Difference between revisions of "PERL/C3/Special-Variables-in-PERL/English-timed"

From Script | Spoken-Tutorial
Jump to: navigation, search
 
(2 intermediate revisions by one other user not shown)
Line 11: Line 11:
 
|00:04
 
|00:04
 
|In this tutorial, we will learn about:
 
|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
 
|00:13
 
|For this tutorial, I am using:
 
|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.
 
You can use any '''text editor''' of your choice.
  
Line 212: Line 212:
 
|-
 
|-
 
|05:05
 
|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
 
|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.
 
|'''Dollar a''' and '''dollar b ''' are '''special package local variables''' in which the values to be compared are loaded.
  
Line 279: Line 279:
 
|-
 
|-
 
|06:48
 
|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
 
|06:59
Line 327: Line 326:
 
|-
 
|-
 
|07:56
 
|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 339: Line 338:
 
|-
 
|-
 
|08:17
 
|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 364: Line 362:
 
|-
 
|-
 
|08:55
 
|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 390:
 
|-
 
|-
 
|09:50
 
|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 414:
 
|-
 
|-
 
|10:25
 
|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.
  
 
|-
 
|-

Latest revision as of 18:15, 20 February 2017

Time
Narration
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 initialized 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 $_ - Dollar Underscore is the default parameter for lot of functions and pattern-searching strings.
01:14 Let us understand the usage of '$_' (Dollar Underscore) 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 dot pl ampersand and press Enter.
01:32 special dot 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. $_ (Dollar Underscore) saves the use of one extra variable $color.
02:03 Press Ctrl+S to save the file.
02:06 Then switch to the terminal and execute the Perl script by typing perl special dot pl and press Enter.
02:13 Here, both foreach loops give the same output.
02:18 Now, let us see another example to demonstrate how '$_' (Dollar Underscore) 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 dot 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 dot txt file.
07:56 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 dot 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 (all in capital )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 dot pl ampersand and press Enter.
09:39 specialconstant dot 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 summarize.
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 $_ (Dollar Underscore).
10:45 Save and execute the program.
10:47 Now check the result.
10:49 The video at the following link summarizes 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.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14