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

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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.
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11:17 This is Nirmala Venkat from IIT Bombay, signing off. Thanks for watching.

Contributors and Content Editors

PoojaMoolya, Sandhya.np14