HP 50g Graphing manual Iferr then Else END Structure, To enter Iferr then Else END in a program

The IFERR THEN ELSE END Structure

The syntax for this structure is

«… IFERR trapclause

THEN errorclause ELSE normalclause END … »

The commands in the errorclause are executed only if an error is generated during execution of the trapclause. If an error occurs in the trapclause, the error is ig nored, the remainder of the trapclause is skipped, and program execution jumps to the errorclause. If no errors o ccur in the trapclause, execution jumps to the nor malclause at the completion of the trapclause.

To enter IFERR THEN ELSE END in a program:

Press !°LL %ERROR% @%IFERR%

Example: The following program prompts for two numbers, then adds them. If only one number is supplied, the program displays an error message and prompts again.

Program:	Comments:
«
DO	Begins the main loop.
"KEY IN a AND b" " "	Prompts for two numbers.
INPUT OBJ→
UNTIL	Starts the loop test clause.
IFERR	The error trap contains only the +
+	command.
THEN	If an error occurs, recalls and displays the
ERRM 5 DISP	Too Few Arguments message for 2
2 WAIT	seconds, then puts 0 (false) on the stack for
0	the main loop.
ELSE	If no error occurs, puts 1 (true) on the stack
1	for the main loop.
END	Ends the error trap.
END	Ends the main loop. If the error trap left 0
	(false) on the stack, the main loop repeats —
	otherwise, the program ends.
»

136 RPL Programming