One of the enhancements IBM introduced in i5/OS V5R3 is the %SUBARR built-in function. This built-in function is helpful to the growing number of programmers who use dynamic array techniques. In addition, %SUBARR is very useful when a traditional array is used and only a portion of the array's elements is needed.
The %SUBARR built-in function allows you to subscript an array by specifying both the starting array element and the number of elements you want to manipulate. You then pass this value to a SORTA, %LOOKUP, %XFOOT, or just about any other operation code or built-in function. The scope of the array elements is limited to what you have specified, rather than the entire array. This is good news for all of us.
For example, if you have an array that contains the sales figures for any period of time up to 10 years, you might have an array with several thousand elements. But what happens when you need to store and sort only a year's worth of data? Some people initialize the arrays to all 0s or all 9s, depending on the sort criteria. With %SUBARR, you simply determine how many array elements you want to sort and then issue the SORTA opcode.
The syntax for %SUBARR is shown below. The first parameter is the name of the array. The second parameter is the starting element. Normally, this is set to 1, but it may be any element in the array. The third and final parameter is the number of elements you want to subscript. This is an optional parameter that, if not specified, defaults to the end of the array.
Here's an example:
// Populate a year's worth of elements.
// Sort the array: Sort a year's worth of data only.
C sorta %subarr(DailySales:1:366)
Notice that the starting array element and the number of elements are specified as the second and third parameters of %SUBARR, respectively.
As I mentioned, in addition to traditional arrays, %SUBARR may be used with dynamic arrays. With the rise in new CGI programs and efficient storage utilization, using an array with a dynamic number of elements is becoming standard practice for RPG IV programmers. One of the shortcomings of dynamic arrays used to be that you could not subscript the array--that is, scope the manipulation of the array elements to those that you have allocated memory for. But now you can.
The same technique used for traditional fixed-element arrays can be used for dynamic arrays. In fact, it was designed in part because of the increase in dynamic array techniques being used by RPG IV developers.
Using %SUBARR with a dynamic array isn't all that different from using it with fixed-element arrays. In fact, it is virtually identical. The code below shows the DailySales array set up as a dynamic array.
D DailySales S 7P 2 Dim(3660) BASED(pArr)
// First, allocate the memory for 1 year of sales.
C eval pArr = %Alloc(%Size(DailySales)*366)
// Populate the 366 elements using traditional techniques.
C**.. TODO: Insert code here to populate the array.
// Now sort the array, but only sort a year's worth of data
C sorta %subarr(DailySales:1:366)
In this example, I use the %ALLOC built-in function to allocate the necessary storage for a year's worth of sales data. Then, the program fills in the array with the sales figures and calls the SORTA opcode to order the sales figures in ascending sequence.
The %SUBARR built-in function may be used in an expression on the EVAL and EVALR opcodes, and it will only impact the elements within the scope of the %SUBARR or extract those elements. In addition, the SORTA and RETURN support %SUBARR in the Extended Factor 2.
The %XFOOT built-in function supports a %SUBARR array as its parameter. But %LOOKUPxx does not because %LOOKUPxx already has the capabilities (starting element and element count) as %SUBARR.
One interesting byproduct of %SUBARR is the ability to add the elements of one array to the elements of another array. The %SUBARR built-in function gives you the ability to add elements 4, 5, and 6 to elements 1, 2, and 3. For example:
D ArrB S 7P 2 Dim(104)
** Add ArrA(4,5,6) to ArrB(1,2,3)
C eval ArrB += %subarr(ArrA:4:3)
** Add ArrA(4,5,6) to ArrB(56,57,58)
C eval %subarr(ArrB:56:3) += %subarr(ArrA:4:3)
In this example, the short-form math operators are used to add the three elements from ARRA to ARRB.
Let your imagination run with this new capability. You'll probably have plenty of time for that, because unfortunately most of us won't be onV5R3 until at least next year.
Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.
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