1. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 1 Lecture 8: Perl Basics Fundamentals of Web Programming Lecture 8: Perl Basics

2. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 2 Lecture 8: Perl Basics Introduction to Perl Basic data and variable types. Scalars, arrays, hashes. Basic control structures: if-then, while, foreach. Example: generating HTML tables. Adding your own pages/scripts to Apache.

3. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 3 Lecture 8: Perl Basics Perl characteristics Scripting language. Interpreted. Highly portable. Easy to learn. Suitable for prototyping. Good support of CGI. Also a bit eclectic...

4. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 4 Lecture 8: Perl Basics Tedious task: HTML tables Imagine you’ve been hired by a company selling PCs on the Web. You’re responsible for creating a page that describes the products. You start with three models, but then the number grows… Some models are special.

5. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 5 Lecture 8: Perl Basics Required table

6. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 6 Lecture 8: Perl Basics What do we need to do? Store information about the PCs. Write a script that would generate the table in a way that allows for quick changes. Install the script on the server. Test it.

7. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 7 Lecture 8: Perl Basics Files, variables, data types Usually, information is stored in files. For now we will use variables. Variable: a piece of memory with a name. Variables have types: they can hold data of a particular type.

8. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 8 Lecture 8: Perl Basics Basic Data Types in Perl Scalars: include numbers and strings. Arrays or ordered lists. Hashes. A literal is the way a value is represented in the text of a program, e.g., “string”.

9. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 9 Lecture 8: Perl Basics Scalars Scalars: –Integer literals, e.g. 123, 11111 ; –Float literals (‘real numbers’), e.g. 3.14, 1.2e10; –Single-quoted strings: ‘hello’, ‘don\’t’; –Double-quoted strings: “hello”, “this string contains \””.

10. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 10 Lecture 8: Perl Basics Scalar variables Scalar variables can hold only scalars. The name of a scalar variable begins with $. $pi = 3.14; $s1 = “hello”; = denotes assignment.

11. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 11 Lecture 8: Perl Basics Arrays Arrays and lists: –A list is an ordered collection of data. –It has elements that are identified by indices. –Each element is a scalar. –Since lists are ordered, there is a first and a last element. –Can be empty (with no elements).

12. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 12 Lecture 8: Perl Basics Array literals and variables The name of an array variable begins with @, e.g., @my_array. In a program, lists are enclosed in (), e.g., @numbers = (1,2,3); @strings = (“CMU”, “UCLA”, “MIT”); @empty_list = ();

13. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 13 Lecture 8: Perl Basics Array Element Access You use an index to access an element in an array. Since arrays contain scalars, the resulting element will be a scalar. So, if you assign @numbers = (1,2,3); $numbers[0] contains 1.

14. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 14 Lecture 8: Perl Basics Array Element Access You can use a scalar variable to access an array element: $I = 1; $numbers[$I] equals 2. For every array @a Perl provides a special scalar variable $#a that contains the index of the last element: e.g. $#numbers is 2; $#empty_list is -1.

15. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 15 Lecture 8: Perl Basics Printing a table row Let’s define a few variables: $pcName = “PC1”; $memory = 128; $disk = 6; $modem = 0; print “ \n”; print “ $pcName $memory $disk $modem \n”; print “ \n”; PC1 128 6 0

16. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 16 Lecture 8: Perl Basics Variable interpolation When a variable name appears in a double-quoted string, the value of the variable is substituted for the name: print “ $pcName ” prints PC1

17. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 17 Lecture 8: Perl Basics Same with arrays @names = (“PC1”, “PC2”, “PC3”); @memory = (128, 128, 256); @disks = (6, 6, 10); @modems = (0, 1, 1); $I = 1; print “ \n”; print “ $names[$I] $memory[$I] $disks[$I] $modems[$I] \n”; print “ \n”;

18. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 18 Lecture 8: Perl Basics Control Statements Control statements such as if, unless, foreach, while, etc, change the control flow in the program. They usually evaluate an expression and their behavior depends on the value of the expression.

19. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 19 Lecture 8: Perl Basics Conditional if (expression) { statement1; statement2; } else { statement3; statement4; } if ($modem[$I] == 0) { print “No”; } else {print “Yes”;}

20. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 20 Lecture 8: Perl Basics Iteration: for statement for ($I = 1; $I <= 10; $I++) { print “$I “; } evaluate $I = 1; repeat print “$I “; $I++ as long as $I <= 10 is true.

21. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 21 Lecture 8: Perl Basics Iteration: while statement The above is equivalent to: $I = 1; while ($I <= 10) { print “$I “; } The body of the loop is executed as long as the conditon holds.

22. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 22 Lecture 8: Perl Basics Iteration: foreach statement foreach takes a list of values. Assigns them one at a time to a scalar variable and executes the body. @numbers = (1, 2, 3); foreach $number (@numbers) { print $number, “ “; }

23. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 23 Lecture 8: Perl Basics Complete example @names = (“PC1”, “PC2”, “PC3”); @memory = (128, 128, 256); @disks = (6, 6, 10); @modems = (0, 1, 1); for ($I = 0; $I \n”; print “ $names[$I] $memory[$I] $disks[$I] $modems[$I] \n”; print “ \n”; }

24. 20-753: Fundamentals of Web Programming Copyright © 1999, Carnegie Mellon. All Rights Reserved. 24 Lecture 8: Perl Basics Common mistakes The while loop requires correct initialization and control expression: watch out for array bounds. ($I = 0; $I <= $#array) When you traverse an array in a loop, make sure that the index doesn’t get out of bounds; results in an error message.