HP HP-28S - Page 226

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.
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