References

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References
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References

• Analagous (somewhat) to pointers in C/C
• Far less messy, and definitely less dangerous
• Assign a reference to a variable to a scalar
  variable.
• Use the \ to create a reference
• _at_foo (1, 2, 3)
• aref \_at_foo
• aref now contains a reference to the array _at_foo
• Changes to _at_foo will affect array referenced by
  aref
• Changes to the array referenced by aref will
  affect _at_foo

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De-Referencing

• De-reference a reference using the appropriate
  variable symbol (_at_ for array, for hash, etc)
• aref \_at_foo
• _at_new_array _at_aref
• contents of a simple scalar ? can drop them
  _at_aref
• _at_aref can be used like any other array
• push _at_aref, 'val1', 'val2'
• _at_new_array is a different array, which contains
  the same values of members that the array
  referenced by aref contained.
• same as if you'd done _at_new_array _at_foo
• Changes to _at_foo or aref do NOT affect _at_new_array
• Changes to _at_aref DO affect _at_foo

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Referencing Other Types

• You can also reference other kinds of variables
• hash('Paul'gt26, 'Gavin'gt20)
• h \hash
• bar "hello world\n"
• bar_ref \bar

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Anonymous References

• A value need not be contained in a defined
  variable to create a reference.
• To create an anonymous array reference
• use square brackets, instead of parens
• arr 20, 30, 50, "hi!!"
• _at_a _at_arr
• _at_a ? (20, 30, 50, "hi!!")
• For hash references, use curly brackets, instead
  of parens
• hsh'sky'gt'blue','grass'gt'green'
• h hsh
• h ? ('sky' gt 'blue', 'grass' gt 'green')
• To de-reference specific element of references.

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TMTOWTDI

• a_ref 'Hi', 'Hiya', 'Hello'
• _at_a_ref ('Hi', 'Hiya', 'Hello')
• a_ref2 'Hello'
• a_ref2 'Hello'
• a_ref-gt2 'Hello'
• h_refkey value
• h_refkey value
• h_ref-gtkey value
• These are all valid and acceptable. The form you
  choose is whatever looks the best to you.

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Why bother?

• Weeks ago, we briefly mentioned two-dimensional
  arrays
• The mention was "Don't Do That."
• To create a 2D array, create an array of array
  references
• _at_two_d (1, 2, 3, 4, 5, 6)
• two_d1 is a reference to an array containing
  (3, 4)
• _at_two_d1 is an array containing (3, 4)
• two_d10 is the scalar value 3.
• two_d1-gt0 is the same thing
• Any time an arrow separates two brackets ( or
  ), it can be dropped completely
• two_d10 is also the scalar value 3.
• Other arrows CANNOT be dropped
• foo-gt10 is NOT equivalent to foo10

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More Complicated

• Using similar methods, you can create arrays of
  hashes, hashes of arrays, hashes of hashes,
  arrays of arrays of hashes, hashes of hashes of
  arrays, arrays of hashes of arrays, . . . . .
• letters ( lower gt 'a' .. 'z', upper gt
  'A' .. 'Z')
• letterslower is an array reference
• _at_letterslower is an array
• letterslower1 is the scalar value 'b'.
• letterslower-gt1 is the scalar value 'b'
• letterslower1 is the scalar value 'b'

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Schwartzian Transform

• Randal Schwartz devised a method to efficiently
  sort a list based on a computation-intense
  calculation
• To sort a list of files based on file size
• sort -s a ltgt -s b _at_files
• The stat has to be done twice every time two
  files are compared. That's costly!
• _at_sorted map _-gt0sort a-gt1 ltgt
  b-gt1map _, -s _ _at_files
• Now stat is done only once for each filename

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Help available

• perldoc perlreftut
• tutorial on references
• perldoc perllol
• "lists of lists" very inaccurate
• perldoc perldsc
• Data Structures Cookbook building complicated
  structures
• perldoc perlref
• Reference reference.