Exception Handling ☆☆☆

An error that occurs during the execution of a program is called an exception.
Exception handling enables the program to continue by describing the process that shall be executed when an exception has occurred.

Example 1. A program without exception handling.

10 OPTION ANGLE DEGREES  
20 SET WINDOW 0,180,-10,10
30 FOR x=0 TO 180 STEP 0.1
40    PLOT LINES: x,TAN(x);
50 NEXT x
60 END

This program stops at x=90 for an exception.

Example 2. A program with exception handling.

10 OPTION ANGLE DEGREES
20 SET WINDOW 0,180,-10,10
30 FOR x=0 TO 180 STEP 0.1
40    WHEN EXCEPTION IN
50       PLOT LINES: x,TAN(x)
60    USE
70       PLOT LINES
80    END WHEN
90 NEXT x
100 END

If an exception occurs on the 50-line, the control jumps to the 70-line.
That is, on this program, the 70-line is the process that is executed when an exception has occurred on the 50-line.