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System iPortal Home ContentLeave, Iterate, and LeaveSRArticle
ID: 57524Posted December 4th, 2008 in RPG ProgrammingBy:Bob
CozziHoliday OpCodesI wanted a holiday theme for this issue;
however, I couldn't think of any. So, Isimply called it, "Holiday
OpCodes." I hope you like it.
There are so many Structured Programming opcodes in RPG IV that
sometimes we forget all the pieces. We have things like FOR, DO,
DOW, DOU, IF, SELECT/WHEN and CASxx; but we also have LEAVE, ITER
and LEAVESR. Most programmers know how to usethe FOR, DO, and so
on; but few seem to understand how LEAVE, ITER, and LEAVESR
work.
Glorified GOTO--But in a Good WayWhen we started to move away
from the GOTO operation in programming some 25 years ago, new
structured operation codes were introduced. The DO, DOWHILE, and
DOUNTIL opcodes helped countless programmers write code that was
easier to read anddebug. The internal MI code that was generated
(often called "IRP") used the GOTO operation under the covers to
enable this capability in RPG III. But since wedidn't need to
modify the MI code, who cares what it generated?
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The instruction in MI was "B" or Branch--not GOTO--and it caused
the control tojump to another location (identified by a tag or
label) in the program. The DO,DOWHILE, and DOUNTIL opcodes were
looping opcodes that had one entry and one exit point and were a
key theme in structure programming.
Then came the issue of nesting. To avoid running sections of a
DO loop, nested IF statements were used, or in rare cases even a
GOTO was inserted. In the case of the nested IF statements, it was
to skip sections of the DO loop and loop backup to the top of the
loop. In the case of the embedded GOTO, it was to exit theDO loop
abnormally instead of at the natural, single exit point. This was a
painful time for programmers; we had structured opcodes but had to
resort to gruntmethods to make something work.
Enter LEAVEIn the late 1980s, three new opcodes entered the
scene. These opcodes helped reduce the use of nested IFs in DO
loops and effectively (permanently) eliminated the use of GOTO in
most RPG code. Those new opcodes are:
ITER--Iterate a loop back up to the topLEAVE--Exit a
loopLEAVESR--Exit a SubroutineITER--InterateThe ITER opcode (does
anyone else wish they would spell it out in RPG IV?) causes a jump
to the logical top of the Do loop, whether it is a DO, DOW, or DOU.
It
also works with the FOR opcode. This causes the code in the DO
loop following the ITER opcode to be skipped. Therefore,
conditioning the ITER opcode is virtually mandatory.
A few things happen when the ITER is performed, depending on
which opcode is looping. Branching to the top of the loop is the
same for any of them, but individually here are the
differences:
DO--The ITER opcode causes the increment counter (result field
of the DO opcodeitself) to be incremented. Thus, if you have a
simple "DO" opcode with a limit in Factor 2, then ITER branches to
the top of the Do loop and increments the counter. The Do loop
checks the counter to see if it has exceeded the value in Factor 2;
if it has, it exits the DO loop. Obviously, if you specify "DO 1"
(do one t
ime) or don't put a value in Factor 2, an ITER results in the
loop exiting. Butthe counter will be incremented first.
DOU (Do Until)--The ITER opcode causes a branch to the top of
the Do loop, the conditional statement is tested and, if met, the
program branches to the statement following the ENDDO opcode.
Although logically the test for the DOU is performed at the bottom
of the DOU loop, it is actually performed at the top of the loop.
This is because the difference between DOU and DOW in RPG is that
DOU is always performed once before its condition is tested. The
other difference is that DOU means continue while the condition is
NOT met, whereas DOW means continue while the condition is met.
DOW (Do While)--The ITER opcode causes a branch to the top of
the Do loop, the c
onditional statement is tested and, if not met, the program
branches to the statement following the ENDDO opcode.
FOR--The ITER opcode causes a branch to the top of the FOR loop,
the counter isincremented, and the limit condition is tested. If
the limit condition has not been met, then the next loop through
the FOR loop begins. Otherwise, control branches to the ENDFOR
opcode.
LEAVE--Exit a LoopThe LEAVE opcode causes a jump to the
corresponding ENDDO or ENDFOR statements.
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ed, it applies to the innermost loop that is being performed.
So, in our example, the LEAVE opcode applies to the FOR loop, not
to the DOU loop. Some programming languages have a way to indicate
on the LEAVE opcode that you want to exit 2 or more levels at once,
but RPG supports only one level of LEAVE.
--------------------------------------------------------------------------------
Bob Cozzi produces the popular Tuesday Tips video podcast on the
2nd and 4th Tuesday of each month for System iNetwork. He is author
of "RPG TnT: 101 Dynamic Tips 'n Techniques with RPG IV" and
produces RPG World, the most popular RPG IV developer conference of
the year. The next RPG World conference is scheduled for May 2009
in Las Vegas. Visit RPGWorld.com for details. Be sure to use
Promotion Code RPGCODER to save $100 off registration (promo code
valid until February 1, 2009).
Join Bob Cozzi and friends at noon Eastern every Friday for
iWeekly, a live streaming video podcast sponsored by RPG World.
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