PERL/C2/while-do-while-loops/English

Title Of Script: while and do-while loops in Perl

Author: Amol Brahmankar

Keywords: Loops in Perl, while loop in perl, do-while loop in perl, video tutorial.

Visual Clue	Narration
Slide	Welcome to the spoken tutorial on while and do-while loops in Perl
Slide: Learning Objectives	In this tutorial, we will learn about; while loop in Perl do-while loop in Perl
Slide: System Requirements	I am using Ubuntu Linux12.04 operating system and Perl 5.14.2 I will also be using the gedit Text Editor. You can use any text editor of your choice.
Slide: Prerequisites	You should have basic knowledge of Variables and Comments in Perl Knowledge of for and foreach loops in Perl will be an added advantage. Please go through the relevant spoken tutorials on the spoken tutorial website.
Slide	while loop in Perl The while loop executes a block of code while a condition is true.
Slide while (condition) { Piece of code to be executed while the condition is true }	The syntax of while loop is as follows - while space open bracket condition close bracket Open curly bracket Piece of code to be executed while the condition is true Close curly bracket
Highlight while (condition)	So, what happens if the condition is not satisfied? Then, the while loop will exit, without executing the code within, even once.
	Now let us look at an example of while loop.
Switch to the Terminal and type gedit whileLoop.pl & and press Enter.	Open the Terminal and type gedit whileLoop dot pl space ampersand and press Enter
Point to the filename whileLoop.pl in the Titlebar of gedit. Type the piece of code. #!/usr/bin/perl $i = 0; while ($i<=4) { print “Value of i: $i\n”; $i++; }	This will open the whileLoop.pl file in gedit. Now Type the following code hash exclamation mark slash u s r slash bin slash perl Press Enter. dollar i is equal to zero semicolon Press Enter while open bracket dollar i less than or equal to four close bracket space Open curly bracket press enter and type print space double quote Value of i colon space dollar i slash n doule quote complete semicolon Press Enter dollar i plus plus semicolon press enter and Close the curly bracket
Highlight $i=0;	Let me explain the while loop in detail. We have initialized the variable i to 0.
	Now we have specified the condition for while loop as $i less than or equal to 4. If the condition is true, the code within the while loop will get executed.
Highlight print	This means, first time our while loop will print 'Value of i: 0' on the terminal.
Highlight $i++	Then the $i++ increments the value of variable i by one.
	Now again, the loop condition $i<=4 will be evaluated. And the loop will exit once the value of i becomes 5.
	In this case, while loop will get executed for i equal to 0, 1, 2, 3, 4.
Press Ctrl S	Press ctrl+s to save the file.
Switch to terminal	Now, switch to the terminal.
perl -c whileLoop.pl	Type the following to check for any compilation or syntax error - perl hyphen c whileLoop dot pl and press Enter.
Highlight the below line on terminal- whileLoop.pl syntax OK	The following line will be displayed on the terminal whileLoop.pl syntax OK
Execute Perl script perl whileLoop.pl	As there are no compilation or syntax error, we will execute the Perl script by typing - perl whileLoop dot pl and press Enter
Highlight the output of the perl script on terminal Value of i: 0 Value of i: 1 Value of i: 2 Value of i: 3 Value of i: 4	The following output will be displayed on the terminal.
	Now, let us look at do-while loop
Slide	The do...while statement will always execute the piece of code at-least once. It will then check the condition and repeat the loop while the condition is true.
Slide do { Piece of code to be executed while the condition is true } while (condition);	The syntax for do-while loop is as follows - do space open curly bracket Piece of code to be executed while the condition is true close curly bracket then space while space within brackets condition and then semicolon
Switch to the Terminal and type gedit doWhileLoop.pl & and press Enter.	Open the Terminal and type; gedit doWhileLoop dot pl space ampersand and then press Enter
Point to the filename doWhileLoop.pl in the Titlebar of gedit.	This will open the doWhileLoop.pl file in gedit.
Type the piece of code and save the file #!/usr/bin/perl $i = 0; do { print “Value of i: $i\n”; $i++; } while ($i<=4);	Type the following piece of code - hash exclamation mark slash u s r slash bin slash perl Press Enter dollar i equals to zero semicolon press enter do space open curly bracket enter type print space double quote Value of i colon space dollar i slash n double quote complete semicolon Press Enter dollar i plus plus semicolon press enter close curly bracket space while space open bracket dollar i less than or equal to four close bracket semicolon
Highlight $i=0;	Here is the detail explanation of a do-while loop. We have initialized the variable i to 0.
Highlight print	First time, the do-while loop will print the output as 'Value of i colon 0' without checking for condition
Highlight $i++;	$i++ will increment the value of variable i by one each time loop gets executed.
	Second time, the condition $i less than or equal to 4 will be checked. If the condition is true, the loop will get executed again. In our case, second time the output displayed on terminal will be 'Value of I colon 1'.
	The loop will get executed till the condition becomes false that is when variable i becomes 5.
Press Ctrl+S	Press ctrl+s to save the file.
Switch to terminal perl -c doWhileLoop.pl	Now, switch to terminal and type the following to check for any compilation or syntax error. perl hyphen c doWhileLoop dot pl and press Enter.
Highlight the below line on terminal - doWhileLoop.pl syntax OK	The following line will be displayed on the terminal doWhileLoop.pl syntax OK
Execute Perl script perl doWhileLoop.pl	As there are no compilation or syntax errors, we will now execute the Perl script. Type perl doWhileLoop dot pl and press Enter
Highlight the output of the perl script on terminal Value of i: 0 Value of i: 1 Value of i: 2 Value of i: 3 Value of i: 4	The following output will be displayed on the terminal.
	Now, let us see the actual difference between while and do-while loops.
Switch to the Terminal and type gedit loop.pl & and press Enter.	Open the Terminal and type - gedit loop dot pl space ampersand and press Enter
#!/usr/bin/perl $count = 0; # while loop while ($count > 0) { print “I am in while loop\n”; } # do-while loop do { print “I am in do-while loop\n”; } while ($count > 0);	This will open loop dot pl file in gedit. Now type the piece of code shown.
Highlight $count = 0;	We have declared a variable count and initialized it to zero
Highlight while ($count > 0)	In the while loop condition, we are checking if the variable count is greater than zero. The condition is not true. So, the while loop code will not be executed even once.
Highlight do {	In the do...while loop, we are first executing the code and then checking the condition.
Highlight print “I am in do-while loop\n”;	So, the code will be executed at least once.
Highlight } while ($count > 0);	Then the condition whether the variable count is greater than zero, is checked.
	The condition is not true. So the loop will exit.
Press Ctrl S	Now, press ctrl+s to save the file.
Switch to terminal type perl -c loop.pl	Now, switch to terminal and type the following to check for any compilation or syntax errors perl hyphen c loop dot pl and press Enter.
Highlight the below line on terminal; loop.pl syntax OK	The following line will be displayed on the terminal loop dot pl syntax OK
Execute perl script perl loop.pl	As there are no compilation or syntax errors, let us execute the Perl script. By Typing perl loop dot pl and press Enter.
Highlight the output of the perl script on terminal I am in do-while loop	The following output will be displayed on the terminal. I am in do-while loop
	Here we can see, there is no output message displaying 'I am in while loop' This message was what we printed inside the while loop.
Slide	This implies that, do-while loop executes at-least once before evaluating the condition whereas while loop does not get executed even once when the condition specified is false.
	I hope the difference is clear to you now. That's all there is to while and do-while loops.
Summary	Let us summarize. In this tutorial, we have learnt - while loop and do-while loop in Perl using sample programs.
Assignment	Here is assignment for you - Print 'Hello Perl' till the count of variable reaches 10 by using while loop and do-while loops
About the Project	Watch the video available at the following link It summaries the Spoken Tutorial project If you do not have good bandwidth, you can download and watch it
Spoken Tutorial Workshops	The Spoken Tutorial Project Team Conducts workshops using spoken tutorials Gives certificates to those who pass an online test For more details, please write to contact at spoken hyphen tutorial dot org
Acknowledgment http://spoken-tutorial.org\NMEICT-Intro	Spoken Tutorial Project is a part of the Talk to a Teacher project It is supported by the National Mission on Education through ICT, MHRD, Government of India. More information on this Mission is available spoken hypen tutorial dot org slash NMEICT hyphen Intro
	Hope you enjoyed this Perl tutorial. This is Amol Brahmankar signing off. Thanks for joining

Contributors and Content Editors

AmolBrahmankar, PoojaMoolya, Sneha