Perl

Practical Extraction and Report Language

Larry Wall developed Perl because awk was inadequate for a report-generating job he was doing.

• Perl is an interpreted language
• 1st line indicates interpreter to use:
  #!/usr/local/bin/perl
• Interpreter parses and compiles the program before executing
• Sources for Perl:
  http://www.perl.com/CPAN

Perl Basics

• Source file is executable (after chmod)
• # starts a comment (continues to end of line)
• Free-format language: whitespace (space, tab, newline) is usually optional
• Perl statements terminated by ;
• Program automatically gets 3 filehandles:
  STDIN, STDOUT, STDERR
• Variables are defined as they are used

Scalar Data

Scalars are numbers and strings:

• All numbers are represented internally as double- precision floats
• Floating point literals the same as for C
• Integer literals include decimal, octal (number begins with 0), and hexadecimal (number begins with 0x or 0X)
• Strings are sequences of characters
• Single-quoted strings may contain any character including newline
• Double-quoted strings interpret variables and escape characters

Scalar Operators

Perl converts values to proper types for operator

• Numeric operators
• Arithmetic operators
  + - * / ** %
• Relational operators
  < <= == >= > !=
• Boolean operators
  || &&
• Auto increment and decrement
  ++ --
• String operators
• Concatenation with ‘.’
  $greeting = ‘Welcome’ . $number;
• String comparisons
  eq ne lt gt le ge
• String repetition with ‘x’
  “la” x 3 # produces “lalala”
• Chop & chomp remove last character of a string
  chop($a); # remove last character
  chomp($a); # only remove newline

Scalar Variables

Scalar variables are always preceded by $

• $ must be followed by a letter
• may thereafter be followed by letters, digits, underscores (up to 255 characters)
• case sensitive

Scalar variables are assigned values as in C

• Regular assignment
  $a = 17 + $b;
  $a = $b = $c = 5;
• Binary assignment
  += -= *= /= .=
• Variables that have not been assigned a value have a value of undef

Lists

A list is ordered scalar data

• Represented literally in ( )
  (1, 2, 3) # list of three values
  ($a + $b, $c . " " . $d) # list of two values
  ( ) # empty list
• Ranges may be represented
  (1 .. 5)       # same as (1, 2, 3, 4, 5)
  (1.3 .. 5.1) # same as (1.3, 2.3, 3.3, 4.3)
  (5 .. 1)       # same as ( )
• Quote word function creates a list of strings cleanly
  qw(Elmo Zoe Telly)

Arrays

An array is a variable that holds a list (i.e., 0 or more scalar values)

• Array variable names always begin with @; follow other rules for scalar variables
• Assignment copies the list over
  @fred = (1, 2, 3);
  @barney = @fred;
  @wilma = (0, @fred, 4);
• Arrays that have not been assigned a value have a value of ( )
• Individual elements of the array are addressed as scalar values, starting at index 0
  $fred[0] = 0; # now @fred is (0,2,3)
• $#fred returns the index value of the last element of @fred
• Assignment to an element beyond the ends of the current array extends the array, filling in unassigned elements with undef

Input and Output

<STDIN> may be used as a scalar value

• Reads a string from standard input up to (and including) the newline character
  $a = <STDIN>;

print prints to standard output

• Prints out argument; parentheses optional
  print ("Hi There!");
  print "Hi There!";
• Concatenates list items
  print ("You are #", $nextnum, " on the list");
• Converts numeric values to strings