PERL/C3/Exception-and-error-handling-in-PERL/English

Title of script: Exception and Error handling in PERL

Author: Nirmala Venkat

Keywords: Exception, Error handling, ward(), die(), eval(), gedit, video tutorial

Visual Cue	Narration
Slide 1:	Welcome to the Spoken Tutorial on Exception and error handling in PERL
Slide 2:	In this tutorial we will learn to Catch errors and Handle exceptions
Slide 3:	For this tutorial, I am using Ubuntu Linux 12.04 operating system Perl 5.14.2 and the gedit Text Editor You can use any text editor of your choice.
Slide 4:	To follow this tutorial, you should have working knowledge of Perl Programming. If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.
Slide 5:	When an error occurs: Exception handling deviates the execution of a program from the normal execution path. Error handling helps to recover the program, without terminating the application.
	We can identify and trap an error in a number of ways. We will see few commonly used methods in Perl.
Slide 6: Warn() Function Example: chdir('/downloads') or warn “Cannot change directory”	The warn function only raises a warning message without taking further action.
Slide 7: Die() Function Example: chdir('/downloads') or die “Cannot change directory”	The die function immediately terminates the execution and displays the error message.
	Let us understand the die function using a sample program, which I have already saved.
Switch to the Terminal and type gedit die.pl &	Go to the terminal and type gedit die dot pl ampersand and press Enter
Point to the filename die.pl in the Titlebar of gedit.	This is code in die.pl file. Let us understand the code now.
#!/usr/bin/perl sub divide { my( $numerator, $denominator ) = @_;	Here we have defined a function divide which takes two input parameters i.e dollar numerator and dollar denominator At the rate underscore (@_) is a special variable used to pass the parameter list to the function.
if ($denominator == 0) {die "Can't divide by zero!";} else { print " Result: ",($numerator / $denominator),"\n";} }	If the denominator is zero, the die function will quit the script. It will also display the error message for the user to read. Else it will print the output.
divide(121, 2); divide(360, 4);	These are the function call statements. The first two times, the function is executed because the second parameter is not zero.
divide(412, 0);	The third time, the denominator value is zero, so the die function is executed.
divide(525, 7);	The last divide function will not be executed as the die function quits the script.
Press Ctrl + S	Press Ctrl + S to save the program.
	Let us execute the program.
On the terminal type perl die dot pl >> press Enter	Switch back to the terminal and type, perl die dot pl and press Enter.
Highlight Output Result: 60.5 Result: 90 Can't divide by zero! at die.pl line 6.	The output is displayed as shown here. Can't divide by zero! - This is the error message we have given in the program in the die statement. <<PAUSE>>
	Next, we will see how to use eval function in error handling.
Slide 8: eval() Function	eval function is used for handling run-time errors or exceptions. For example, built-in errors such as out of memory, divide by zero or user defined errors.
Slide 9: Syntax for eval(): eval {….} if ($!) { # deal with error here }	The general syntax for eval function is shown here. The dollar exclamation($!) special variable holds the error message, if any. Otherwise, dollar exclamation( $!) holds an empty string. That means it is evaluated as false.
Switch to the terminal.	Let us understand the eval function using a sample program. Go to the terminal.
Type gedit eval dot pl ampersand >> press Enter	Type gedit eval dot pl ampersand and press Enter
	In the eval dot pl file, type the following code as displayed on the screen. Let me explain the code now.
Example: 1 #!/usr/bin/perl my $f = 'test.dat'; eval { open FILE, $f or die "Cannot open $f: $!"; };	Here in our example, open FILE invokes the die statement, if it has trouble in opening a file “test.dat”
if ($!) { print "An error occurred ($!) \n"; }	Perl gives the system error message from the last eval block to the variable dollar exclamation( $!)
Press Ctrl+S	Press Ctrl + S to save the file.
Switch to terminal perl eval dot pl >> press Enter	Switch back to the terminal and type, perl eval dot pl and press Enter.
Highlight Output An error occurred (No such file or directory)	The system error message is displayed as shown here.
	Let us see another example. This time we will see an error message returned from eval function using $@ (dollar at the rate).
	Let us switch back to the eval dot pl file.
Example : 2 average(123, 5); average(356, 41); average(42,0); average(500, 7); eval { $average = $total / $count }; sub average { my( $total, $count ) = @_;	Type the code as shown on the screen. We are passing $total, $count as input parameters to the function average.
if ($count == 0){ die "Can't divide by zero!: $@";} else{ print " Average: ",($total / $count),"\n";} }	We have a possibility of getting an error if the count is zero. Here, that is handled with the die statement. The error message returned from eval is displayed using $@ ( dollar at the rate) If not, it will print the Average value.
	Press Ctrl +S to save the file. Let us execute the program.
	Switch back to the terminal and type, perl eval.pl and press Enter.
Highlight Output Average: 24.6 Average: 8.68292682926829 Can't divide by zero! : at eval.pl line 15.	The output is as shown here.
Slide 10: Summary	This brings us to the end of this tutorial. Let us summarise. In this tutorial, we have learnt how to Catch errors and Handle exceptions
slide 11: Assignment	As an assignment do the following. On your Linux machine, create a file emp.txt with 5 employee names. Change permission of emp.txt to READ only. Note: Go through the relevant Linux spoken tutorials on the spoken tutorial website for change permission option. Write a Perl program to open the emp.txt file in WRITE mode and add few employee names to it. Using "eval", print appropriate error message if open/write operation fails.
Slide 12: About the Spoken Tutorial Project	The video at the following link summarises the Spoken Tutorial project. Please download and watch it.
Slide 13: Spoken Tutorial Workshops	The Spoken Tutorial Project Team conducts workshops using spoken tutorials and gives certificates on passing online tests. For more details, please write to us.
Slide 14: Acknowledgement	Spoken Tutorial project is funded by NMEICT, MHRD, Government of India. More information on this mission is available at this link.
	This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

Nancyvarkey, Nirmala Venkat