Question

I’m going through a Fortran code, and one bit has me a little puzzled.

There is a subroutine, say

SUBROUTINE SSUB(X,...)
REAL*8 X(0:N1,1:N2,0:N3-1),...
...
RETURN 
END

Which is called in another subroutine by:

CALL SSUB(W(0,1,0,1),...)

where W is a ‘working array’. It appears that a specific value from W is passed to the X, however, X is dimensioned as an array. What’s going on?

Answer 1

This is non-uncommon idiom for getting the subroutine to work on a (rectangular in N-dimensions) subset of the original array.

All parameters in Fortran (at least before Fortran 90) are passed by reference, so the actual array argument is resolved as a location in memory. Choose a location inside the space allocated for the whole array, and the subroutine manipulates only part of the array.

Biggest issue: you have to be aware of how the array is laid out in memory and how Fortran’s array indexing scheme works. Fortran uses column major array ordering which is the opposite convention from c. Consider an array that is 5×5 in size (and index both directions from 0 to make the comparison with c easier). In both languages 0,0 is the first element in memory. In c the next element in memory is [0][1] but in Fortran it is (1,0). This affects which indexes you drop when choosing a subspace: if the original array is A(i,j,k,l), and the subroutine works on a three dimensional subspace (as in your example), in c it works on Aprime[i=constant][j][k][l], but in Fortran in works on Aprime(i,j,k,l=constant).

The other risk is wrap around. The dimensions of the (sub)array in the subroutine have to match those in the calling routine, or strange, strange things will happen (think about it). So if A is declared of size (0:4,0:5,0:6,0:7), and we call with element A(0,1,0,1), the receiving routine is free to start the index of each dimension where ever it likes, but must make the sizes (4,5,6) or else; but that means that the last element in the j direction actually wraps around! The thing to do about this is not use the last element. Making sure that that happens is the programmers job, and is a pain in the butt. Take care. Lots of care.