HP HP-28S - Page 255

Techniques:
• DO ... UNTIL ... END (indefinite loop). The DO clause contains
the steps to be repeated; the UNTIL clause contains the test that
determines whether to repeat both clauses again (if false) or to exit
(if true).
•
Programs as arguments. Although programs are commonly named
and then executed by calling their names, programs can also be put
on the stack and used as arguments to other programs.
• Evaluation of local variables.
The program argument to be executed
repeatedly is stored in a local variable. It's handy to store an object
in a local variable
when you don't know beforehand how many
copies you'll need.
MULTI demonstrates one of the differences between global and lo-
cal variables: if a global variable contains a
name or program, the
contents of the variable are evaluated when the name is evaluated;
but the contents of a local variable are always simply recalled. Con-
sequently, MULTI uses the local name to put the program argu-
ment on the stack and then executes an explicit EVAL command to
evaluate
the program.
Program
DO
DUP
F' E',)AL
UtHIL
DUP
POT
::;At'1E
Comments
Begin the program.
Create a local variable p that con-
tains the program argument.
Begin the defining program.
Begin the DO clause.
Make a copy of the object.
Apply the program to the object,
returning a n~w version. (The
EV AL command is necessary to
execute
the program because lo-
cal variables
always return their
contents to the stack
unevaluated.)
Begin
the UNTIL clause.
Make a copy of the new version
of
the object.
Move the old version to level 1.
Test whether the old version and
the new version are the same.
254 28: Programming Examples