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

Time	Narration
00:01	Welcome to the Spoken Tutorial on Exception and error handling in PERL.
00:06	In this tutorial, we will learn to: Catch errors and Handle exceptions.
00:12	For this tutorial, I am using: Ubuntu Linux 12.04 operating system Perl 5.14.2 and the gedit Text Editor.
00:23	You can use any text editor of your choice.
00:27	To follow this tutorial, you should have working knowledge of Perl programming.
00:32	If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.
00:39	When an error occurs, Exception handling deviates the execution of a program from the normal execution path.
00:47	Error handling helps to recover the program, without terminating the application.
00:53	We can identify and trap an error in a number of ways. We will see few commonly used methods in Perl.
01:01	The warn function only raises a warning message without taking further action.
01:07	The die function immediately terminates the execution and displays the error message.
01:13	Let us understand the die function using a sample program which I have already saved.
01:20	Go to the terminal and type: gedit die dot pl ampersand and press Enter.
01:29	This is the code in 'die.pl' file. Let us understand the code now.
01:35	Here, we have defined a function divide which takes two input parameters i.e dollar numerator and dollar denominator.
01:46	At the rate underscore (@_) is a special variable used to pass the parameter list to the function.
01:53	If the denominator is zero, the die function will quit the script.
01:57	It will also display the error message for the user to read. Else, it will print the output.
02:05	These are the function call statements.
02:08	The first two times, the function is executed because the second parameter is not zero.
02:15	The third time, the denominator value is zero. So, the die function is executed.
02:23	The last divide function will not be executed as the die function quits the script.
02:29	Press Ctrl + S to save the program.
02:32	Let us execute the program.
02:35	Switch back to the terminal and type: perl die dot pl and press Enter.
02:43	The output is displayed as shown here. "Can't divide by zero!"
02:49	This is the error message we have given in the program, in the die statement.
02:54	Next, we will see how to use eval function in error handling.
03:00	eval function is used for handling run-time errors or exceptions.
03:06	For example, built-in errors such as out of memory, divide by zero or user defined errors.
03:14	The general syntax for eval function is shown here.
03:19	The dollar exclamation($!) special variable holds the error message, if any.
03:25	Otherwise, dollar exclamation( $!) holds an empty string. That means it is evaluated as false.
03:33	Let us understand the eval function using a sample program. Go to the terminal.
03:40	Type: gedit eval dot pl ampersand and press Enter.
03:47	In the eval dot pl file, type the following code as displayed on the screen. Let me explain the code now.
03:54	Here, in our example,open FILE invokes the die statement, if it has trouble in opening a file “test.dat”.
04:05	Perl gives the system error message from the last eval block to the variable dollar exclamation( $!).
04:13	Press Ctrl + S to save the file.
04:17	Switch back to the terminal and type: perl eval dot pl and press Enter.
04:25	The system error message is displayed as shown here.
04:30	Let us see another example. This time, we will see an error message returned from eval function using '$@' (dollar at the rate).
04:40	Let us switch back to the eval dot pl file.
04:44	Type the code as shown on the screen.
04:48	We are passing $total, $count as input parameters to the function average.
04:56	We have a possibility of getting an error if the count is zero.
05:00	Here, that is handled with the die statement.
05:04	The error message returned from eval is displayed using $@ ( dollar at the rate).
05:11	If not, it will print the Average value.
05:15	Press Ctrl +S to save the file. Let us execute the program.
05:22	Switch back to the terminal and type: perl eval.pl and press Enter.
05:31	The output is as shown here.
05:35	This brings us to the end of this tutorial. Let us summarize.
05:41	In this tutorial, we have learnt how to: Catch errors and Handle exceptions.
05:47	As an assignment do the following. On your Linux machine, create a file 'emp.txt' with 5 employee names.
05:57	Change permission of 'emp.txt' to READ only.
06:02	Note: Go through the relevant Linux spoken tutorials on the spoken tutorial website for change permission option.
06:10	Write a Perl program to open the 'emp.txt' file in WRITE mode and add few employee names to it.
06:19	Using "eval", print appropriate error message if open/write operation fails.
06:26	The video at the following link summarizes the Spoken Tutorial project. Please download and watch it.
06:33	The Spoken Tutorial Project team: conducts workshops using spoken tutorials and gives certificates on passing online tests.
06:42	For more details, please write to us.
06:46	Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.
06:53	More information on this mission is available at this link.
06:58	This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14