Re: Using CHAIN instead of SETLL AND READE to begin a loop whichreads a group of records.

[Scott Klement <klemscot@xxxxxxxxxxxx>]

On Thu, 9 May 2002, Mark Adkins wrote:
>
> If there is a table of order detail which has an index on Order #, and UPC.
> Will these two pieces of code execute identically.
>
> 1.   MYORD#    CHAIN     ORDDTL    99
>      *IN99          DOWEQ*OFF
>                <SOMECODE>
>      MYORD#    READE     ORDDTL          99
>                ENDDO
>
> 2.   MYORD#    SETLL     ORDDTL
>      MYORD#    READE     ORDDTL          99
>      *IN99          DOWEQ*OFF
>                <SOMECODE>
>      MYORD#    READE     ORDDTL          99
>                ENDDO
>
> What if the index is not unique?  What if I set my pointer past the end of
> file before execution?

These two pieces of code will do the same thing.   If you added more to
it (such as using READ instead of READE, checking indicators on the SETLL,
etc, etc) then they wouldn't.

> Ever wonder why the 1st array element in most programming languages is 0?
> Or is it just me...
>

Because of the way arrays work at a lower level.   If you have an address
in memory that signifies the start of an array, and then you need to find
out the address of a given element in the array, you multiply the
array index times the size of each element to get an offset that you
can add to that first address.

With an array that starts with 0, this logic works.   With an array
that starts with 1, you have to first subtract 1 from the array index
before doing the calculation.

i.e. for an array starting with 0:

         element_addr = base_addr + (index * element_size)

     for an array starting with 1:

         element_addr = base_addr + ((index - 1) * element_size)

So, it's (slightly) easier to code an array that starts with 0.  Plus, the
code will run (slightly) faster.  If the person working with the array
would prefer to start with 1, they still can -- it just wastes one element
of the array.