PERL/C3/Special-Variables-in-PERL/English-timed - Revision history

PoojaMoolya at 12:45, 20 February 2017

2017-02-20T12:45:04Z

Sandhya.np14 at 06:27, 11 February 2016

2016-02-11T06:27:57Z

Sandhya.np14 at 07:37, 10 February 2016

2016-02-10T07:37:31Z

Sandhya.np14 at 07:28, 9 February 2016

2016-02-09T07:28:41Z

Sandhya.np14 at 12:56, 28 December 2015

2015-12-28T12:56:15Z

Sandhya.np14 at 09:34, 28 December 2015

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PoojaMoolya at 05:58, 26 November 2015

2015-11-26T05:58:28Z

PoojaMoolya: Created page with " {| Border = 1 | '''Time''' | '''Narration''' |- |00:01 |Welcome to the''' Spoken Tutorial''' on''' special variables '''in '''Perl.''' |-..."

2015-11-24T04:34:04Z

Created page with " {| Border = 1 | <center>'''Time'''</center> | <center>'''Narration'''</center> |- |00:01 |Welcome to the''' Spoken Tutorial''' on''' special variables '''in '''Perl.''' |-..."

New page

{| Border = 1
| <center>'''Time'''</center>
| <center>'''Narration'''</center>

|-
|00:01
|Welcome to the''' Spoken Tutorial''' on''' special variables '''in '''Perl.'''

|-
|00:04
|In this tutorial we will learn about
* '''Global special variables'''
* '''Special command line variables'''
* '''Global special constants'''

|-
|00:13
|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.

|-
|00:27
|As a prerequisite, 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.

|-
|00:38
|What are''' special variables'''?

|-
|00:41
|'''Special variables''' are '''predefined''' variables that have a special meaning in '''Perl'''.

|-
|00:46
|These do not need to be initialised before use.

|-
|00:50
|These are used to hold the results of searches, '''environment variables''' and '''flags''' to control '''debugging'''.

|-
|00:58
|First, we will learn about''' Global special variables.'''

|-
|01:02
|'''$_ : (Dollar Underscore)'''. This is a widely used''' special variable'''.

|-
|01:06
|'''$_''' - '''Dollar Underscore '''is the default parameter for lot of functions and pattern-searching strings.

|-
|01:14
|Let us understand the usage of '''$_ variable''' using a sample program.

|-
|01:20
|I will open the''' special dot pl file''' which I have already created.

|-
|01:26
|Go to the '''terminal''' and type''' gedit special.pl ampersand '''and press '''Enter.'''

|-
|01:32
|'''special.pl''' file is now open in '''gedit'''. Type the code as displayed on the screen.
Let me explain the code now.

|-
|01:42
|There are 2 '''foreach '''loops here. Both these''' foreach''' loops will execute the same result.

|-
|01:49
|In each iteration of the loop, the current string is placed in '''$_'''.

|-
|01:54
|And it is used by the '''print statement, '''by default. '''$_ '''saves the use of one extra variable '''$color'''.

|-
|02:03
|Press''' Ctrl+S''' to save the file.

|-
|02:06
|Then switch to the''' terminal''' and execute the '''Perl''' script by typing '''perl special dot pl''' and press''' Enter'''.

|-
|02:13
|Here both '''foreach''' loops give the same output.

|-
|02:18
|Now, let us see another example to demonstrate how '''$_ variable '''is '''implicit'''.
Go back to the '''special dot pl file'''.

|-
|02:27
|Type the piece of code shown on the screen.

|-
|02:30
|This program reads a '''text file first.txt '''line by line. Then it loops through the '''DATA''' file, till all lines are read.

|-
|02:40
|'''print $_ '''variable prints the contents of the current line from the''' first.txt''' file.
In the ''''while'''' loop, the use of '''$_''' is '''implicit.'''

|-
|02:51
|We will see more about this in future tutorials.

|-
|02:55
|'''At the rate underscore''' is the '''special variable''' used to store '''subroutine parameters.'''

|-
|03:01
|'''Arguments''' for a '''subroutine''' are stored in this array variable.

|-
|03:06
|'''Array operations''' like '''pop/shift '''can be done on this '''variable,''' as we do in normal '''arrays'''.

|-
|03:13
|I will show an example for this. Let us switch back to '''special dot pl file''' once again.

|-
|03:19
|Type the code as displayed on the screen.

|-
|03:22
|This program will return the '''maximum value '''between two numbers.
'''@_ - At the rate underscore''' is a '''local array '''which stores the two '''arguments dollar a comma dollar b.'''

|-
|03:35
|That is, it is stored under '''dollar underscore index of zero''' and '''dollar underscore index of one'''

|-
|03:43
|The '''print statement '''prints the '''maximum '''of the two given numbers.

|-
|03:47
|Press''' Ctrl+S''' to save the file.

|-
| 03:51
| Switch to the''' terminal''' and execute the '''Perl '''script by typing '''perl special dot pl''' and press''' Enter'''.

|-
|03:58
|The maximum value is displayed as output. Let’s move on.

|-
|04:02
|'''Environment variables '''are represented by '''percentage '''followed by '''capital ENV'''.

|-
|04:10
|'''Environment variables '''contain a copy of the current '''environment variables, '''such as the following.

|-
|04:17
|Let us understand''' %ENV''' variable using a sample program.

|-
|04:23
|We will switch back to the''' special dot pl file'''.

|-
|04:26
|Type the following code as displayed on the screen.

|-
|04:30
|Press''' Ctrl+S''' to save the file. Switch to the''' terminal''' and execute the '''Perl '''script.

|-
|04:37
|Type''' perl special dot pl''' and press''' Enter'''.

|-
|04:42
|We can see the current environment details such as '''PWD (present working directory)''', username, language etc.

|-
| 04:51
|Next we will see about another '''special variable''' '''dollar zero'''

|-
|04:55
|The '''special variable dollar zero''' ('''$0''') contains name of the current '''Perl '''program that is being executed.

|-
|05:02
|This is generally used for logging purpose.

|-
|05:05
|For example: I have a file named '''First.pl '''within which I am using '''$0 '''variable''' '''as shown here.
|-
|05:14
|On executing, it will print the filename '''First.pl'''.

|-
|05:19
|'''Perl''' has a''' built-in function''' called '''sort''' that sorts an '''array'''.

|-
|05:24
|A '''comparison function''' will compare its parameters using the''' numerical comparison operator.'''

|-
|05:30
|This operator is represented by''' lesser than equal to greater than symbols, '''as shown here.

|-
|05:38
|Let us see an example for this.

|-
|05:40
|Open the '''terminal''' and type '''gedit sort.pl ampersand '''and press''' Enter.'''
|-
|05:47
|'''sort.pl''' file is now open in '''gedit Text Editor'''. Type the following code as displayed on the screen.

|-
|05:56
|Let me explain the code. The first line declares an '''array '''of numbers.

|-
|06:02
|The '''numerical comparison operator '''will compare the two values as numbers.

|-
|06:08
|''''Dollar a '''and '''dollar''' '''b '''are '''special package local variables''' in which the values to be compared are loaded.

|-
|06:16
|And this '''sort''' function will sort the numbers in ascending order.

|-
|06:21
|Let us now save and execute the program.

|-
|06:25
|Switch back to the '''terminal''' and type, '''perl sort.pl''' and press''' Enter'''.

|-
|06:31
|We can see that the numbers are sorted in ascending order.

|-
|06:35
|Let’s see another special variable '''dollar exclamation.'''

|-
|06:39
|''''dollar exclamation''' if used in '''string '''context, returns the '''system error string.'''
Here is an example of its usage.

|-
|06:48
|If the file '''hello.txt '''doesn't exist, it will print the error message,like:
'''Cannot open file for reading : No such file or directory.'''
|-
|06:59
|Let’s now look at another '''special variable''' namely, '''dollar at the rate.'''

|-
|07:04
|'This is another widely used '''variable'''. It returns an error message, returned from''' eval''' or''' require''' command.

|-
|07:12
|'This example will print: '''could not divide Illegal division by zero. '''

|-
|07:17
|'''dollar dollar''' is yet another '''special variable.''' This holds the '''process ID''' of the '''Perl interpreter''' running this script.

|-
|07:26
|The '''diamond operator''' is used to read every line, from the files specified on the '''command line'''.

|-
|07:32
|Let us see an example for this.

|-
|07:35
|Open the '''terminal''' and type''' gedit commandline.pl ampersand '''and press''' Enter.'''

|-
|07:42
|'''commandline.pl''' file is now open in '''gedit'''.

|-
|07:46
|Type the code as displayed on the screen.

|-
|07:49
|Save the file.

|-
|07:51
|Let me show you the text that I have in a file named '''sample.txt''' file.

|-
|07:56
|Now, run the program from the '''command line '''by typing: '''perl commandline.pl space sample.txt '''and press''' Enter.'''

|-
|08:07
|This is the text we had in '''sample.txt''' file.

|-
|08:11
|If no files are specified, it reads from the standard input i.e. from the keyboard.

|-
|08:17
|'''Perl''' has an '''array''' '''at the rate capital A R G V '''special variable.
This holds all the values from the '''command line'''.

|-
|08:27
|When using '''array''' '''at the rate capital A R G V, '''there is no need to declare the '''variables.'''

|-
|08:33
|'The values from the '''command line''' are automatically placed in this '''variable'''.

|-
|08:37
|Let’s now move on to '''Global Special Constants.'''

|-
|08:41
|'''underscore underscore E N D ('''in capital letters)'''underscore underscore.''' indicates the logical end of the program.

|-
|08:50
|Any text following this '''special variable '''is ignored after this statement.

|-
|08:55
|'''underscore underscore FILE '''(in capital letters)''' underscore underscore''' represents the filename of the program at the point where it is used.

|-
|09:06
|'''underscore underscore LINE '''(in capital letters) '''underscore underscore''' represents the current line number

|-
|09:13
|'''underscore underscore PACKAGE '''(in capital letters) '''underscore underscore''' represents the current package name at compile time, or undefined if there is no current package.

|-
|09:25
|We will see a sample program on how '''Global Special Constants '''are used.
|-
|09:30
|Open the '''terminal''' and type '''gedit specialconstant.pl ampersand ''' and press '''Enter.'''

|-
|09:39
|'''specialconstant.pl''' file is now open in '''gedit'''.

|-
|09:44
|'Type the following code as displayed on the screen. Let me explain the code now.

|-
|09:50
|The '''special literals PACKAGE, FILE, LINE ''' represent the package name, current filename and
line number respectively at that point in the program.

|-
|10:00
|Let us execute the program.

|-
|10:02
|Switch back to the '''terminal '''and type: '''perl specialconstant.pl''' and press''' Enter'''.

|-
|10:09
|We can see the current package name, filename and line number of our program.

|-
|10:15
|This brings us to the end of this tutorial. Let us summarise.

|-
|10:19
|In this tutorial, we learnt about some commonly used '''special variables''' in '''Perl'''.

|-
|10:25
|As an assignment do the following. Write a '''Perl''' script to sort the following array of numbers in ascending and descending order.

|-
|10:34
| Note: For descending order, use the below code for comparison.

|-
|10:39
| Print the sorted result using '''‘while loop’''' and '''special variable $_.'''

|-
|10:45
|Save and execute the program.

|-
|10:47
|Now check the result.
|-
|10:49
|The video at the following link summarises the Spoken Tutorial project.

Please download and watch it
|-
|10:56
|The Spoken Tutorial Project Team
conducts workshops and gives certificates on passing online tests.
|-
|11:03
| For more details, please write to us.
|-
|11:06
|Spoken Tutorial project is funded by NMEICT, MHRD, Government of India.

|-
|11:13
|More information on this mission is available at this link.
|-
|11:17
|This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.
|}

@@ Line 11: / Line 11: @@
 |00:04
 |In this tutorial, we will learn about:
-* '''Global special variables'''
+'''Global special variables'''
-* '''Special command line variables'''
+'''Special command line variables'''
-* '''Global special constants'''.
+'''Global special constants'''.
 |-
 |00:13
 |For this tutorial, I am using:
-* '''Ubuntu Linux 12.04''' operating system
+'''Ubuntu Linux 12.04''' operating system
-* '''Perl 5.14.2''' and the
+'''Perl 5.14.2''' and the
-* ''' gedit''' Text Editor.
+''' gedit''' Text Editor.
 You can use any '''text editor''' of your choice.
@@ Line 279: / Line 279: @@
 |-
 |06:48
-|If the file 'hello.txt' doesn't exist, it will print the error message, like:
+|If the file 'hello.txt' doesn't exist, it will print the error message, like: "Cannot open file for reading : No such file or directory".
 "Cannot open file for reading : No such file or directory".
 |-
 |06:59
@@ Line 339: / Line 338: @@
 |-
 |08:17
-|'''Perl''' has an array '''at the rate capital A R G V '''special variable.
+|'''Perl''' has an array '''at the rate capital A R G V '''special variable. This holds all the values from the '''command line'''.
 This holds all the values from the '''command line'''.
 |-

@@ Line 212: / Line 212: @@
 |-
 |05:05
-|For example: I have a file named 'First.pl ' within which I am using '$0' variable, as shown here.
+|For example: I have a file named 'First.pl' within which I am using '$0' variable, as shown here.
 |-
 |05:14
@@ Line 249: / Line 249: @@
 |-
-|06:08'
+|06:08
 |'''Dollar a''' and '''dollar b ''' are '''special package local variables''' in which the values to be compared are loaded.

@@ Line 327: / Line 327: @@
 |-
 |07:56
-|Now, run the program from the '''command line '''by typing: '''perl commandline dot pl space sample dot txt '''and press''' Enter.'''
+|Now, '''run''' the program from the '''command line '''by typing: '''perl commandline dot pl space sample dot txt '''and press''' Enter.'''
 |-
@@ Line 364: / Line 364: @@
 |-
 |08:55
-|'''underscore underscore FILE '''(in capital letters)''' underscore underscore''' represents the filename of the program at the point where it is used.
+|'''underscore underscore FILE '''(in capital letters)''' underscore underscore''' represents the filename of the program, at the point where it is used.
 |-
@@ Line 392: / Line 392: @@
 |-
 |09:50
-|The special literals  '''PACKAGE, FILE, LINE ''' represent the  package name, current filename and line number respectively, at that point in the program.
+|The special literals "PACKAGE, FILE, LINE" represent the  package name, current filename and line number respectively, at that point in the program.
 |-
@@ Line 416: / Line 416: @@
 |-
 |10:25
-|As an assignment do the following. Write a '''Perl''' script to '''sort''' the following array of numbers in ascending and descending order.
+|As an assignment, do the following. Write a '''Perl''' script to '''sort''' the following array of numbers in ascending and descending order.
 |-

@@ Line 6: / Line 6: @@
 |-
 |00:01
-|Welcome to the''' Spoken Tutorial''' on''' special variables '''in '''Perl.'''
+|Welcome to the''' Spoken Tutorial''' on''' Special variables in Perl.'''
 |-
@@ Line 73: / Line 73: @@
 |-
 |01:32
-|'''special dot pl''' file is now open in '''gedit'''. Type the code as displayed on the screen.
+|'''special dot pl''' file is now open in '''gedit'''. Type the code as displayed on the screen. Let me explain the code now.
 Let me explain the code now.
 |-
@@ Line 439: / Line 438: @@
 |-
 |10:56
-|The '''Spoken Tutorial''' project team
+|The '''Spoken Tutorial''' project team conducts workshops and gives certificates on passing online tests.
 conducts workshops and gives certificates on passing online tests.
 |-
 |11:03

@@ Line 190: / Line 190: @@
 |04:30
 |Press''' Ctrl+S''' to save the file. Switch to the terminal and execute the '''Perl '''script.
 |-
@@ Line 455: / Line 454: @@
 |-
 |11:17
-|This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.
+|This is Nirmala Venkat from '''IIT Bombay''', signing off. Thanks for watching.
 |}

@@ Line 61: / Line 61: @@
 |-
 |01:14
-|Let us understand the usage of '''$_ variable''' using a sample program.
+|Let us understand the usage of '''$_ (Dollar Underscore) variable''' using a sample program.
 |-
@@ Line 69: / Line 69: @@
 |-
 |01:26
-|Go to the '''terminal''' and type''' gedit special.pl ampersand '''and press '''Enter.'''
+|Go to the '''terminal''' and type''' gedit special dot pl ampersand '''and press '''Enter.'''
 |-
 |01:32
-|'''special.pl''' file is now open in '''gedit'''. Type the code as displayed on the screen.
+|'''special dot pl''' file is now open in '''gedit'''. Type the code as displayed on the screen.
 Let me explain the code now.
@@ Line 86: / Line 86: @@
 |-
 |01:54
-|And it is used by the '''print statement, '''by default. '''$_ '''saves the use of one extra variable '''$color'''.
+|And it is used by the '''print statement, '''by default. '''$_  (Dollar Underscore)''' saves the use of one extra variable '''$color'''.
 |-
@@ Line 102: / Line 102: @@
 |-
 |02:18
-|Now, let us see another example to demonstrate how '''$_ variable '''is '''implicit'''.
+|Now, let us see another example to demonstrate how '''$_ (Dollar Underscore) variable '''is '''implicit'''.
 Go back to the '''special dot pl file'''.
@@ Line 145: / Line 145: @@
 |03:22
 |This program will return the '''maximum value '''between two numbers.
-'''@_ - At the rate underscore''' is a '''local array '''which stores the two '''arguments dollar a comma dollar b.'''
+'''@_ (At the rate underscore)''' is a '''local array '''which stores the two '''arguments dollar a comma dollar b.'''
 |-
@@ Line 217: / Line 217: @@
 |-
 |05:14
-|On executing, it will print the filename '''First.pl'''.
+|On executing, it will print the filename '''First dot pl'''.
 |-
@@ Line 325: / Line 325: @@
 |-
 |07:51
-|Let me show you the text that I have in a file named '''sample.txt''' file.
+|Let me show you the text that I have in a file named '''sample dot txt''' file.
 |-
 |07:56
-|Now, run the program from the '''command line '''by typing: '''perl commandline.pl space sample.txt '''and press''' Enter.'''
+|Now, run the program from the '''command line '''by typing: '''perl commandline dot pl space sample dot txt '''and press''' Enter.'''
 |-
 |08:07
-|This is the text we had in '''sample.txt''' file.
+|This is the text we had in '''sample dot txt''' file.
 |-
@@ Line 358: / Line 358: @@
 |-
 |08:41
-|'''underscore underscore E N D ('''in capital letters)'''underscore underscore.'''  indicates the logical end of the program.
+|'''underscore underscore E N D ''' (all in capital )'''underscore underscore.'''  indicates the logical end of the program.
 |-
@@ Line 379: / Line 379: @@
 |09:25
 |We will see a sample program on how '''Global Special Constants '''are used.
 |-
 |09:30
-|Open the '''terminal''' and type '''gedit specialconstant.pl ampersand ''' and press '''Enter.'''
+|Open the '''terminal''' and type '''gedit specialconstant dot pl ampersand ''' and press '''Enter.'''
 |-
 |09:39
-|'''specialconstant.pl''' file is now open in '''gedit'''.
+|'''specialconstant dot pl''' file is now open in '''gedit'''.
 |-
@@ Line 393: / Line 394: @@
 |-
 |09:50
-|The '''special literals  PACKAGE, FILE, LINE ''' represent the  package name, current filename and
+|The '''special literals  PACKAGE, FILE, LINE ''' represent the  package name, current filename and line number respectively at that point in the program.
 line number respectively at that point in the program.
 |-
@@ Line 426: / Line 426: @@
 |-
 |10:39
-| Print the sorted result using '''‘while loop’''' and '''special variable $_.'''
+| Print the sorted result using '''‘while loop’''' and '''special variable $_ (Dollar Underscore)'''
 |-
@@ Line 454: / Line 454: @@
 |11:13
 |More information on this mission is available at this link.
 |-
 |11:17
 |This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.
 |}