PERL/C3/Exception-and-error-handling-in-PERL/English-timed
From Script | Spoken-Tutorial
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| 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. |
