Difference between revisions of "PERL/C3/Exception-and-error-handling-in-PERL/English"

• Catch errors and
• Handle exceptions

• Ubuntu Linux 12.04 operating system
• Perl 5.14.2
• and the gedit Text Editor

You can use any text editor of your choice.

If not, then go through the relevant Perl spoken tutorials on the spoken tutorial website.

1. Exception handling deviates the execution of a program from the normal execution path.

1. Error handling helps to recover the program, without terminating the application.

We will see few commonly used methods in Perl.

Warn() Function

Example:

chdir('/downloads') or warn “Cannot change directory”

Die() Function

Example:

chdir('/downloads') or die “Cannot change directory”

gedit die.pl &

gedit die dot pl ampersand and press Enter

Let us understand the code now.

sub divide {

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

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";}

}

It will also display the error message for the user to read.

Else it will print the output.

divide(121, 2);

divide(360, 4);

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

divide(412, 0);

divide(525, 7);

perl die dot pl >> press Enter

perl die dot pl and press Enter.

Output

Result: 60.5

Result: 90

Can't divide by zero! at die.pl line 6.

Can't divide by zero! -

This is the error message we have given in the program in the die statement.

<<PAUSE>>

eval() Function

For example, built-in errors such as out of memory, divide by zero or user defined errors.

Syntax for eval():

eval {….}

if ($!) {

# deal with error 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.

Go to the terminal.

Let me explain the code now.

#!/usr/bin/perl

my $f = 'test.dat';

eval {

open FILE, $f

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

};

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

if ($!) {

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

}

perl eval dot pl >> press Enter

perl eval dot pl and press Enter.

An error occurred (No such file or directory)

This time we will see an error message returned from eval function using $@ (dollar at the rate).

average(123, 5);

average(356, 41);

average(42,0);

average(500, 7);

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

sub average {

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

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";}

}

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.

perl eval.pl and press Enter.

Average: 24.6

Average: 8.68292682926829

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

Summary

In this tutorial, we have learnt how to

• Catch errors and
• Handle exceptions

Assignment

1. On your Linux machine, create a file emp.txt with 5 employee names.
2. 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.

3. Write a Perl program to open the emp.txt file in WRITE mode and add few employee names to it.
4. Using "eval", print appropriate error message if open/write operation fails.

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Slide 14:

Acknowledgement

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