Perl

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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

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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

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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';

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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

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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

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Pluralities

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

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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

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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";

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Grade Example

• A set of scores for each student:

  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"; }

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Filehandles

• Filehandles are similar to streams
• Use the open function to create a filehandle and attach it to a file
  

  open(SESAME, "filename"); # read from existing file open(SESAME, "<filename"); # (same thing, explicitly) open(SESAME, ">filename"); # create file and write to it open(SESAME, ">>filename"); # append to existing file open(SESAME, "| output-pipe-command"); # set up an output filter open(SESAME, "input-pipe-command |"); # set up an input filter

• The angle operator encloses the filehandle you want to read lines from
  

  print STDOUT "Enter a number: "; # ask for a number $number = <STDIN>; # input the number print STDOUT "The number is $number\n"; # print the number

  • The empty angle operator, <>, reads lines from all the files
    specified on the command line, or STDIN, if none were
    specified
• read does not automatically remove the newline from its input
  line

  chop($number = <STDIN>); # input number and remove newline

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Operators

• Some binary operators
  

  Binary Arithmetic Operators

  Example	Name	Result
  $a + $b	Addition	Sum of $a and $b
  $a * $b	Multiplication	Product of $a and $b
  $a % $b	Modulus	Remainder of $a divided by $b
  $a ** $b	Exponentiation	$a to the power of $b

• String operators
  

  $a = 123; $b = 456; $c = 3; print $a + $b; # prints 579 print $a . $b; # prints 123456 $b = 3; print $a * $c; # prints 369 print $a x $c; # prints 123123123

• Assignment Operators
• Autoincrement and Autodecrement Operators
• Logical Operators
• Comparison Operators
• File Test Operators
  

  Some File Test Operators

  Example	Name	Result
  -e $a	Exists</td	True if file named in $a exists
  -r $a	Readable</td	True if file named in $a is readable
  -w $a	Writable</td	True if file named in $a is writable
  -d $a	Directory</td	True if file named in $a is a directory
  -f $a	File</td	True if file named in $a is a regular file
  -T $a	Text File</td	True if file named in $a is a text file

• Examples
  

  -e "/usr/bin/perl" or warn "Perl is improperly installed\n"; -f "/vmunix" and print "Congrats, we seem to be running BSD Unix\n";

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Control Structures

• What Is Truth?
  • Any string is true except for “” and “0”
  • Any number is true except for 0
    • Any reference is true.
    • Any undefined value is false

  0 # would become the string "0", so false 1 # would become the string "1", so true 10 - 10 # 10-10 is 0, would convert to string "0", so false 0.00 # becomes 0, would convert to string "0", so false "0" # the string "0", so false "" # a null string, so false "0.00" # the string "0.00", neither empty nor exactly "0", so true "0.00" + 0 # the number 0 (coerced by the +), so false. \$a # a reference to $a, so true, even if $a is false undef() # a function returning the undefined value, so false

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Control Structures

• The if, else, elsif and unless statements
  

  unless ($destination eq $home) { print "I'm not going home.\n"; }

  • There is no “elsunless” though
• Iterative (Looping) Constructs
  • The while and until statements
  • The for statement
  • The foreach statement
  • Breaking out: next and last
    

    foreach $user (@users) { if ($user eq "root" or $user eq "lp") { next; } if ($user eq "special") { print "Found the special account.\n"; # do some processing last; } }

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Regular Expressions

• The simplest use of regular expressions is to match a literal expression
  

  while ($line = <FILE>) { if ($line =~ /http:/) { print $line; } }

• The special variable $_
  

  while () { print if /http:/; print if /ftp:/; print if /mailto:/; # What next? }

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Regular Expressions

• Quantifiers – placed after the item being quantified
  • Match at least seven digits, but no more than eleven digits
    

    /\d{7,11}/

• Minimal Matching – use of a question mark after any quantifier
  • Stops at the first colon rather than the last
    

    /.*?:/

• Nailing Things Down – matching a special kind of “nothing” that depends on
  its surroundings
  • \b matches at a word boundary
  • Matches “Fred the Great” but not “Frederick the Great”:
    

    /\bFred\b/

  • Go to the next iteration of LINE loop if this line begins with #
    

    next LINE if $line =~ /^#/;

• Backreferences – use parentheses to remember bits and pieces of what you
  matched
  • Swap the first two words of a string
    

    s/(\S+)\s+(\S+)/$2 $1/

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Regular Expressions Examples

• Examples:
  

  s/^([^ ]+) +([^ ]+)/$2 $1/; # swap first two words /(\w+)\s*=\s*\1/; # match "foo = foo" /.{80,}/; # match line of at least 80 chars /^(\d+\.?\d*|\.\d+)$/; # match valid number

• Pull fields out of a line
  

  if (/Time: (..):(..):(..)/) { $hours = $1; $minutes = $2; $seconds = $3; }

• Pull a substring out of a line
  

  if (/Version: *([0-9.]+)/) { $version = $1; }

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List Processing

• The list on the right side contains exactly four values:
  

  @array = (1 + 2, 3 - 4, 5 * 6, 7 / 8);

• Some operators can produce either a scalar or a list value, depending on
  their context

  sort @guys, @gals, other();

  • Each of the LIST elements produces its list in turn, then
  • all the separate lists are joined together, end to end, into a single
    list.
  • The one-dimensional list is handed off to the function that wanted a
    LIST in the first place – sort
• Filters and such
  

  print reverse sort map {lc} keys %hash;

  • Takes the keys of %hash and returns them to the map function, which
  • lowercases all the keys by applying the lc operator to each of
    them, and passes them to the sort function, which
  • sorts them, and passes them to the reverse function, which
  • reverses the order of the list elements, and passes them to the
    print function, which prints them.

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