[Info-vax] Redimensioning formal parameter arrays in OpenVMS BASIC

[T. Kurt Bond]

On Thursday, May 8, 2003 at 4:11:28 PM UTC-4, T. Kurt Bond wrote:
> The BASIC for OpenVMS Reference Manual says, in the fifth item of the 
> Remarks section:
> The executable DIM statement cannot be used to dimension virtual arrays,
> arrays received as formal parameters, or arrays declared in COMMON, MAP,
> or nonexecutable DIM statements.
> http://h71000.www7.hp.com/doc/73final/cobol/bas_ref_013.htm#noisn
> The "no formal parameters" rule is inconvienent. It means that you 
> can't pass an array to a function, redimension it, fill it with values,
> and then use LBOUND and UBOUND in the caller to find out its new size.
> Presumably it is illegal because there is no way at compile time to 
> know if the function will be called with a dynamic array created with 
> the executable DIM statement or a static array created with a 
> non-executable DIM statement.
> However, the programmer *can* know, so it ought to be safe to
> redimension the array directly when the programmer knows it was 
> created by an executable DIM statement. 
> Using Alpha BASIC V1.4-000 under OpenVMS V7.2 and looking at the
> listing of some code with some executable dimension statements 
> compiled with /LIST/MACHINE revealed the existance of DBASIC$RT_DIM
> and after a little experimentation lead to a program that used
> DBASIC$RT_DIM directly to redimension dynamically dimensioned arrays
> in functions[.]

> (Interestingly, a slightly different approach was necessary using VAX
> BASIC V3.5 under VMS V5.5-2: using BY VALUE in function
> definition statements is not allowed by this version of VAX BASIC,
> and BAS$RT_DIM had to be used instead of DBASIC$RT_DIM, of course.)
> -- 
> T. Kurt Bond, t... at tkb.mpl.com

I needed this again for a program I was writing on a VAX.  (The VAX is  running VMS V5.5-2; It's like I am in 1992 all over again!)

Anyway, I ended up figuring out how to pass the lower bound (call BAS$RT_DIM_BOUNDS instead) and how to use the same redim procedure with different types, plus simplified things a little.  Here's my new version:

program vax_redim_best
    option type = explicit, constant type = integer, &
        size = integer long, size = real double
    record rec
        string s = 13
        long   i
    end record rec

    ! Don't declare the parameters, and everything works, even passing
    ! different types.
    external sub redim

    declare long stat, i
    i = 1
    dim rec rec(1 to i)
    print "lbound (rec) at start: "; lbound (rec)
    print "ubound (rec) at start: "; ubound (rec)
    call redim (rec(), 3, 10)
    print "lbound (rec) after redim: "; lbound (rec)
    print "ubound (rec) after redim: "; ubound (rec)

    dim string s(1 to i)
    print "lbound (s) at start: "; lbound (s)
    print "ubound (s) at start: "; ubound (s)
    call redim (s(), 3, 10)
    print "lbound (s) after redim: "; lbound (s)
    print "ubound (s) after redim: "; ubound (s)

    dim long l(1 to i)
    print "lbound (l) at start: "; lbound (l)
    print "ubound (l) at start: "; ubound (l)
    call redim (l(), 3, 10)
    print "lbound (l) after redim: "; lbound (l)
    print "ubound (l) after redim: "; ubound (l)


end program ! vax_redim_best

function long redim (long s, long low, long high)
    option type = explicit, constant type = integer, &
        size = integer long, size = real double
    external long function bas$rt_dim_bounds (long by value, long by value, long by value)
    declare long r
    r = bas$rt_dim_bounds (loc (s), low, high)
end function r ! redim