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An Array of ArraysJuly 2006 | by Jon Parisand Susan Gantner

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An Array of Arrays

The majority of the code is straightforward, so we won't discuss it in detail. At (A) you can see the definition of the array in which the customer

numbers will be stored. The entries are defined as being "LIKE" the customer number field. In addition, the array is loaded in key sequence, so we

also specified the ASCEND keyword, which ensures that the %LOOKUP BIF will use an efficient binary search. This approach will work as long as

we have fewer than 32,767 active customers.

At (B) we use the DSPLY op-code to ask the user to supply the number of test loops to run. Once the user has supplied this value, we start the timer

and invoke the sub-procedure that loads the customer numbers into the array. The procedure returns a count of the customer numbers loaded, whichis used later to limit the range of the search. Details of the array load subprocedure are at (D) above.

Within the test loop, we begin by calling GetRandom to obtain a random customer number. We then use the %LOOKUP BIF to search for that

customer in the array. Notice that we're using the count of the number of elements loaded into the array as the fourth parameter. This has the effect of

reducing the search time by limiting the search to the number of active elements in the array. Based on the result of the search, we then simply

increment the appropriate count and repeat the loop.

Once we've completed the required number of iterations, we exit the loop, stop the timer and display the results of the test (C). We'll look at the

results of the different methods later in this article.

As we noted earlier, our second program, CUSTSRCH2, uses an array where each element represents the status of a customer number. If the indicator

at a specific position is *On, then we consider the customer number to be valid. If it's *Off, then the customer number is invalid. As with the first

program, we're limited in this approach by RPG's array limit of 32,767 elements, but as long as the highest customer number in use is within that

range, this approach is simple to implement. So simple, in fact, that we won't take the space to describe it here - you can study the source if you wish

when you download the package. (Note: Although the program still reads the file in key sequence, this isn't strictly necessary when using thisapproach, and the load time could be shortened by reading the file in physical sequence.)

The third program, CUSTSRCH3, demonstrates an approach to the problem of RPG's array size limits. There are many techniques that we could use

as an alternative, including creating multiple arrays, each of which would relate to a specific range of account numbers. However, we've used a far

simpler (and it turns out much faster!) technique by simply employing a little bit of pointer arithmetic. If your pointer skills are rusty, or you are a little

"pointer-phobic", you might want to start out with the basic introduction to the use of based variables (read "Some Pointers on Using Pointers in RPG

IV"). Effectively what we'll do is the same as the compiler does "under the covers" to access an array element. We'll allocate sufficient memory to

accommodate all of our indicators, and then manipulate a pointer (pvalidCustomer) to position our indicator "view" over the appropriate one. This

almost certainly sounds more complicated than it really is, as you'll see when you study the code. We're effectively using the pointer as the array

index, but because we didn't have to define the storage as an array to RPG, we're no longer bound by the language's limits. In fact, we can easily

accommodate numbers up to 16 million with this technique (and with a little more work we can even extend that number). The first thing to change is

the definition of the "array" to:


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D validCustomer s n Based(pvalidCustomer)

Notice that it now appears to be a single named indicator. But the use of the BASED keyword will allow us to increment the pointer pvalidCustomer

to move its position through the "array." Naturally, the LoadCustomerNumbers routine also needs to be modified. The first significant change is to

reserve the required memory. In our example, we've used %ALLOC to reserve 100,000 bytes of dynamic memory. This is sufficient to accommodate

the full range of our five-digit customer number. We could've used a User Space to hold the "array," which has the advantage of allowing a single load

of the data to be shared between multiple jobs.

The next step is to initialize our "array" by setting all of the locations to *Off. This is necessary because we can't guarantee the content of dynamic

memory and some locations might already contain the value of *On. We've used the C function memset to do this - it's the fastest method we know of.

We won't detail its usage here - hopefully it's obvious from the code.

Next page: >>

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Jon Paris is a technical editor with IBM Systems Magazine and co-owner of Partner400.

More Articles From Jon Paris

Susan Gantner is a technical editor with IBM Systems Magazine and co-owner of Partner400.

More Articles From Susan Gantner

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