HP HP-28S - Page 237

displays an error message if the key is undefined. In the listing below,
the indentation marks the outer structure, the clauses, the inner struc-
tures,
and their clauses.
Sequence
"Apple" "Banana"
II Cher t-I~ II
DO
CLLCD
" [AJ felt" Applre"
2 DISP
" [B J f Ot" B.:.nan.:."
" [CJ fot" Chet"t"y"
DO UNTIL KEY END
UtHIL
Comments
This list contains the possible out-
comes. It remains on the stack until
the following DO ... UNTIL ... END
structure returns 1, 2, or 3, indicating
the user's choice.
Begin
the outer loop clause. This
clause displays
option messages,
which tell the user what the choices
are,
and gets a response from the user.
Clear the display.
The option message for line 1.
Display the message.
The option message for line 2.
Display the message.
The option message for line 3.
Display
the message.
The option message for line 4.
Display
the message.
This
inner indefinite loop repeats until
the user presses a key. The command
KEY returns ~3 if no key was pressed,
or a string (representing the key) and
1 if a key was pressed. When the loop
ends, the string is left on the stack.
Begin
the outer test clause. This clause
checks
whether the key pressed was a
defined key.
236 27: Interactive Progran ... s
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