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Another trick with arrays is sizing them. This article shows you how to size your arrays dynamically.

In an earlier article, I showed you how to initialize arrays and how to sort them based on a subfield. I did this with fairly small arrays, where you could easily define the values for the array in your D-specs. The next trick is trying load arrays from disk. You don't know how many you will load, and you really don't want to allocate the memory for the entire array.

Expanding on the Original Store-Sorting Program

The original program had a few stores hardcoded into the D-specs as initialized variables. In this version of the program, I'm going to create an array of 1000 stores and then load that array from disk.

A    H OPTION(*NODEBUGIO:*SRCSTMT)

These are just a couple of standard H-spec keywords that I use all the time. They both make it a little easier to debug programs.

B    FSTORES    IF   E             DISK    RENAME(STORES:RSTORES)

Here is the file specification for the STORES database file. I created it using DDL, so the record format name is the same as the file name; RPG requires us to rename the format.

C    d cStores         ds                  based(pStores)

C    d  aStores                            dim(1000)

C    d   aStoreID                     3    overlay(aStores)

C    d   aStoreCity                  20    overlay(aStores:*NEXT)

C    d   aStoreState                  2    overlay(aStores:*NEXT)

C    d   aStoreZip                    5    overlay(aStores:*NEXT)

C    d pStores         s               *

C    d nStores         s              5u 0

C    d x               s              5u 0

This is essentially the same code as from the previous program, except without the hard-coded values. I've defined a data structure and then an array of data structures within the data structures. I've done this (using the overlay *NEXT syntax) in order to be able to sort by subfields. The only problem here is the limitation of 65535 for the total size of the array.

One other thing you should note is that the data structure is defined as "based," using the keyword based(pStores). This means that the array has no memory assigned; instead, I need to create memory for it. At the end of the array definition, you see that I defined a pointer variable named pStores. Technically, I don't need to define the variable--the based keyword does that--but adding the additional definition line doesn't hurt. Either way, the pStores pointer is at an unknown state, probably null, when the program begins. Whatever is in it, it doesn't point to my memory, so the array is unusable. Before using the array, I need to allocate some memory.

      /free

D      setll *start STORES;

D      nStores = 0;

D      read STORES;

D      dow not %eof(STORES);

D        if HasDeli = 'Y';

D          nStores += 1;

D        endif;

D        read STORES;

D      enddo;

D      if nStores > 0;

Next, I check to see how many records I actually need. In this case, I simply loop through the file, looking for records that match. With SQL, it would have been much easier; I could have used a single exec sql statement to set the value of nStores to the count of records that matched my criteria. I hope to be able to show you that and a few other SQL techniques in the next installment of array processing.

E        pStores = %alloc( nStores * 30);

E        setll *start STORES;

E        x = 0;

E        read STORES;

E        dow not %eof(STORES);

E          if HasDeli = 'Y';

E            x += 1;

E            aStoreID(x) = StoreID;

E            aStoreCity(x) = StoreCity;

E            aStoreState(x) = StoreState;

E            aStoreZip(x) = StoreZip;

E          endif;

E          read STORES;

E        enddo;

Now for the meat of today's exercise. This section of code is quite simple: I read each record, and if it matches the selection criteria, I add the record to the array. In this case, I'm directly moving database fields from the external file to the subarrays one at a time.

It's a little more complex, though. As I observed earlier, the data structure actually doesn't have any memory yet, so neither do the arrays; if you were to use it, you would get some serious errors. They might not even show up immediately as errors; instead, you'd notice strange problems that you couldn't readily diagnose; this is one of the symptoms of memory corruption.

Note the first line of this section, which sets the value of pStores. The %alloc built-in function (BIF) will allocate the amount of memory you ask for and return a pointer. In this case, I'm asking for 30 characters per record found. The number of records found is in nStores, so the logical step is to allocate that number times 30--nothing fancy, and now pStores has a value.

Now I can read data into the array. I read a record and once again test for the selection criteria (that HasDeli is a 'Y'). If it passes, I increment the index and add the fields from the data record to the subarrays. There's a subtle opportunity for error here, though. If a new matching record was added between the time I computed the number of matching records and the time I began loading the array, I would go past the end of the array. I don't show the code for that here, because what you might end up doing is simply going back and recomputing the count and reattempting the load. Or you could simply ignore any additional records and have an incomplete list.

F        x = %lookup( '002': aStoreID: 1: nStores );

F        sorta %subarr( aStoreCity: 1: nStores);

F        sorta %subarr( aStoreState: 1: nStores);

F        sorta %subarr( aStoreZip: 1: nStores);

F        sorta %subarr( aStoreID: 1: nStores);

The primary difference now is how you process the arrays. When doing a lookup, it's relatively straightforward: you add two new parms that identify the starting element and the number of elements that you want to include in the lookup.

The sorta opcode is a little different. The sorta allows you to use a relatively new BIF called %subarr, which allows you to define a subsection of an array, or "subarray." The subarray can then be sorted just like any other array. The %subarr BIF isn't implemented everywhere; you can't, for example, use it in a %lookup BIF. I would have liked to see %subarr extended there to allow a consistent syntax for all subarray processing. No matter; use the appropriate syntax in the appropriate place and you're fine.

D      endif;

This is the endif to avoid trying to process an empty set.

G      *inlr = *on;

      /end-free

And this is how you get out. You may have noticed that I didn't execute a %dealloc BIF. That's because there is no %dealloc BIF. There is a dealloc opcode, but that's a little different. I don't like the fact that I use a BIF in one place and an opcode in the other. You'll have to make your own call. Any allocated memory is released when the activation group ends, so if you're using activation groups for proper housekeeping, you may choose to go that route as well.

That's it for dynamic arrays. The last part of this mini-series will be to implement dynamic arrays in conjunction with embedded SQL. Until then, keep coding!