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Gotcha: Differences With DO Between S/36 and AS/4

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Duty calls for ninety-nine repetitions of the loop in the program in 1.

Duty calls for ninety-nine repetitions of the loop in the program in Figure 1.

The S/36 runs this program without trouble, and so does the AS/400 in the S/36E. Both understand very well their orders: repeat the loop ninety-nine times, then end the program.

The AS/400 native environment, on the other hand, tries to go beyond the call of duty, and this gets it into trouble very quickly: RPG0121 is issued, which reads "The array index is invalid (C G S D)."

Taking option D (for Dump) generates a printout that lets you examine the values of all variables. Variable X has a value of... zero! Obviously, the program tried to loop for the hundredth time: this zero is a number 100 truncated to two digits.

If you are converting programs to native, beware.

Bill Robins Riverside, California

Editor's Note:

What is really disturbing about this bug is that if you remove the array reference from within the loop (ARR,X), the program will loop forever, even though it goes against common sense. This problem was reported to IBM Level 2 as a compiler bug on January 21, 1991. Their reply was FAXed to us the next day:

"The user obtains different results because of the differences between RPG II and RPG III compilers. In the user's program, the limit value (Factor 2) of the DO op-code is 99 while the length of the index variable (Result Field) is only 2. In the S/36 RPG II compiler, the DO-END statement will loop from the starting value (Factor 1) to 99 (Factor 2) and then stop. After the END statement, the index variable is set to 00. In the RPG III compiler, since the index variable will never be greater than the limit value, the DO-END statement will loop forever or issue a message RPG0121.

"In the AS/400, the RPG/36 compatible compiler behaves the same as the S/36 RPG II compiler while RPG/400 and RPG/38 behave the same as the S/38 RPG III compiler. As a result, no changes will be made to the compiler.

"This difference will be documented in Appendix B, 'AS/400 RPG/400 and AS/400 RPG II System/36-Compatible Functions,' in the RPG/400 User's Guide (SC09-1161)."

For the record, we must refer the reader to page 11-53 of the RPG/400 Reference Guide, which states the following near the bottom of the page:

"3. If the index value is greater than the limit value, control passes to the calculation following the associated END statement (step 7). Otherwise, control passes to the first operation after the DO statement (step 4)."

The implication from IBM's "solution" to this bug seems to be that it is not really a bug, only a documentation issue, so they have nothing to fix. This is the stuff of which "Gotchas" are made. This one may remain a "Gotcha!" until the end of time.


Gotcha: Differences With DO Between S/36 and AS/4

Figure 1 Program to perform 99 repetitions

 Figure 1: Program to Perform Ninety-Nine Repetitions ....1.... ....2.... ....3.... ....4.... ....5.... ....6.... ....7 E ARR 99 7 2 * C Z-ADD0 TOTAL 92 C 1 DO 99 X 20 C ADD ARR,X TOTAL C END * C SETON LR ....1.... ....2.... ....3.... ....4.... ....5.... ....6.... ....7 
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