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

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 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.
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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