Perl

Interpreters

• Interpreter - a program that executes other programs.
• perl - a stable, cross platform programming language, and the name of the interpreter
• make - a program for directing recompilation (and other rerun of processes).
• Compiler vs. Interpreter models
  
  

Perl "hello" example

• Simply say

  print "Howdy, world!\n";

• To execute:

  perl -e "print \"Howdy, world\!\n\";"

• Or, edit the file hello to contain the one-liner program above, and type:

  perl hello

Perl language

• Designed to work smoothly in the same ways that natural language works smoothly
• Easy things should be easy, and hard things should be possible
• On the other hand, the camel has not evolved to smell good. Neither has Perl

Perl Poetry

• A perl poem by christo, chris lacy-hulbert, October 12th 2003

  #!/usr/bin/perl untie $me; open $doors; for($ever){ join $me, 'and', $you; last or sleep and die;} map $our $future or kill my $hopes; my %hope = ('to','be','with','you'); open $doors unless $I& break; $Read= each %hope and wait; times; shift and tell; warn my $friends; warn for($they_are_not_safe); $I& sort and study time; wait for($me); close $doors and sin bind (You_and, me); $please= wait and $see; if ($you){ chop( my $words);} my %hopes& break and die ; if($you){split 'from', $me;} $I= 'cry' and die 'inside';

Perl Variables

• Scalar and others

  Variable Syntax

  Type	Character	Example	Is a name for:
  Scalar	$	$cents	An individual value (number or string)
  Array	@	@large	A list of values, keyed by number
  Hash	%	%interest	A group of values, keyed by string
  Subroutine	&	&how	A callable chunk of Perl code
  Typeglob	*	*struck	Everything named struck

• Spring into existence with nothing in them when needed
  • No need to declare
  • Initialized with 0, null, "", depending on the context

Singularities

• Scalars are assigned a new value with the = operator

  $answer = 42; # an integer $pi = 3.14159265; # a "real" number $avocados = 6.02e23; # scientific notation $pet = "Camel"; # string $sign = "I love my $pet"; # string with interpolation $cost = 'It costs $100'; # string without interpolation $thence = $whence; # another variable $x = $moles * $avocados; # an expression $cwd = `pwd`; # string output from a command $exit = system("vi $x"); # numeric status of a command $fido = new Camel "Fido"; # an object

Pluralities

• Array - look something by the number
• Hash - look something by the name

Arrays

• Ordered list

  @home = ("couch", "chair", "table", "stove"); # initialize ($potato, $lift, $tennis, $pipe) = @home; # list ($alpha,$omega) = ($omega,$alpha); # swap

• 0 based

  $home[0] = "couch"; $home[1] = "chair"; $home[2] = "table"; $home[3] = "stove";

• Can push and pop to and from

Hashes

• Each pair of items is interpreted as a key/value pair
• Can be assigned from a list

  %longday = ("Sun", "Sunday", "Mon", "Monday", "Tue", "Tuesday", "Wed", "Wednesday", "Thu", "Thursday", "Fri", "Friday", "Sat", "Saturday");

• Alternatively:

  %longday = ( "Sun" => "Sunday", "Mon" => "Monday", "Tue" => "Tuesday", "Wed" => "Wednesday", "Thu" => "Thursday", "Fri" => "Friday", "Sat" => "Saturday", );

• Its keys can be extracted from using the keys function
  • The keys can sorted using the sort function
• Intuitive access:

  $wife{"Adam"} = "Eve";

Grade Example

• A set of scores for each student in a file called grades:

  Noël 25
  Ben 76
  Clementine 49
  Norm 66
  Chris 92
  Doug 42
  Carol 25
  Ben 12
  Clementine 0
  Norm 66
  ...

• Need to gather all their scores together, determine each student's average, and print them all out in alphabetical order.

  #!/usr/bin/perl open(GRADES, "grades") or die "Can't open grades: $!\n"; while ($line = <GRADES>) { ($student, $grade) = split(" ", $line); $grades{$student} .= $grade . " "; } foreach $student (sort keys %grades) { $scores = 0; $total = 0; @grades = split(" ", $grades{$student}); foreach $grade (@grades) { $total += $grade; $scores++; } $average = $total / $scores; print "$student: $grades{$student}\tAverage: $average\n"; }