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Perl

Colin Brown COMP 205. Perl. Note: Several slides taken/adapted from slides for COMP 461, Spring 2004, by the author. Perl. “Practical Extraction and Report Language” AKA: “Pathologically Eclectic Rubbish Lister” (and others) ‏ Invented by Larry Wall

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Perl

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  1. Colin Brown COMP 205 Perl Note: Several slides taken/adapted from slides for COMP 461, Spring 2004, by the author.

  2. Perl • “Practical Extraction and Report Language” • AKA: “Pathologically Eclectic Rubbish Lister” (and others)‏ • Invented by Larry Wall • Wall’s goal: “Easy things should be easy and hard things should be possible.” • Eclectic blend of features from several languages • Often used as an active Web-server scripting language • Responds to an HTTP request by generating the HTML for the response page • Typically also saves user data and/or retrieves data for the response. • Also popular for building system administration tools • Influence on recent languages such as Ruby

  3. Note All double quotes in these slides should be ", not “ and ”, which may have been accidentally inserted by PowerPoint or OOO Impress

  4. The Role of Perl in Server-Side Response Handling and Page Generation An Aside Browser Server POST or GET HTTP (Web) Server Form Response Page ValidationResult ValidationRequest URL HTTP Parameters Response Page (HTML)‏ JavaScript Code Perl

  5. Checking and Running Your Script • To “compile” your script to check out its syntax: perl –c filename • To run your script: perl filename [ arguments ]

  6. Documentation • http://www.perl.com/doc/manual/html/pod/ • Commonly used: • perldata: data types • perlfunc: built in functions • permod: how modules work • permodinstall: how to install modules • perlop: operators • perlsub: subroutines • perlsyn: general syntax • perlvar: predefined variables Perl Online Documentation

  7. Hello World print "Hello world!\n"; # This is a comment. # This entire line is a comment. Source: http://www.perl.com/pub/a/2000/10/begperl1.html

  8. Perl Variables • $abc – a scalar variable called “abc” • @def – an array variable called “def” • %ghi – a hashtable variable called “ghi” scalar: having a single value

  9. Perl Scalars $i = 5; $pie_flavor = 'apple'; $constitution1776 = "We the People, "; "References" and "file handles" are also scalars Source: http://www.perl.com/pub/a/2000/10/begperl1.html

  10. Arrays @months = ("July", "August", "September"); print $months[0]; # This prints "July". $months[2] = "Smarch"; #We just renamed Sept Array members must be scalars. Note Arrays of more complex stuff must use references. Source: http://www.perl.com/pub/a/2000/10/begperl1.html

  11. Hashs %days_in_month = ( "July" => 31, "August" => 31, "September" => 30 ); print $days_in_month{"September"}; # 30 $days_in_month{"February"} = 29; # leap year. Note Source: http://www.perl.com/pub/a/2000/10/begperl1.html

  12. Statement Delineation print "This is "; print "two statements.\n"; print "But this ", "is only one statement.\n"; Also note: two arguments to the print function. Source: http://www.perl.com/pub/a/2000/10/begperl1.html

  13. For Loops for $i (1,2,3,4,5) { print $i, "\n"; } $ perl for.pl 1 2 3 4 5 Often want $i to be a local variable: for my $i (1,2,3,4,5) ...

  14. Other Forms of for $ perl for.pl a: 1 a: 2 a: 3 b: 1 b: 2 b: 3 c: 1 c: 2 c: 3 d: 1 d: 2 d: 3 for $i (1,2,3) { print "a: $i\n"; } for ($i = 0; $i < 3; $i++ ) { print "b: ", $i+1, "\n"; } for $i ( 1 ... 3) { print "c: $i\n"; } @a = ( 1... 3); for $i (@a) { print "d: $i\n"; }

  15. Sorting • See Sort in perlfunc* • Syntax forms: • sort SUBNAME LIST • sort BLOCK LIST • sort LIST • See <=> and cmp operators. • When comparison subroutine or block is run, $a and $b are set to values to be compared. numeric stringwise * http://www.perl.com/doc/manual/html/pod/perlfunc.html

  16. $ perl sort.pl -1 1 205 3 42 1000 205 Dog cat dog 1000 205 cat Dog dog 1000 205 cat Dog dog Sorting @nums = ( 3, 42, 205, 1, -1); @strs = ( "Dog","cat","dog", "205", "1000"); for $i (sort @nums ) { print "$i\n"; } print "\n"; for $i (sort @strs ) { print "$i\n"; } print "\n"; sub ci { uc($a) cmp uc($b) } for $i ( sort ci @strs ) { print "$i\n"; } print "\n"; for $i ( sort {uc($a) cmp uc($b)} @strs ) { print "$i\n"; }

  17. Parentheses and Function Calls print 1+2+4; # Prints 7. print(1+2) + 4; # Prints 3. print (1+2)+4; # Also prints 3! print +(1+2)+4; # Prints 7. print ((1+2)+4); # Prints 7. "It LOOKS like a function, therefore it IS a function, and precedence doesn't matter." Source: http://www.perl.com/doc/manual/html/pod/perlfunc.html

  18. Here Documents $ perl here.pl The price is 2.3. The price is $Price. hi there lo there I said foo. I said bar. $Price = 2.30; print <<EOF; The price is $Price. EOF print <<'EOF'; The price is $Price. EOF #backquotes: print <<`EOC`; echo hi there echo lo there EOC print <<"foo", <<"bar"; I said foo. foo I said bar. bar Source: http://www.perl.com/doc/manual/html/pod/perldata.html

  19. Here Documents – Not Just for Print Here's a line or two. 23 and here's another. sub myfunc { for my $i (@_) { print "$i\n"; } } myfunc(<<"THIS", 23, <<'THAT'); Here's a line or two. THIS and here's another. THAT What's @_? It's an array containing the arguments to the subprogram. Source: http://www.perl.com/doc/manual/html/pod/perldata.html

  20. Modules • http://www.perl.com/CPAN/modules/index.html • If you're writing a module: the “package” statement • package Abc::Def #file is Abc/Def.pm • If you're using a package: the “use” or “require” statement use Abc::Def; #loads the module require Abc::Def; #makes it accessible #through references like Abc::Def::myFunc

  21. Objects in Perl use XML::XPath; use XML::XPath::XMLParser; my $xpath = XML::XPath->new(filename => 'accounts.xml'); #find all <acct> tags inside <account> my $nodes = $xpath->find('/accounts/acct'); for my $n ($nodes->get_nodelist) { print "Acct:", $n->getAttribute("name"), "\n"; } For doc on Xpath module: http://search.cpan.org/~msergeant/XML-XPath-1.13/XPath.pm

  22. Perl Data Structures • Relatively gentle introduction: perlreftut • Lots of cookbook examples: perldsc • All the details: perlref • Arrays and hashes always contain scalars • Array of hashes: really array of references to hashes • Hash of arrays: really hash of references to arrays

  23. Making a Reference References are scalars. • @a = (1,2,3); • $aref1 = \@a; • $aref2 = [ @a ]; #copies the array • $aref3 = [1,3,4]; #anonymous array • %h = ( a=>1, b=>3, c=>9); • $href1 = \%h; • $href2 = { %h };

  24. “Use Rules” for References • Use Rule 1: Use {$aref} where ever array/hash name can be used. • Example: if $aref = \@a then may use ${$aref}[0] instead of $a[0] • Use Rule 2 • ${$aref}[3] can be abbreviated $aref->[3] • Arrow Rule (multi-dimensional array/hash)‏ • $a[1]->[2] can be abbreviated $a[1][2] • May omit {...} when they contain an atomic scalar • @{$aref} can be abbreviated @$aref • ${$aref}[1] can be abbreviated $$aref[1]

  25. Some Examples from perldsc @AoA = ( [ "fred", "barney" ], [ "george", "jane", "elroy" ], [ "homer", "marge", "bart" ], ); push @AoA, [ split ]; @tmp = somefunc($i); $AoA[$i] = [ @tmp ]; print "elt $i $j is $AoA[$i][$j]\n";

  26. Some Examples from perldsc %HoA = ( flintstones => [ "fred", "barney" ], jetsons => [ "george", "jane", "elroy" ], simpsons => [ "homer", "marge", "bart" ], ); $HoA{$who} = [ @fields ]; push @{ $HoA{"flintstones"} }, "wilma", "betty"; foreach $family ( keys %HoA ) { print "family: "; foreach $i ( 0 .. $#{ $HoA{$family} } ) { print " $i = $HoA{$family}[$i]"; } print "\n"; }

  27. Some Examples from perldsc @AoH = ({ Lead => "fred", Friend => "barney", },{ Lead => "george", Wife => "jane", Son => "elroy", },{ Lead => "homer", Wife => "marge", Son => "bart", } ); $rec = {}; $rec->{$key} = $value; $AoH[0]{pet} = "dino"; $AoH[0]{lead} = "fred"; print "$role=$AoH[$i]{$role} ";

  28. Perl Assignment • Choice of presentation • Generate HTML code • Generate formatted text • Pop up a window using a portable GUI toolkit (e.g., TK)‏ • Choice of data representation • One of the complex data structures • Initialized in code • Read from a file • Parse your XML file from the XSLT assignment • NOT: data hard-coded in print statements

  29. Use Rules for References • Use Rule 1: {$aref} can be used anywhere that the thing pointed to by $aref can be used. • If: $aref = \@a; • Then: ${$aref}[3] is same as $a[3] • Use Rule 2: ${$aref}[3] can be written $aref->[3] • Arrow Rule: $a[1]->[2] can be written $a[1][2] • May omit {...} if content is ref scalar: • @{$aref} can be written @$aref

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