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(g77-295.info)Arrays

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Arrays (DIMENSION)
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   Fortran uses "column-major ordering" in its arrays.  This differs
from other languages, such as C, which use "row-major ordering".  The
difference is that, with Fortran, array elements adjacent to each other
in memory differ in the *first* subscript instead of the last;
`A(5,10,20)' immediately follows `A(4,10,20)', whereas with row-major
ordering it would follow `A(5,10,19)'.

   This consideration affects not only interfacing with and debugging
Fortran code, it can greatly affect how code is designed and written,
especially when code speed and size is a concern.

   Fortran also differs from C, a popular language for interfacing and
to support directly in debuggers, in the way arrays are treated.  In C,
arrays are single-dimensional and have interesting relationships to
pointers, neither of which is true for Fortran.  As a result, dealing
with Fortran arrays from within an environment limited to C concepts
can be challenging.

   For example, accessing the array element `A(5,10,20)' is easy enough
in Fortran (use `A(5,10,20)'), but in C some difficult machinations are
needed.  First, C would treat the A array as a single-dimension array.
Second, C does not understand low bounds for arrays as does Fortran.
Third, C assumes a low bound of zero (0), while Fortran defaults to a
low bound of one (1) and can supports an arbitrary low bound.
Therefore, calculations must be done to determine what the C equivalent
of `A(5,10,20)' would be, and these calculations require knowing the
dimensions of `A'.

   For `DIMENSION A(2:11,21,0:29)', the calculation of the offset of
`A(5,10,20)' would be:

       (5-2)
     + (10-1)*(11-2+1)
     + (20-0)*(11-2+1)*(21-1+1)
     = 4293

So the C equivalent in this case would be `a[4293]'.

   When using a debugger directly on Fortran code, the C equivalent
might not work, because some debuggers cannot understand the notion of
low bounds other than zero.  However, unlike `f2c', `g77' does inform
the GBE that a multi-dimensional array (like `A' in the above example)
is really multi-dimensional, rather than a single-dimensional array, so
at least the dimensionality of the array is preserved.

   Debuggers that understand Fortran should have no trouble with
non-zero low bounds, but for non-Fortran debuggers, especially C
debuggers, the above example might have a C equivalent of `a[4305]'.
This calculation is arrived at by eliminating the subtraction of the
lower bound in the first parenthesized expression on each line--that
is, for `(5-2)' substitute `(5)', for `(10-1)' substitute `(10)', and
for `(20-0)' substitute `(20)'.  Actually, the implication of this can
be that the expression `*(&a[2][1][0] + 4293)' works fine, but that
`a[20][10][5]' produces the equivalent of `*(&a[0][0][0] + 4305)'
because of the missing lower bounds.

   Come to think of it, perhaps the behavior is due to the debugger
internally compensating for the lower bounds by offsetting the base
address of `a', leaving `&a' set lower, in this case, than
`&a[2][1][0]' (the address of its first element as identified by
subscripts equal to the corresponding lower bounds).

   You know, maybe nobody really needs to use arrays.