1, 1) ! scalar (rank zero) a(1:1, 1) ! array section (rank one)depending on the circumstances or requirements. By qualifying objects of derived type, we obtain elements or sections depending on the rule stated earlier:
tar%u ! array section (structure component) tar(1, 1)%u ! component of an array element
DOT_PRODUCT Dot product of 2 rank-one arrays MATMUL Matrix multiplication
ALL True if all values are true ANY True if any value is true. Example: IF (ANY( a > b)) THEN COUNT Number of true elements in array MAXVAL Maximum value in an array MINVAL Minimum value in an array PRODUCT Product of array elements SUM Sum of array elements
ALLOCATED Array allocation status LBOUND Lower dimension bounds of an array SHAPE Shape of an array (or scalar) SIZE Total number of elements in an array UBOUND Upper dimension bounds of an array
MERGE Merge under mask PACK Pack an array into an array of rank SPREAD Replicate array by adding a dimension UNPACK Unpack an array of rank one into an array under mask
RESHAPE Reshape an array
CSHIFT Circular shift EOSHIFT End-off shift TRANSPOSE Transpose of an array of rank two
MAXLOC Location of first maximum value in an array MINLOC Location of first minimum value in an array