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

Nancyvarkey at 05:58, 8 August 2015

2015-08-08T05:58:56Z

Nirmala Venkat: Created page with "'''Title of script''':''' Exception and Error handling in PERL''' '''Author: Nirmala Venkat''' '''Keywords: Exception, Error handling, ward(), die(), eval(), gedit, video tu..."

2015-08-03T09:31:33Z

Created page with "'''Title of script''':''' Exception and Error handling in PERL''' '''Author: Nirmala Venkat''' '''Keywords: Exception, Error handling, ward(), die(), eval(), gedit, video tu..."

New page

'''Title of script''':''' Exception and Error handling in PERL'''

'''Author: Nirmala Venkat'''

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

{| style="border-spacing:0;"
| style="background-color:#c0c0c0;border:1pt solid #000000;padding:0.097cm;"| <center>'''Visual Cue'''</center>
| style="background-color:#c0c0c0;border:1pt solid #000000;padding:0.097cm;"| <center>'''Narration'''</center>

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 1:

| style="border:1pt solid #000000;padding:0.097cm;"| Welcome to the''' Spoken Tutorial''' on''' Exception and error handling in PERL'''

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 2:

| style="border:1pt solid #000000;padding:0.097cm;"| In this tutorial we will learn to

* Catch errors and
* Handle exceptions
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 3:
| style="border:1pt solid #000000;padding:0.097cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 4:
| style="border:1pt solid #000000;padding:0.097cm;"| 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.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 5:

| style="border:1pt solid #000000;padding:0.097cm;"| 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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| We can identify and trap an error in a number of ways.

We will see few commonly used methods in '''Perl'''.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 6:

'''Warn() Function'''

'''Example''':

'''chdir('/downloads') or warn “Cannot change directory”'''
| style="border:1pt solid #000000;padding:0.097cm;"| The''' warn''' function only raises a warning message without taking further action.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 7:

'''Die() Function'''

'''Example''':

'''chdir('/downloads') or die “Cannot change directory”'''
| style="border:1pt solid #000000;padding:0.097cm;"| The''' die''' function immediately terminates the execution and displays the error message.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us understand the''' die''' function using a sample program, which I have already saved.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the '''Terminal and type'''

'''gedit die.pl &'''
| style="border:1pt solid #000000;padding:0.097cm;"| Go to the''' terminal''' and type

'''gedit die dot pl ampersand '''and press''' Enter'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Point to the filename die.pl in the Titlebar of gedit.'''
| style="border:1pt solid #000000;padding:0.097cm;"| This is code in''' die.pl''' file.

Let us understand the code now.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''<nowiki>#!/</nowiki>usr/bin/perl'''

'''sub divide {'''

'''my( $numerator, $denominator ) = @_;'''

'''if ($denominator == 0)'''

'''{die "Can't divide by zero!";}'''

'''else'''

'''{ print " Result: ",($numerator / $denominator),"\n";}'''

'''}'''

'''divide(121, 2);'''

'''divide(360, 4);'''

'''divide(412, 0);'''

'''divide(525, 7);'''
| style="border:1pt solid #000000;padding:0.097cm;"| 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 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.

These are the '''function call statements'''.

The first two times, the '''function''' is executed because the '''second parameter''' is not zero.

The third time, the '''denominator''' value is zero, so the '''die function''' is executed.

The last '''divide function '''will not be executed as the '''die function '''quits the script.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl + S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl + S''' to save the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| On the terminal type

'''perl die dot pl >> '''press''' Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the''' terminal''' and type,

'''perl die dot pl''' and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight

'''Output'''

'''Result: 60.5'''

'''Result: 90'''

'''Can't divide by zero! at die.pl line 6.'''
| style="border:1pt solid #000000;padding:0.097cm;"| 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.

<nowiki><<</nowiki>PAUSE>>

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Next, we will see how to use''' eval function '''in error handling.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 8:

'''eval() Function'''
| style="border:1pt solid #000000;padding:0.097cm;"| '''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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 9:

'''Syntax for eval():'''

eval {….}

if ($!) {

<nowiki># </nowiki>deal with error here
}
| style="border:1pt solid #000000;padding:0.097cm;"| 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'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to the '''terminal.'''
| style="border:1pt solid #000000;padding:0.097cm;"| Let us understand the''' eval''' function using a sample program.
Go to the''' terminal.'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Type '''gedit eval dot pl ampersand '''>> press''' Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Type '''gedit eval dot pl ampersand '''and press''' Enter'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| In the''' eval dot pl''' file, type the following code as displayed on the screen.

Let me explain the code now.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Example: 1'''

'''<nowiki>#!/</nowiki>usr/bin/perl'''

'''my $f = 'test.dat';'''

'''eval {'''

'''open FILE, $f'''

'''or die "Cannot open $f: $!";'''

'''};'''

'''if ($!) {'''

'''print "An error occurred ($!) \n";'''

'''}'''

| style="border:1pt solid #000000;padding:0.097cm;"| Here in our example,''' '''

'''open FILE''' invokes the''' die statement, '''if it has trouble in opening a file''' “test.dat”'''

'''Perl''' gives the '''system error message''' from the last '''eval''' block to the variable '''dollar exclamation( $!)'''

'''Dollar exclamation( $!)''' returns the error string along with the filename and line number.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+S'''
| style="border:1pt solid #000000;padding:0.097cm;"| Press '''Ctrl + S''' to save the file.

Let us execute the program
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Switch to terminal

'''perl eval dot pl''' >> press '''Enter'''
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the''' terminal''' and type,

'''perl eval dot pl''' and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight''' '''Output

'''An error occurred (No such file or directory)'''
| style="border:1pt solid #000000;padding:0.097cm;"| The '''system error message''' is displayed as shown here.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us see another example.

This time we will see an error message returned from '''eval''' function using '''$@''' '''(dollar at the rate).'''
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us switch back to the '''eval dot pl''' file.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| '''Example : 2'''

'''average(123, 5);'''

'''average(356, 41);'''

'''average(42,0);'''

'''average(500, 7);'''

'''eval { $average = $total / $count };'''

'''sub average {'''

'''my( $total, $count ) = @_;'''

'''if ($count == 0){'''

'''die "Can't divide by zero!: $@";}'''

'''else{'''

'''print " Average: ",($total / $count),"\n";}'''

'''}'''
| style="border:1pt solid #000000;padding:0.097cm;"| Type the code as shown on the screen.

We are passing''' $total''', '''$count''' as input parameters to the '''function average'''.

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.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| Switch back to the''' terminal''' and type,

'''perl eval.pl''' and press''' Enter'''.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Highlight Output

'''Average: 24.6'''

'''Average: 8.68292682926829'''

'''Can't divide by zero! : at eval.pl line 15.'''

| style="border:1pt solid #000000;padding:0.097cm;"| The output is as shown here.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 10:

Summary
| style="border:1pt solid #000000;padding:0.097cm;"| 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
|-
| style="border:1pt solid #000000;padding:0.097cm;"| slide 11:

'''Assignment'''

| style="border:1pt solid #000000;padding:0.097cm;"| 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.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 12:

About the Spoken Tutorial Project

| style="border:1pt solid #000000;padding:0.097cm;"| The video at the following link summarises the Spoken Tutorial project.

Please download and watch it.
|-
| style="border:1pt solid #000000;padding:0.097cm;"| Slide 13:
Spoken Tutorial Workshops

| style="border:1pt solid #000000;padding:0.097cm;"| The''' Spoken Tutorial Project''' Team

* conducts workshops using spoken tutorials and
* gives certificates on passing online tests.

For more details, please write to us.
|-
| style="border:1pt solid #000000;padding:0.097cm;"|

Slide 14:

Acknowledgement

| style="border:1pt solid #000000;padding:0.097cm;"|

Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.
More information on this mission is available at
this link.

|-
| style="border:1pt solid #000000;padding:0.097cm;"|
| style="border:1pt solid #000000;padding:0.097cm;"| This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

|}

@@ Line 93: / Line 93: @@
 '''my( $numerator, $denominator ) = @_;'''
 '''if ($denominator == 0)'''
@@ Line 103: / Line 111: @@
 '''}'''
-'''divide(121, 2);'''
+It will also display the error message for the user to read.
-'''divide(360, 4);'''
+Else it will print the output.
-'''divide(525, 7);'''
+|-
-| style="border:1pt solid #000000;padding:0.097cm;"| Here we have defined a '''function divide''' which takes two '''input parameters'''
+| style="border:1pt solid #000000;padding:0.097cm;"|
-i.e '''dollar numerator '''and''' dollar''' '''denominator'''
+'''divide(121, 2);'''
-'''At the rate underscore '''(@_) is a '''special variable '''used to pass the '''parameter list''' to the '''function'''.
+'''divide(360, 4);'''
-If the '''denominator''' is zero, the '''die''' function will quit the script.
+The first two times, the '''function''' is executed because the '''second parameter''' is not zero.
-It will also display the error message for the user to read.
+|-
-Else it will print the output.
+'''divide(412, 0);'''
-These are the '''function call statements'''.
+|-
-The first two times, the '''function''' is executed because the '''second parameter''' is not zero.
+'''divide(525, 7);'''
+| style="border:1pt solid #000000;padding:0.097cm;"| The last '''divide function '''will not be executed as the '''die function '''quits the script.
-The third time, the '''denominator''' value is zero, so the '''die function''' is executed.
-−
 The last '''divide function '''will not be executed as the '''die function '''quits the script.
 |-
@@ Line 221: / Line 230: @@
 '''};'''
 '''if ($!) {'''
@@ Line 227: / Line 242: @@
 '''}'''
-| style="border:1pt solid #000000;padding:0.097cm;"| Here in our example,''' '''
+| style="border:1pt solid #000000;padding:0.097cm;"| '''Perl''' gives the '''system error message''' from the last '''eval''' block to the variable '''dollar exclamation( $!)'''
-−
-'''open FILE''' invokes the''' die statement, '''if it has trouble in opening a file''' “test.dat”'''
-−
 '''Perl''' gives the '''system error message''' from the last '''eval''' block to the variable '''dollar exclamation( $!)'''
 |-
 | style="border:1pt solid #000000;padding:0.097cm;"| Press''' Ctrl+S'''
 | style="border:1pt solid #000000;padding:0.097cm;"| Press '''Ctrl + S''' to save the file.
 |-
 | style="border:1pt solid #000000;padding:0.097cm;"| Switch to terminal
@@ Line 276: / Line 285: @@
 '''my( $total, $count ) = @_;'''
 '''if ($count == 0){'''
@@ Line 286: / Line 307: @@
 '''}'''
-| style="border:1pt solid #000000;padding:0.097cm;"| Type the code as shown on the screen.
+| style="border:1pt solid #000000;padding:0.097cm;"|We have a possibility of getting an error if the count is zero.
-−
-We are passing''' $total''', '''$count''' as input parameters to the '''function average'''.
-−
 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 '''$@''' '''( dollar at the rate)'''
+The error message returned from '''eval''' is displayed using '''$@ ( dollar at the rate)'''
 If not, it will print the '''Average''' value.
@@ Line 300: / Line 317: @@
 |-
 | style="border:1pt solid #000000;padding:0.097cm;"|
-| style="border:1pt solid #000000;padding:0.097cm;"| Let us execute the program.
+| style="border:1pt solid #000000;padding:0.097cm;"| Press Ctrl +S to save the file.  Let us execute the program.
 |-
 | style="border:1pt solid #000000;padding:0.097cm;"|
@@ Line 320: / Line 337: @@
 Summary
-| style="border:1pt solid #000000;padding:0.097cm;"| This brings us to the end of this tutorial. Let us summarise
+| style="border:1pt solid #000000;padding:0.097cm;"| This brings us to the end of this tutorial. Let us summarise.
 In this tutorial, we have learnt how to
@@ Line 333: / Line 350: @@
 | style="border:1pt solid #000000;padding:0.097cm;"| As an assignment do the following.
-# On your linux machine, create a file '''emp.txt''' with 5 employee names.
+# 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.
+#:'''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.
+# 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.
+# Using "'''eval'''", print appropriate error message if '''open/write''' operation fails.
 |-
 | style="border:1pt solid #000000;padding:0.097cm;"| Slide 12: