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HP -28S Manual Page 226

Advanced scientific calculator.
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Flags. The numbers 1 and 0 that are returned by test commands are
called
stack flags.
Because they represent the truth or falsity of the
test, 1 is called a
true flag,
and 0 is called a
false flag.
The term Uflag" also refers to the built-in
user flags.
They are num-
bered 1 through 64; flags 31 through 64 have specific meanings to the
calculator, while flags 1 through 30 can represent any true/false dis-
tinction you wish. You can effectively store a stack flag in a user flag,
since both represent a truth value. For example, the sequence
IF A B
<
THEN 12 SF ELSE 12 CF END
sets flag 12 if A
<
B, or it clears flag 12 if A
~
B. You can later test
whether flag 12 is set by the sequence
IF 12 FS? THEN
which returns the same truth value as the original test
A B
<.
The
advantage to this technique is that the truth value of the original test
is preserved, even if the values of A and B have changed. The com-
mands for changing and testing user flags appear in uPROGRAM
TEST" in the Reference Manual. For the remainder of this chapter,
Uflag" refers to a stack flag.
Clauses. The objects and commands between two program-structure
commands are called a
clause.
Each clause is handled as a single en-
tity by the program structure. A clause is labeled by its logical role or
by the command that precedes it. In the first example:
• The clause between IF and THEN
(X 0
;t:) is called the
test clause
or
IF clause.
• The clause between THEN and ELSE (X
SIN
X /)
is called the
true clause
or
THEN clause.
• The clause between ELSE and END (1) is called the
false clause
or
ELSE clause.
26: Program Structures
225

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