VSI OpenVMS Forum

Code: Select all

> I'm trying to read a file in reverse [...]

   Is this an existing file (with some kind of structure), or some new
file which you'll be creating in some way or other, or what?

   It might be helpful to describe in more detail the actual problem
which you are trying to solve, rather than asking how to implement some
particular "solution" or technique ("read a file in reverse").

> [...] in Guide to OpenVMS File Applications: [...]
> There was an example but it was written in C [...]

   Have you looked at the BASIC documentation?

      https://docs.vmssoftware.com/   (Look for "COMPILERS"?)

   The "VSI BASIC for OpenVMS User Manual" includes "Chapter 13. File
Input and Output", which includes section 13.2.4 on "Indexed Files".

Code: Select all

> [...] I've tried to use the convert utility to no avail.

   "tried" is not a very detailed description of what you did, and "to
no avail" is no better as a description of what happened when you did
it.

> [...] read the entire file from start to finish in key order, store
> the keys in an array in memory [...]

   I know nothing about that array stuff, but it did leap out at me as I
was skimming the BASIC docs.

Code: Select all

$ type gen.pas
program gen(input,output);

type
   fixstring = packed array [1..8] of char;
   mydata = record
               nam : [KEY(0)] fixstring;
               v   : integer;
            end;

var
   f : file of mydata;
   d : mydata;

begin
   open(f, 'fwdrev.isq', new, organization := indexed, access_method := keyed);
   rewrite(f);
   d.nam := 'A A';
   d.v   := 1;
   f^ := d;
   put(f);
   d.nam := 'B B';
   d.v   := 2;
   f^ := d;
   put(f);
   d.nam := 'C C';
   d.v   := 3;
   f^ := d;
   put(f);
   d.nam := 'D D';
   d.v   := 4;
   f^ := d;
   put(f);
   close(f);
end.
$ pas gen
$ link gen
$ run gen
$ type fwd.bas
program fwd

record mydata
    string nam = 8
    integer v
end record
map (buf) mydata d

open "fwdrev.isq" as file #1, indexed fixed, recordtype none, map buf, primary key d::nam
handler eof_handler
end handler
when error use eof_handler
    while 1 = 1
        get #1
        print d::nam,d::v
    next
end when
close #1

end program
$ bas fwd
$ link fwd
$ run fwd
A A            1
B B            2
C C            3
D D            4
$ type rev.bas
program rev

record mydata
    string nam = 8
    integer v
end record
map (buf) mydata d
declare string target
declare integer i

handler eof_handler
end handler

open "fwdrev.isq" as file #1, indexed fixed, recordtype none, map buf, primary key d::nam
for i = 1 to 26
    target = chr$(91 - i)
    when error use eof_handler
        find #1, key #0 ge target
        get #1
        while mid(d::nam, 1, 1) = target
            print d::nam,d::v
            get #1
        next
    end when
next i
close #1

end program
$ bas rev
$ link rev
$ run rev
D D            4
C C            3
B B            2
A A            1