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Introduction to Perl Learning Objectives: 1. To introduce the features provided by Perl 2. To learn the basic Syntax & simple Input/Output control in Perl 3. To learn how to combine Unix Command in Perl using backquotes
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COMP111 Lecture 9 / Slide 2 Introduction to Perl Table of Content Introduction to Perl Basic Syntax Perl Example 1 Scalar Variables Assigning Scalar Variables Numerical Scalar Variable User Input Printing Special Symbol - $ Numerical Example Backquotes: Command Substitution Backquote Example 2
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COMP111 Lecture 9 / Slide 3 Introduction to Perl Perl is a scripting language that makes manipulation of text, files, and processes easy. Perl is a cross between shell programming and the C programming language. C (numbers) Shell programming (text) Smalltalk (objects) C++ (numbers, objects) Perl (text, numbers) Java (objects)
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COMP111 Lecture 9 / Slide 4 Introduction to Perl Perl provides a more concise and readable way to do many tasks compared to C++ and shell scripts. Perl has replaced shell programming as the most popular programming language for text processing and Unix system administration. Perl was originally designed under Unix, but now also runs under all operating systems (including Windows). Many libraries available (e.g. database, internet) Perl is also a popular language for CGI (for web server) and GUI programming.
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COMP111 Lecture 9 / Slide 5 Introduction to Perl The perl command on our system invokes Perl 4.0, but we want to use the latest version Perl 5.0. You can run a perl 5.0 script explicitly with perl5: $ cat simple $ perl5 simple $ You can run the script directly if you make the script executable, and the first line is of the following form ( #!/usr/... must start from the first column): $ chmod +x simple $ cat simple #!/usr/local/bin/perl5 -w $ simple $
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COMP111 Lecture 9 / Slide 6 Basic Syntax (1) The -w option tells Perl to produce extra warning messages about potential dangers. Use -w in all your Perl programs for now. #!/usr/local/bin/perl5 -w Whitespace doesn't matter in Perl (like C++), except for #!/usr/local/bin/perl5 -w which must start from column 1 on line 1.
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COMP111 Lecture 9 / Slide 7 Basic Syntax (2) All perl statements end in a semicolon ; (like C++) In Perl, comments begin with # (like shell scripts) everything after the # to the end of the line is ignored. # need not be at the beginning of the line. there are no C++-like multiline comments: /* */
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COMP111 Lecture 9 / Slide 8 Perl Example 1 (1) Here is a “ hello world ” Perl program: $ ls -l -rwxr-xr-x 1 horner cs 52 Mar 2 15:50 hello* $ cat hello #!/usr/local/bin/perl5 -w # comment lines start with the # character print "Hello world\n"; $ hello Hello world $ The print command sends the string to the screen, and “ \n “ adds a newline.
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COMP111 Lecture 9 / Slide 9 Perl Example 1 (2) You can optionally use parenthesis around the argument in print : print ("Hello world\n"); or, if your prefer the C++ function style: print("Hello world\n");
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COMP111 Lecture 9 / Slide 10 Scalar Variables A scalar variable can hold a single number or string (like shell variables), including integers and floating- point numbers (unlike shell variables). Scalar variables begin with “ $ ” followed by a letter, and then possibly more letters, digits, or underscores. (e.g., $n, $n1, $name, $first_name ). Scalar variables are case sensitive.
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COMP111 Lecture 9 / Slide 11 Assigning Scalar Variables Scalars are assigned using “ = “ $scalar = expression; To assign a value to a scalar variable: $number = 25; $name = "Bill Clinton"; Unlike shell scripts, use the “ $ ” both when the variable is used and assigned: $ cat test1 #!/usr/local/bin/perl5 -w $number = 25; $name = "Bill Clinton"; print "$number $name\n"; $ test1 25 Bill Clinton $
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COMP111 Lecture 9 / Slide 12 Numerical Scalar Variables Internally, all numerical scalar values are stored as floats (so you don ’ t have to worry about integer division in Perl like you do in C++). Perl supports the usual C++ numerical operations: $a = 25;# $a is now 25 $a += 5; # $a is now 30 $a *= 3; # $a is now 90 $a++; # $a is now 91 --$a; # $a is now 90 $result = ($a + 2) * 3.4; # $result is 312.8
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COMP111 Lecture 9 / Slide 13 User Input (1) Use to get input from the user: $ cat test2 #!/usr/local/bin/perl5 -w print "Enter name: "; $name = ; chomp ($name); print "How many girlfriends do you have? "; $number = ; chomp($number); print "$name has $number girlfriends!\n"; $ test2 Enter name: Bill Clinton How many girlfriends do you have? more than you Bill Clinton has more than you girlfriends!
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COMP111 Lecture 9 / Slide 14 User Input (2) grabs one line of input, including the newline character. So, after: $name = ; if the user typed “ Bill Clinton[ENTER] ”, $name will contain: “ Bill Clinton\n ”. To delete the newline, the chomp() function takes a scalar variable, and removes the trailing newline if present. (If there is no newline at the end, it does nothing.) A shortcut to do both operations in one line is: chomp($name = );
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COMP111 Lecture 9 / Slide 15 Printing Special Symbol - $ As with the shell scripts, use a backslash before $ if you really want to print the dollar sign: $ cat test4 #!/usr/local/bin/perl5 -w print "Enter amount: "; $cost = ; print "The total is: \$$cost"; $ test4 Enter amount: 18.50 The total is $18.50 No need to use chomp() if the newline on $cost can be used when it is printed.
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COMP111 Lecture 9 / Slide 16 Numerical Example $ cat test6 #!/usr/local/bin/perl5 -w print "Enter height of rectangle: "; $height = ; print "Enter width of rectangle: "; $width = ; $area = $height * $width; print "The area of the rectangle is $area\n"; $ test6 Enter height of rectangle: 10 Enter width of rectangle: 5 The area of the rectangle is 50 $ test6 Enter height of rectangle: 10.1 Enter width of rectangle: 5.1 The area of the rectangle is 51.51
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COMP111 Lecture 9 / Slide 17 Backquotes: Command Substitution You can use command substitution in Perl like in shell scripts: $ whoami clinton $ cat test7 #!/usr/local/bin/perl5 -w $user = `whoami`; chomp($user); $num = `who | wc -l`; chomp($num); print "Hi $user! There are $num users logged on.\n"; $ test7 Hi clinton! There are 6 users logged on. Command substitution will usually include a newline, so use chomp().
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COMP111 Lecture 9 / Slide 18 Backquote Example 2 $ cat big1 #!/usr/local/bin/perl5 -w $dir = `pwd`; chomp($dir); $big = `ls -l | sort +4 | tail -1 | cut -c55-70`; chomp($big); $nline = `wc -l $big | cut -c6-8`; chomp($nline); $nword = `wc -w $big | cut -c6-8 `; chomp($nword); $nchar = `wc -c $big | cut -c6-8 `; chomp($nchar); print "The biggest file in $dir is $big.\n"; print "$big has $nline lines, $nword words, $nchar characters.\n " ; $ big1 The biggest file in /homes/horner/111/perl is big1. big1 has 14 lines, 66 words, 381 characters.
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