I've read several articles regarding how to create dynamic arrays using RPG. While the results have been helpful, they have also been limited. I speak now of the inherent 32,767 (x'FF') length limitation of any field or file on the iSeries. Granted, if you're planning to use an array to hold more than 32,767 elements, you should probably rethink it. However, if you want to have truly dynamic "arrays" and allocate memory only for those elements that are being used, then the solution is a linked list.
If you search for the definition of a linked list (as I did), you will find something like "a linked list is a complex data structure," which meant exactly nothing to me. Simply put, a linked list is a list of items in which each item points to at least one other item in the list. Fortunately for me, the information I found had a graphical representation. (Oh good! I can understand pictures!) The structure of a singly linked list, one in which each item points only to the next item, looks like the picture shown below.
The last item in a given list will typically have a NULL pointer to signal the end of the list. There are other possibilities, such as a circular list, which I will discuss later. The singly linked list is the simplest form of linked list. A doubly linked list, one in which each item has pointers to both the next item and the previous item, is a little more complicated. The figure below represents a doubly linked list with two items in it.
In the case of a doubly linked list, both the first item and the last item will contain a NULL pointer to signal the beginning and the end of the list.
When I was writing the functions that are included with this article, one of my fellow developers (one who works primarily with C/C++) asked me, "Why would you use RPG? It's so much easier to do in C." When he said it, I agreed. But now that I have completed the functions, I am not so sure.
In RPG, we have to alter the basic form of a linked list structure a bit in order to make it function with variable-length data. You can see in the /COPY member (@LNKLSTFNC) that, instead of using a single data structure, I use three. The first is the actual "linked list" structure. This is the structure that holds the pointers to the item data, the next item, and, because it is a doubly linked list, the next item. Then, there is the data structure used to contain the actual data. This is used so that you can create a single list that contains multiple pieces of data, as in the example I have included. Finally, I used a data structure (the "control" data structure) to hold some important information about the list. This information is used by some of the functions and simplifies some of the operations.
The beauty of linked lists is that they can be accessed in the same manner as arrays, and they are limited only by the amount of memory available. Another nifty feature of a linked list is that, once it is created, you can access the list in multiple programs within the same activation group simply by passing the pointer to the control data structure.
Remember this when you are using linked lists: When you are done with a list, be sure to "delete" it by deallocating all the memory that it is using. Otherwise, you will have a memory leak. (Microsoft is always looking for people with this skill.) Also, be careful about clearing any pointers, as doing so can lead to orphaned items or lists and also cause memory leaks.
As I mentioned earlier, there are several variations on the linked list. One is the circular list, which simply has the last item pointing back to the first. With a doubly linked circular list, the first item also points back to the last. Another variation is the ordered list. It this case, you put the items into some sequence as they are added to the list. This actually isn't too difficult. You could add two fields to the control structure: one to contain the "key" field location and another to contain the key field length. Then, you need to create some new functions for adding and inserting items in the correct sequence. Once your list is in sequence, you can start trying to improve the performance of your searches. This can get complicated, but there is a lot of open-source C code available on the Internet that will help.
As you can tell, there is much more to linked lists than can easily be covered here. I discovered linked lists because of a specific problem I needed to solve. I think you will find (as I have) that once you add them to your programming arsenal, you'll find more and more uses for linked lists.
Happy linking.
Jeff Olen is a member of the AS/400 development team at Gauss Interprise, a content management software company located in Irvine, California. He can be reached by email at
Jeff Olen is a super-spy now but keeps his cover identity intact by working for video game studios on the East Coast. So when he’s not out killing members of ISIS or rescuing refugees, you can find him playing Wolfenstein II or testing the new Fallout 76 releases at his beach house in Costa Rica. In any case, he can’t be reached. You can email his cat at
MC Press books written by Jeff Olen available now on the MC Press Bookstore.
 |
||
 |
|
The IBM i Programmer’s Guide to PHP Get the scoop on how PHP can—and should—be deployed on IBM systems. List Price $79.95 Now On Sale
|
More than ever, there is a demand for IT to deliver innovation. Your IBM i has been an essential part of your business operations for years. However, your organization may struggle to maintain the current system and implement new projects. The thousands of customers we've worked with and surveyed state that expectations regarding the digital footprint and vision of the company are not aligned with the current IT environment.
TRY the one package that solves all your document design and printing challenges on all your platforms. Produce bar code labels, electronic forms, ad hoc reports, and RFID tags – without programming! MarkMagic is the only document design and print solution that combines report writing, WYSIWYG label and forms design, and conditional printing in one integrated product. Make sure your data survives when catastrophe hits. Request your trial now! Request Now.
Forms of ransomware has been around for over 30 years, and with more and more organizations suffering attacks each year, it continues to endure. What has made ransomware such a durable threat and what is the best way to combat it? In order to prevent ransomware, organizations must first understand how it works.
Disaster protection is vital to every business. Yet, it often consists of patched together procedures that are prone to error. From automatic backups to data encryption to media management, Robot automates the routine (yet often complex) tasks of iSeries backup and recovery, saving you time and money and making the process safer and more reliable. Automate your backups with the Robot Backup and Recovery Solution. Key features include:
Business users want new applications now. Market and regulatory pressures require faster application updates and delivery into production. Your IBM i developers may be approaching retirement, and you see no sure way to fill their positions with experienced developers. In addition, you may be caught between maintaining your existing applications and the uncertainty of moving to something new.
IT managers hoping to find new IBM i talent are discovering that the pool of experienced RPG programmers and operators or administrators with intimate knowledge of the operating system and the applications that run on it is small. This begs the question: How will you manage the platform that supports such a big part of your business? This guide offers strategies and software suggestions to help you plan IT staffing and resources and smooth the transition after your AS/400 talent retires. Read on to learn:
LATEST COMMENTS
MC Press Online