PERL

1
PERL
David Buckley - DavidBuckley_at_bigfoot.com
2
Resources

• Web
• http//www.perl.com/ - Perl homepage
• http//www.perl.org/ - Perl institute
• http//www.perlmonks.com/ - Perl Monks
• Books
• Learning Perl (O'Reilly)
• - Schwartz
• Programming Perl (O'Reilly)
• - Wall, Christiansen, Schwartz
• Usenet
• comp.lang.perl.misc
• comp.lang.perl.modules

3
Basic Perl

• Similar syntax to Java curly brackets for
  scope etc.
• Perl is NOT strongly typed casts are
  automatically applied by Perl for you.
• Perl is NOT compiled. This means running a Perl
  program you just wrote is a one-step process.
• Perl can look rather like line noise if you
  don't try to make your code look nice.

4
Hello, World!

• !/usr/bin/perl
• Give the path to Perl, so the operating
• system can run the Perl script.
• use strict Force predeclaration of
• variables
• use warnings Turn on basic warnings
• my a Declare a
• a "Hello, World!\n"
• Assign the value
• "Hello, World\n" to a
• print a Print the value of a to
• STDOUT (the screen)

5
Data Types

• Perl has very few data types
• Scalar
• Holds a single value
• Example a
• Array
• Holds a list of values
• Example _at_a
• Hash
• Holds a list of "named" values
• Example a

6
Scalars

• A scalar variable holds a "single value". This
  could be anything of
• A string (e.g. "Hello, World!")
• An integer (e.g. 4)
• A floating point number (e.g. 3.1415926)
• A reference to something (See later!)
• Perl does not mind too much about the range of
  values. Strings can be as long as you like,
  numbers can take pretty much any value, etc.

7
Arrays

• Arrays hold a list of scalars. The scalars do not
  have to be the same type. Example
• my _at_a ("Example", 2)
• print "a0 a1\n"
• Will output "Example 2".
• Perl arrays are dynamic they automatically grow
  as you add more elements
• a2 "Example"
• a3 3
• print "a2 a3\n"

8
Hashes

• Hashes (also called "associative arrays") hold a
  list of "named" values. The previous example
  might have been written something like
• my a (
• Text gt "Example",
• Value gt 4,
• )
• print "aText aValue\n"
• Hashes are also dynamic we add new elements like
  this
• aNewElement "Example 5"
• print "aNewElement\n"

9
Array Functions

• push _at_array, value1, value2, ...
• Add values to the end of an array.
• value pop _at_array
• Remove values from the end of an array.
• value join(string, _at_array)
• Join all the values of an array together.
• "_at_array" eq join(" ", _at_array)

10
Lists

• Similar to arrays.
• Can be used to assign to many variables
• my (a, b, c) ("Something", 1, 2)
• All functions simply take a list.
• Arrays can act like lists
• push _at_array1, _at_array2 Concatenate _at_array2
• onto _at_array1
• (a, b) _at_array3

11
Comparisons

• Because there's no distinction between a string
  and an integer, Perl has two sets of comparison
  operators
• Strings eq ne ge gt le lt cmp
• Integers ! gt gt lt lt ltgt
• Note that
• 0.0 ne 0 Same as "0.0" ne "0"
• "foobar" "notfoobar" Same as 0 0

12
Flow Control

• First, conditionals
• if (a 1)
• print "\a is 1\n"
• else
• print "\a is not 1\n"
• unless (a 2)
• print "\a is not 2\n"

13
Loops - While

• Perl has a while that works just like the one in
  most other languages
• while (a lt 5)
• a a 1
• print "a\n"
• Equivalently
• until (a 5)
• a a 1
• print "a\n"

14
Loops For

• Perl has a for statement that works just like for
  in a lot of other languages
• for (a 0 a lt 5 a)
• print "a\n"

15
Loops Foreach

• Perl also has an additional constuct, called
  foreach. It loops over each element of a list
• foreach a (0 .. 4)
• print "a\n"
• If you omit the loop variable (a), _ is
  automatically used
• foreach (_at_array)
• print "_\n"

16
Short Syntax

• Shorthand for simple loops.
• Loop goes after statement.
• Examples
• print "\a is not 1\n" unless a 1
• print "\a is \"foo\"\n" if a eq "foo"
• print "a\n" while a lt 5
• print "_\n" foreach (_at_array)

17
References

• Damn useful.
• Like C pointers and Java references.
• References are scalar values.
• \ symbol
• You can make a reference to anything
• ref1 \hash hash ref1
• ref2 \_at_array _at_array _at_ref2
• ref3 \scalar scalar ref3
• A core part of object orientation and data
  structures in Perl.

18
Data Structures

• Implemented using references.
• Can use _at_ etc to dereference, or var-gt.
• Use list and key gt value,key gt value
  to create "anonymous" arrayrefs and hashrefs.
• Example
• my a
• Key gt "This is" ,
• 4,
• "Example", 6 ,
• print "a-gt0Key _at_a-gt2\n"

19
Subroutines

• Simple to use.
• Parameters passed as an array called _at__.
• sub doStuff
• print "I was passed\n"
• print "\t_\n" foreach(_at__)
• doStuff("Parameter 1", "Another Parameter")
• Parameters are lvalues
• sub addOne
• _0 1 return _0
• a 0
• print addOne(a) print a
• print addOne(0) error!

20
A more complex example !/usr/bin/perl use
warnings use strict use DataDumper Include
data structure dumper my players Bucko
gt RealName gt "David Buckley",
Score gt 20, , DeathKiller gt
RealName gt "Fred Bloggs", Score gt
100000, , my _at_inf grep _-gtRealName
/David/ values players print
DataDumperDumper( \_at_inf )
21
Perl -e

• Immensely useful for testing things.
• Examples
• perl -e 'print "_\n" foreach (0 .. 4)'
• perl -e 'print chr(_)."\n" foreach(0..255)'
• perl -e 'sub a print "Hello, World" a()'
• Also -n
• ls perl -ne 'print if /a/'
• And -p
• ls perl -pe 's/foo/bar/'

22
Manual Pages

• perldoc ltmodulegt
• DataDumper Dumps of data structures
• perldoc -f ltfunctiongt
• grep Search through lists
• print Output information
• perldoc ltmanpagegt
• perlre Regular expressions
• perldsc Data structures
• perlreftut References