Making an Error Trap

You can construct an error trap with one of the following conditional structures:

IFERR … THEN … END.

IFERR … THEN … ELSE … END.

The IFERR 7 THEN 7 END Structure

The syntax for this structure is

«… IFERR trap-clause THEN error-clause END … »

The commands in the error-clause are executed only if an error is generated during execution of the trap-clause. If an error occurs in the trap-clause, the error is ignored, the remainder of the trap-clause is skipped, and program execution jumps to the error-clause. If no errors occur in the trap-clause, the error-clause is skipped and execution resumes after the END command.

To enter IFERR 7 THEN 7 END in a program:

Press !°LL%ERROR% !%IFERR%.

Example: The following program takes any number of vectors or arrays from the stack and adds them to the statistics matrix. However, the program stops with an error if a vector or array with a different number of columns is encountered. In addition, if only vectors or arrays with the same number of columns are on the stack, the program stops with an error after the last vector or array has been removed from the stack.

«WHILE DUP TYPE 3 == REPEAT Σ+ END »

In the following revised version, the program simply attempts to add the level 1 object to the statistics matrix until an error occurs. Then, it ends by displaying the message DONE.

Program:

Comments:

 

 

«

 

IFERR

Starts the trap-clause.

WHILE

The WHILE structure repeats

1

indefinitely, adding the vectors

REPEAT

and arrays to the statistics matrix

Σ+

until an error occurs.

END

 

THEN

Starts the error clause. If an error

"DONE" 1 DISP

occurs in the WHILE structure,

1 FREEZE

displays the message DONE in

 

the status area.

END

Ends the error structure.

»

 

RPL Programming 1-35