Practical Extraction & Report Language

Practical Extraction & Report Language Picture taken from http://www.wendy.org/DPW2006/shirt.htm

Agenda • Why Perl? • Getting/Installing Perl • Using Perl • Structure of basic program (Hello world) • Variables & Operators • Regular Expressions • Other Topics

Why Perl • Perl is built around regular expressions • REs are good for string processing • Therefore Perl is a good scripting language • Perl is especially popular for CGI scripts • Perl makes full use of the power of UNIX • Short Perl programs can be very short • “Perl is designed to make the easy jobs easy, without making the difficult jobs impossible.” -- Larry Wall, Programming Perl

Getting/Installing Perl • Windows • www.activestate.com • Download “ActivePerl” • Run installer • Linux • Mostly installed when Linux is installed • userX@machineY$ which perl • Get it from • Linux distribution CDs • Update your installation and during package selection, select perl • ActiveState.com • CPAN

Other Possibilities • Using Virtual Machines • VMWare • Install VMWare workstation on windows • Install Linux under VMware workstation (select perl to be installed) • Cygwin • Install Cygwin on windows • It will provide a Linux interface such that perl can be used • http://www.cygwin.com/mirrors.html

Using Perl • Windows • Write a program and save it with .pl extension • C:\perl\bin>perl program_name.pl • Linux • Write a program and save it with .pl extension • userX@machineY$ perl program_name.pl • userX@machineY$ ./program_name.pl • Same under VMware & Cygwin chmod +x program_name.pl

Structure of a basic program First line is special, Path to perl installation this path can be different e.g., /bin/perl #!/usr/bin/perl # Program to do the obvious print 'Hello world.'; # denotes comment, any thing after # till the end of line is comment Statement ends with semicolon Built in function Function argument, in this case a string constant userX@machineY$ perl hello.pl

Variables • Scalar variables • Only one value at a time • List variables • List of values (Arrays)

Scalar Variables • The scalar variable means that it can store only one value. • They should always be preceded with the $ symbol. e.g., $var1 • There is no necessity to declare the variable before hand. (but recommended) • There are no data types such as character or numeric. If you treat the variable as character then it can store a character. If you treat it as string it can store one word . if you treat it as a number it can store one number.

Example scalar #!/perl/bin $x = "100\n"; print $x; $x = $x + 1; print $x; Output: 100 101

List/Array Variables • They are like arrays. It can be considered as a group of scalar variables. • They are always preceded by the @symbol. • eg @items = (“Apple",“Bell",“Chair"); • Like in C the index starts from 0. • If you want the second name you should use $items[1] • Watch the $ symbol here because each element is a scalar variable. • $# Followed by the list variable gives the length of the list variable. • $#items will provide index of last element @items • $len = @items; #will assign length of array to $len Is the result of two statements same$len = @itemsandprint @items ?

Example List/Array #!/perl/bin @myarray = (1721, 2974, “string"); print @myarray; $myarray[0]= “string”; $myarray[1]= “1234”; $myarray[2]= “5646”; print @myarray; print “$myarray[0]” . “$myarray[1]” . “$myarray[0]”;

Operations on Arrays • Push • push adds one or more things to the end of a list • push (@items, “table", “chair"); • push returns the new length of the list • Pop • pop removes and returns the last element • $myitem = pop(@items); • shift, unshift, reverse

Example (Push & Pop) #!/perl/bin @myarray = (1721, 2974, “string"); print “@myarray\n”; push(@myarray,”newval1”,”newval2”); print “@myarray\n”; $popvalue =pop(@myarray); print “$myarray\n”; print “@myarray”;

Operators • Arithmetic • String • Single and Double quotes • Conditional

Arithmetic in Perl $a = 1 + 2; # Add 1 and 2 and store in $a $a = 3 - 4; # Subtract 4 from 3 and store in $a $a = 5 * 6; # Multiply 5 and 6 $a = 7 / 8; # Divide 7 by 8 to give 0.875 $a = 9 ** 10; # Nine to the power of 10, that is, 910 $a = 5 % 2; # Remainder of 5 divided by 2 ++$a; # Increment $a and then return it $a++; # Return $a and then increment it --$a; # Decrement $a and then return it $a--; # Return $a and then decrement it

String and assignment operators $a = $b . $c; # Concatenate $b and $c $a = $b x $c; # $b repeated $c times $a = $b; # Assign $b to $a $a += $b; # Add $b to $a $a -= $b; # Subtract $b from $a $a .= $b; # Append $b onto $a

Single and double quotes • $a = 'apples'; • $b = 'bananas'; • print $a . ' and ' . $b; • prints: apples and bananas • print '$a and $b'; • prints: $a and $b • print "$a and $b"; • prints: apples and bananas

Conditions StringsNumbers eq == #equal to ne != #not equal to lt < #less than gt > #greater than le <= #less then or equal to ge >= #greater then or equal to Logical && #And || #Or ! #negation

Control structures • Loops • Foreach • For • while • Condition • If / else • Subroutines

foreach # Visit each item in turn and call it $myitem @item = (“item1”,”item2”,”item3”); foreach $myitem (@items) { print "$myitem\n"; }

for loops • for loops are just as in C or Java • for ($i = 0; $i < 10; ++$i){ print "$i\n";}

while loops #!/usr/local/bin/perl $a = 1; while ($a != 10) { $a++;}

do..while loops #!/usr/local/bin/perl $a = 1; do { $a++; } while ($a != 10);

if statements if ($a) { print "The string is not empty\n"; } else { print "The string is empty\n"; }

if - elsif statements if (!$a) { print "The string is empty\n"; } elsif (length($a) == 1) { print "The string has one character\n"; } elsif (length($a) == 2) { print "The string has two characters\n"; } else { print "The string has many characters\n"; }

Calling subroutines • Assume you have a subroutine printargs that just prints out its arguments • Subroutine calls: • printargs(“arg1", “arg2"); • Prints: “arg1 arg2" • $returnvalue =printargs(“arg1", “arg2"); • Prints: “arg1 arg2“ • $returnvalue will be assigned two

Defining subroutines • Here's the definition of printargs: sub printargs{ print "@_\n"; } • Parameters are put in the array @_ which can be accessed using • $_[0], $_[1] etc How many parameters are passed to sub routine?

Returning a result • Use return statement sub maximum { if ($_[0] > $_[1]) { return $_[0]; } else { return $_[1]; } } $biggest = maximum(37, 24);

Basic pattern matching • $sentence =~ /the/ • True if $sentence contains "the" • $sentence = "The dog bites.";if ($sentence =~ /the/) # is false • …because Perl is case-sensitive • !~ is "does not contain"

RE special characters . # Any single character except a newline ^ # The beginning of the line or string $ # The end of the line or string * # Zero or more of the last character + # One or more of the last character ? # Zero or one of the last character

RE examples ^.*$ # matches the entire string hi.*bye # matches from "hi" to "bye" inclusive x +y # matches x, one or more blanks, and y ^Dear # matches "Dear" only at beginning bags? # matches "bag" or "bags" hiss+ # matches "hiss", "hisss", "hissss", etc.

Other Topics • Split() and join() • File handling • Perl 5 • Modules • http://www.pageresource.com/cgirec/index2.htm

Practical Extraction & Report Language