"W.J. Metzger" <[log in to unmask]> wrote: >> Type declarations, e.g., DoublePrecision d >> dimensions, e.g., Dimension d(2) >> common, common /c/ d >> can appear in any order. But giving a name to a constant, e.g., >> Parameter (k=5) >> must appear before the first use of the named constant, i.e., I see your point. The processor needs to go through all the specification statements to decide what a particular name represents anyway. Why should an order be imposed for named constants? I suppose the reason is historical. In FORTRAN II, there are no type declaration statements, and no COMMON. Only the DIMENSION statement is used to declare an array, so there is not much concern here for order of specification statements [Note *1]. FORTRAN IV has type declarations but also retains the DIMENSION statement. Because of this, it is not possible to give all information about a name in a single statement. So, separate statements (COMMON, EXTERNAL, DATA, ...) [Note *2] are used not only for DIMENSION but for all other bits of information about a name. FORTRAN 66 does not specify an ordering of specification statements, perhaps aiming for maximum compatibility between processors [Note *3]. FORTRAN 77 adds the PARAMETER and IMPLICIT statements, and defines particular ordering of these [Note *4]. Having named constant values defined before use would make implementation easier. So, my guess is that FORTRAN 66 allowed arbitrary ordering of specification statements because it was more concerned with portability of then existing code, and FORTRAN 77 imposed an ordering on the new features because it was more concerned with ease of implementation and with actually having the new features added to implementations. [Note *5] ----- [Note *1] In the FORTRAN II Programmer's Reference Manual, it says "the DIMENSION statement must precede the first appearance of the variable" (DIMENSION must be specified before use of array), but "An EQUIVALENCE statement may be placed anywhere in the source program". So apparently an EQUIVALENCE statement can be placed even after the executable statements containing the EQUIVALENCEd variables. In the statement EQUIVALENCE(A,C(5)) C is not necessarily an array. When C is not an array, C(5) is "the 4th storage location in the object program after the cell containing C". If C is an array of rank three (declared by a DIMENSION statement preceding or following the EQUIVALENCE statement), C(5) is the 4th storage location after the cell containing C(1,1,1). [Note *2] In a DEC PDP-9 FORTRAN IV manual from 1968, it says "All SPECIFICATION statements must appear before any executable code generating statement. They must appear in this order: type statements, DIMENSION statements, COMMON statements, and EQUIVALENCE statements. EXTERNAL and DATA statements may appear anywhere after all type statements and before the executable code generating statements". So this implementation does require a particular ordering, and code written without this particular ordering in mind does not run on this processor. The FORTRAN 66 Standard perhaps tried to gain more portability by forcing processors to relax such restrictions. [Note *3] The DATA statement is not a "specification statement" in FORTRAN 66, and a DATA statement must be placed after all the specification statements. Since Fortran 90, the DATA statement may be placed among other specification statements, but the type and array properties must be established before a variable appears in a DATA statement (or the variable must be a scalar of implicit type). So the DATA statement always required a particular order of placement. [Note *4] The two other specification statements added in FORTRAN 77, INTRINSIC and SAVE have no particular ordering defined, perhaps because they work somewhat like FORTRAN 66's existing specification statements. [Note *5] There is another issue that is confusing. The following is not accepted by gfortran and IBM's ifort. SUBROUTINE SUB(A,X) REAL A(X) INTEGER X According to the error messages, X here is an implicit REAL variable and it cannot be in an array-spec, despite the FORTRAN 66 Standard saying "The appearance of a symbolic name in a type-statement serves to inform the processor that it is of the specified data type for all appearances in the program unit" and "In the absence of an explicit declaration, the type is implied by the first character of the name", and the FORTRAN 77 Standard saying essentially the same thing (Sections 8.4 and 4.1.2). The FORTRAN 77 Standard additionally says (in 4.1.1), "Once a particular name is identified with a particular type in a program unit, that type is implied for any usage of the name in the program unit that requires a type". It seems the ordering of specification statements is significant in this context. I cannot figure out what the Fortran 2008 Standard says on this matter. The following is accepted by the two processors (gfortran and ifort): SUBROUTINE SUB(A,X) INTEGER X REAL A(X) and so is SUBROUTINE SUB(A,N) REAL A(N) INTEGER N The latter would appear at first sight that the ordering of the two type declarations doesn't matter, but actually the type of N is determined implicitly, and the INTEGER statement only confirms the implicit type. Writing IMPLICIT NONE here makes the fragment unacceptable. On the same two processors (gfortran and ifort), COMMON /C/I,X DIMENSION I(2) DOUBLE PRECISION I REAL X declares a common block that has five numeric storage units; the first four taken up by the double precision array I. In this context, the processor does look at all three statements to decide what I and X is, and adding an IMPLICIT NONE has no effect. -- Yasuki Arasaki [log in to unmask]