Short Perl tutorial Instructor: Rada Mihalcea

1. Short Perl tutorial Instructor: Rada Mihalcea Note: some of the material in this slide set was adapted from a Perl course taught at University of Antwerp

2. 1987 Larry Wall Develops PERL 1989 October 18 Perl 3.0 is released under the GNU Protection License 1991 March 21 Perl 4.0 is released under the GPL and the new Perl Artistic License Now Perl 5.14 About Perl PERL is not officially a Programming Language per se. Wall’s original intent was to develop a scripting language more powerful than Unix Shell Scripting, but not as tedious as C. PERL is an interpreted language. That means that there is no explicitly separate compilation step. Rather, the processor reads the whole file, converts it to an internal form and executes it immediately. P.E.R.L. = Practical Extraction and Report Language

3. Variables • A variable is a name of a place where some information is stored. For example: • $yearOfBirth = 1976; • $currentYear = 2011; • $age = $currentYear-$yearOfBirth; • print $age; • The variables in the example program can be identified as such because their names start with a dollar ($). Perl uses different prefix characters for structure names in programs. Here is an overview: • $: variable containing scalar values such as a number or a string • @: variable containing a list with numeric keys • %: variable containing a list with strings as keys • &: subroutine • *: matches all structures with the associated name

4. Operations on numbers • Perl contains the following arithmetic operators: • +: sum • -: subtraction • *: product • /: division • %: modulo division • **: exponent • Apart from these operators, Perl contains some built-in arithmetic functions. Some of these are mentioned in the following list: • abs($x): absolute value • int($x): integer part • rand(): random number between 0 and 1 • sqrt($x): square root

5. Input and output • # age calculator • print "Please enter your birth year "; • $yearOfBirth = <>; • chomp($yearOfBirth); • print "Your age is ",2012-$yearOfBirth,".\n"; • # count the number of lines in a file • open INPUTFILE, “<$myfile”; • (-r INPUTFILE) || die “Could not open the file $myfile\n”; • $count = 0; • while($line = <INPUTFILE>) { • $count++; • } • print “$count lines in file $myfile\n”; • # open for writing • open OUTPUTFILE, “>$myfile”;

6. Conditional structures • # determine whether number is odd or even • print "Enter number: "; • $number = <>; • chomp($number); • if ($number-2*int($number/2) == 0) { • print "$number is even\n"; • } • elsif (abs($number-2*int($number/2)) == 1) { • print "$number is odd\n"; • } • else { • print "Something strange has happened!\n"; • }

7. Numeric test operators • An overview of the numeric test operators: • ==: equal • !=: not equal • <: less than • <=: less than or equal to • >: greater than • >=: greater than or equal to • All these operators can be used for comparing two numeric values in an if condition. • Truth expressions • three logical operators: • and: and (alternative: &&) • or: or (alternative: ||) • not: not (alternative: !)

8. Iterative structures • #print numbers 1-10 in three different ways • $i = 1; • while ($i<=10) { • print "$i\n"; • $i++; • } • for ($i=1;$i<=10;$i++) { • print "$i\n"; • } • foreach $i (1,2,3,4,5,6,7,8,9,10) { • print "$i\n"; • } • Stop a loop, or force continuation: • last; # C break • next; # C continue; • Exercise: • Read ten numbers and print the largest, the smallest and a count representing how many of them are dividable by three. • if (not(defined($largest)) or $number > $largest) { $largest = $number; } • if ($number-3*int($number/3) == 0) { $count3++; }

9. A paranthesisPERL philosophy(ies) • There is more than one way to do it • If you want to shoot yourself in the foot, • who am I to stop you? • And a comment: DO write comments in your Perl programs!

10. Basic string operations • - strings are stored in the same type of variables we used for storing numbers • string values can be specified between double and single quotes • !!! in the first one variables will be evaluated, in the second one they will not. • Comparison operators for strings • eq: equal • ne: not equal • lt: less than • le: less than or equal to • gt: greater than • ge: greater than or equal to • Examples: • if ($a eq $b) { • …. • }

11. String substitution and string matching • The power of Perl! • The s/// operator modifies sequences of characters (substitute) • The tr/// operator changes individual characters. (translate) • The m// operator checks for matching (or in short //) (match) • the first part between the first two slashes contains a search pattern • the second part between the final two slashes contains the replacement. • behind the final slash we can put characters to modify the behavior of the commands. • By default s/// only replaces the first occurrence of the search pattern • append a g to the operator to replace every occurrence. • append an i to the operator, to have the search case insensitive • tr translates the characters in the first set of characters into the characters of the second set • if the second set is shorter, the last character is multiplied • if the second set is longer, the exceeding characters are truncated • The tr/// operator allows the modification characters • c (replace the complement of the search class) • d (delete characters of the search class that are not replaced) • s (squeeze sequences of identical replaced characters to one character)

12. Examples • # replace first occurrence of "bug" • $text =~ s/bug/feature/; • # replace all occurrences of "bug" • $text =~ s/bug/feature/g; • # convert to lower case • $text =~ tr/[A-Z]/[a-z]/; • # delete vowels • $text =~ tr/AEIOUaeiou//d; • # replace nonnumber sequences with x • $text =~ tr/[0-9]/x/cs; • # replace all capital characters by CAPS • $text =~ s/[A-Z]/CAPS/g; • Simple example: • Print all lines from a file that include a given sequence of characters • [emulate grep behavior]