Arrays: Under the Bonnet

Memory

• Array elements are not stored in a particular way in memory --- contrast C; the Fortran way is more efficient.

Array Element Ordering

Nevertheless, there is, in practice, array element ordering:

REAL, DIMENSION(1:3,1:3) :: A

corresponds to

A(1,1), A(2,1), A(3,1), A(1,2), A(2,2), A(3,2), A(1,3), A(2,3), A(3,3)

so that

DO J=1, 5000 DO I=1, 5000 A(I,J) = I * (J-1) END DO END DO

is much more efficient than

DO I=1, 5000 DO J=1, 5000 A(I,J) = I * (J-1) END DO END DO

• This array element ordering is "the other way around" from the convention adopted in C --- very important for mixed language programming.