HP HP-28S - Conditional Structures

For example, the user function COT (from chapter 5) was written:
Here the local-variable structure stores one argument in a local vari-
able
}:: (satisfying the first requirement) and evaluates the expression
, I
t,fI..! 0:: TAt·l 0:: ::<::0 ::0 ' (satisfying the second requirement). The user func-
tion O-C (from chapter 14) included a program rather than an
expression but, since the program returned exactly one result, O-C
also satisfied the second requirement.
These requirements apply only to user functions. More generally, local
variables are
used as a substitute for stack manipulations. The follow-
ing example returns the sum and difference of two numbers. Since it
returns two results, it can't be a user function.
« ~ x y « x y + x y - » »
For more examples, see the programs in chapter 28. They use local-
variable
structures more often to avoid stack manipulations than to
create
user functions.
Conditional Structures
Conditional structures enable a program to test a specified condition
and make a decision based on the result of the test. This section first
gives
an example of a conditional structure. It uses that example to
discuss
program structures in general, and then it describes other
types of conditional structures.
Suppose you're writing a program that uses the variable x, and you
want to calculate (sin x)jx. A problem arises because the quotient is
undefined when x = O. The following example returns (sin x)jx if
x =1= 0, or returns 1 if x = O.
IF X 0 ~ THEN X SIN X / ELSE 1 END
26: Program Structures 223