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Administrator> Security

An Array of ArraysJuly 2006 | by Jon Parisand Susan Gantner

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An Array of Arrays

As we read each customer record, we simply add the customer number to the starting address of our storage allocation. We then set the indicator at

the resulting position to *On. The first indicator in our "array" corresponds to customer number 00000, the second to customer 00001 and so on.

/Free

// Allocate space for customer indicators (up to 100,000) and

// set intial value of all entries to *Off

pcustomerArray = %alloc(100000);memset( pcustomerArray: *Off: 100000 );

read custMast;

Dow not %Eof( custmast );

// Increment pointer with customer number to position in array

// Then set the indicator

pvalidCustomer = pcustomerArray + custNo;

validCustomer = *On;

customerCount += 1;

read custMast;

EndDo;

return customerCount;

/end-free

Once the loading of the array is complete, we enter the test loop as we previously did. As you'll see when you study the complete program, the

process of looking up the customer number is basically the same as the approach used to load the array (i.e., we increment the starting pointer by the

customer number).

One Last ThoughtYou may be wondering how the original technique of using an array of customer numbers could be extended beyond RPG's array limits. The answer,

as you may have already surmised, is to adapt the technique that we've just applied to the indicator array. There are two things that we need to

consider. First, the memory allocation will need to be "n" times the maximum number of entries (where "n" is the size in bytes of a single entry - five

in our case). Second, we need to change the search method. The original program used the %LOOKUP BIF, which can only be applied to

conventional RPG arrays, so we must use the C function bsearch instead. If you're not familiar with using bsearch, you might want to check out "

'Sorting' it All Out." We aim to make an example of this approach available on our Web site in the future for those interested in the technique.

When reviewing the performance figures for these various methods, it's important to remember that many RPG applications, particularly batch, are


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I/O bound. Reducing the I/O traffic, even if no physical I/O is involved, can make a far bigger difference to the overall application performance than

these simple timings might indicate.

Table 1 compares the measured performance of the three methods. The figures shown are relative to a base test that used CHAIN against the

customer file. (It may interest you to note that in our tests, CHAIN consistently outperformed SETLL but only by a small margin). As is to be

expected, in a small test of 1,000 lookups, all of our programs took longer than CHAIN due to the overhead of reading the whole file to load the array.

However, as the sample size increased, the advantage quickly swung the other way. Even at a sample size of 10,000 the slowest of the programs was

twice as fast as CHAIN. At a larger sample size the difference is even greater, as you can see.

We hope we've given you something to think about the next time you add a lookup routine to a program. And of course, we hope it goes without

saying that such routines should be packaged in service programs!

In support of these sample programs, we developed a number of small helper subprocedures, the most important of which generates the random

numbers used to fill the test file and generate customer numbers to simulate the transactions. The subprocedure uses the Qc3GenPRNs random

number generator API and although we don't have room to describe it here, we've documented the source code to explain the approach used. You'll

find the routine in the source member BASICTOOLS, which you can download, along with the rest of the programs referenced in the article, from our

Web site (www.Partner400.com/ExtraDownloads.htm).

Page 123

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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