Consider the array declared as
Then we have three aspects of interest:
In most cases the whole array
x refers to all of the elements of the array as a single entity. It may appear in executable statements such as
print *, SUM(x),
print *, SIZE(x) or
A whole array may reference arrays which aren't explicitly shaped (such as
function f(y) real, intent(out) :: y(:) real, allocatable :: z(:) y = 1. ! Intrinsic assignment for the whole array z = [1., 2.,] ! Intrinsic assignment for the whole array, invoking allocation end function
An assumed-size array may also appear as a whole array, but in limited circumstances only (to be documented elsewhere).
An array element is referred to be giving integer indexes, one for each rank of the array, denoting the location in the whole array:
real x(5,2) x(1,1) = 0.2 x(2,4) = 0.3
An array element is a scalar.
An array section is a reference to a number of elements (perhaps just one) of a whole array, using a syntax involving colons:
real x(5,2) x(:,1) = 0. ! Referring to x(1,1), x(2,1), x(3,1), x(4,1) and x(5,1) x(2,:) = 0. ! Referring to x(2,1), x(2,2) x(2:4,1) = 0. ! Referring to x(2,1), x(3,1) and x(4,1) x(2:3,1:2) = 0. ! Referring to x(2,1), x(3,1), x(2,2) and x(3,2) x(1:1,1) = 0. ! Referring to x(1,1) x([1,3,5],2) = 0. ! Referring to x(1,2), x(3,2) and x(5,2)
The final form above uses a vector subscript. This is subject to a number of restrictions beyond other array sections.
Each array section is itself an array, even when just one element is referenced. That is
x(1:1,1) is an array of rank 1 and
x(1:1,1:1) is an array of rank 2.
Array sections do not in general have an attribute of the whole array. In particular, where
real, allocatable :: x(:) x = [1,2,3] ! x is allocated as part of the assignment x = [1,2,3,4] ! x is dealloacted then allocated to a new shape in the assignment
x(:) = [1,2,3,4,5] ! This is bad when x isn't the same shape as the right-hand side
is not allowed:
x(:), although an array section with all elements of
x, is not an allocatable array.
x(:) = [5,6,7,8]
is fine when
x is of the shape of the right-hand side.
type t real y(5) end type t type(t) x(2)
We may also refer to whole arrays, array elements and array sections in more complicated settings.
From the above,
x is a whole array. We also have
x(1)%y ! A whole array x(1)%y(1) ! An array element x%y(1) ! An array section x(1)%y(:) ! An array section x([1,2]%y(1) ! An array section x(1)%y(1:1) ! An array section
In such cases we are not allowed to have more than one part of the reference consisting of an array of rank 1. The following, for example, are not allowed
x%y ! Both the x and y parts are arrays x(1:1)%y(1:1) ! Recall that each part is still an array section