Introduction to Perl

1. Introduction to Perl

2. What is Perl • Perl is an interpreted language. This means you run it through an interpreter, not a compiler. • Similar to shell script but lot easier and more powerful. • Perl is free to download and is also available for Windows and Macintosh. • File name extension .pl • The first line of a perl program should tell where to find the perl intepreter #!/usr/bin/perl

3. Steps to Run a Perl Program • Use your favorite editor to create a Perl program, say test.pl. • Change the file permission and make it executable. chmod 700 test.pl • Run Perl program test.pl

4. Comments • The pound sign "#" is the symbol for comment entry. • Exception: First line, #!/usr/bin/perl, tells where to find the Perl compiler on your system

5. Variables • Three types of variables. • Scalar • Array • Hash

6. Scalars • Scalar means single value • In C/C++, many different kinds of single values: • int, float, double, char, bool • In Perl, Scalar variable can hold all these types, and more, such as string. • In Perl, we do not need to declare the data type of variables

7. Scalars • All Scalar variables begin with a $ • Examples: $foo, $a, $zebra1, $F87

8. Scalar Assignments • Scalars hold any data type: • $foo = 3; • $d = 4.43; • $temp = ‘Z’; #could be double quote • $My_String = “Hello, I’m Paul.” # could be single quote • $value = TRUE;

9. Arithmetic in Perl $a = 1 + 2; # Addition $b = 3 - 4; # Subtraction $c = $a * $b; # Multiplication $d = $a / $b; # Division $a = 9 ** 10; # Exponentiation $b = 5 % 2; # Modulo

10. Single and double quotes • $a = 'apples'; # you can also use double quote • $b = 'bananas'; # you can also use double quote print '$a and $b'; • display: $a and $b print "$a and $b"; • display: apples and bananas • Single quotation marks do not interpret, and double quotation marks do

11. Arrays • Concept is the same as in C/C++ • Groups of other values • much more dynamic than C/C++ • no declaration of size, type • can hold any kinds of value, and multiple kinds of values • All array variables start with the @ character • @array, @foo, @My_Array, @temp34 • Array index stars from 0

12. Array Operations • @a1 = (3, 2, 1, 4); • @a2 = (34, ‘z’, “Hi”, 43.2); • Assignment • $a2[2] = ‘X’; #@a2: (34, ‘z’, ‘X’, 43.2) • Copy array • @a3 = @a1; #@a3: (3, 2, 1, 4); • Merge array: • @a5 = (@a1, @a2); #@a5: (3, 2, 1, 4, 34, ‘z’, “Hi”, 43.2) • How about this operation: • @a1 = (@a1, @a2); #@a1: (3, 2, 1, 4, 34, ‘z’, “Hi”, 43.2)

13. Sort Array • @a1 = (3, 2, 1, 4); • Sort array • @a4 = sort (@a1) #@a4: (1, 2, 3, 4) • How about this one? @a1 = (3, 22, 11, 4); @a4 = sort (@a1) #@a4: (11, 22, 3, 4) This is same as @a4 = sort {$a cmp $b }(@a1); Array is sorted alphabetically (elements are considered as string)

14. Sort Array (continued) • Sorted alphabetically @a1 = (3, 22, 11, 4); @a4 = sort (@a1) ; #or @a4 = sort {$a cmp $b }(@a1); #@a4: (11, 22, 3, 4) • Sort numerically @a1 = (3, 22, 111, 4); @a4 = sort {$a <=> $b }(@a1); #@a4: (3, 4, 11, 22) • Note: • Inside sort block, two variables must be $a and $b • If $a and $b is exchanged, the sorting order is changed

15. More about arrays • special variable for each array: • @foo = (3, 25, 43, 31); • $#foo (a variable: last index of @foo, which is 3). • $#foo+1 (size of array @foo, which is 4).

16. Program Flow: if statement if ($x == $y) { #... } elsif ($x == ($y+1)) { #... } else { #... }

17. Program Flow: Comparing variables Numbers == != < > Strings eq ne lt gt

18. Program Flow: Logical operators • && (logical and) • || (or) • ! (negation)

19. Program Flow (Loops) • for ($t = 0; $t < 100; $t++) • { • #... • } • while ($x == $y) • { • #... • }

20. foreach Statement • This statement takes an array variable and assigns one item at a time to a scalar variable, executing a block of code. • For example, @list an array foreach $var (@list) { # }

21. Basic IO • Output to terminal • The print statement. • Example: print “My name is $name\n”; • Input from keyboard • The <> operator • Example: $input = <>; • read one line from keyboard, and save in variable $input

22. Task 1 • Write a Perl program to ask the user to enter a name, then it will display Hello name_user_rntered

23. Perl Program for Task 1 #!/usr/bin/perl print "Enter your name:"; $name = <>; print "Hello $name\n";

24. Chomp • When reading in, carriage return (“\n”) is included. • Usually don’t want that. • chomp will take off the last character of a string, if it is a “\n”. • chomp ($foo);

25. Perl Program for Task 1 (revised) #!/usr/bin/perl print "Enter your name:"; $name = <>; chomp($name); print "Hello $name\n";

26. Read / Write to Files • To read and write to files we should create something called handles which refer to the files.

27. Read from Files • To create a file handle for reading • Use the OPEN command • Example open(filehandle1,"filename1");

28. Read from Files (continued) • Once the file handles have been obtained, we can read data (line by line) from the file. • Example: @lines = <filehandle1>; • This will result in each line being read from the file pointed by the file handle and all lines are stored in the array variable @lines, where index 0 ($lines[0]) contains first line of the file, index 1 ($lines[1]) contains second line of the file, and so on.

29. Read from Files (continued) • After read file, we should close the file. close(filehandle1);

30. Task 2 • Write a Perl program that can read a file (test.cpp) and display each line with a line number

31. Perl program for Task 2 #!/usr/bin/perl open(fh1, "test.cpp"); @input_lines=<fh1>; chomp(@input_lines); close(fh1); $i=1; foreach $line(@input_lines) { print "$i: $line\n"; $i=$i+1; }

32. Write to Files • To create a file handle for writing • Use the OPEN command • Example open(filehandle2, ">filename2");

33. Write to Files (continued) • Once the file handles have been obtained, we can write data to the file. • Example: print filehandle2 "$linevalue"; • This will result in the value of $linevalue being written to the file pointed by the filehandle2.

34. Write to Files (continued) • After write to file, we should close the file. close(filehandle2);

35. Task 3 • Rewrite the Perl program for Task 2 so that the result will be write to a file (test_c.cpp) instead of displaying on the screen.

36. Perl program for Task 3 #!/usr/bin/perl open(fh1, "test.cpp"); @input_lines=<fh1>; chomp(@input_lines); close(fh1); $i=1; open(fh2, ">test_c.cpp"); foreach $line(@input_lines) { print fh2 "$i: $line\n"; $i=$i+1; } close(fh2);

37. Subroutines (functions) • To define your own subroutine, use the keyword sub • Can be defined anywhere in your program sub function_name { #commands }

38. Function Calls • $Name = getname(); #return a value • getname(); #not returning a value

39. Parameters of Functions • Parameters are passed in a function as an array. • The parameter is taken in as an array which is denoted by @_ inside the function. • So if you pass only one parameter the size of array @_ will be one. If you pass two parameters then the @_ size will be two and the two parameters can be accessed by $_[0],$_[1] , and so on.

40. Subroutines #!/usr/bin/perl $result = max(11, 12); Print “The largest number is: $result \n”; sub max { if($_[0] > $_[1]) { return $_[0]; } else { return $_[1]; } } Output: The largest number is: 12

41. More About Functions • The variables declared in the main program are by default global so they will continue to have their values in the function also. • Local variables are declared by putting my key word while declaring the variable.

42. Subroutines: local variable example #!/usr/bin/perl sub max { my @num = @_ if($num[0] > $num[1]){ return $num[0]; } else{ return $num[1]; } } $result = max(11, 12); Print “The largest number is: $result \n”;

43. A routine (user defined) to read web pages sub getweb { my $url = $_[0]; require LWP::UserAgent; my $ua = LWP::UserAgent->new; $ua->timeout(10); $ua->env_proxy; my $response = $ua->get($url); return $response->content; } This routine takes one parameter (a web address) and returns the contents of a web page as one string

44. Task 4 • Display the html code of www.google.com

45. Perl program for task 4 #!/usr/bin/perl $google = getweb("http://www.google.com"); print $google; #the entire page is saved as one string sub getweb { my $url = $_[0]; require LWP::UserAgent; my $ua = LWP::UserAgent->new; $ua->timeout(10); $ua->env_proxy; my $response = $ua->get($url); return $response->content; }

46. Reading Assignment • Textbook: Chapter 11