Hashes

1
Hashes

• a hash is another fundamental data structure,
  like scalars and arrays.
• Hashes are sometimes called associative arrays.
  
• Basically, a hash associates a key with a value.
  A hash is composed of a set of key-value pairs.
• A key is a string any collection of characters,
  generally enclosed in quotes. Any scalar can be
  a key, but they are all converted to strings.
• A value can be almost anything the values are
  just scalar variables.
• One hash oddity neither the keys nor the values
  is sorted or stored in a useful order. The order
  you enter hash items is not related to the order
  with which you retrieve them.

2
Why Use Hashes?

• The C language doesnt have anything like a hash
  in it, and clearly C can do just about anything
  you need to do in programming.
• The point of Perl is to make your life easier, to
  include useful tools, even if they are messy and
  clutter up the language.
• Examples of hash usage
• --keeping count of the number of times a
  word is used in a text, or that a particular
  sub-sequence appears in DNA. Use the word as the
  key and the number of appearances as the value.
• --Associating ID numbers with peoples names
• --Associating protein names with their
  properties.
• And lots more. The hash is a tool that gets used
  very frequently once you understand them.

3
Hash Basics

• The punctuation mark used to denote a hash is
  (percent sign).
• Note that hashes, arrays, and scalars are
  completely different variables. The variables
  cat, _at_cat, and cat are all different and
  independent variables. I dont recommend using
  the same names for different variables, but it is
  legal.
• Hash elements are accessed by enclosing the key
  in curly braces. For example, the hash
  stoplight is populated as follows
• stoplightred stop
• stoplightyellow caution
• stoplightgreen go
• In this hash, red, yellow, and green
  are the keys, and stop, caution, and go are
  the values.
• Each key can refer to only a single value. You
  cant have duplicate keys. If you try, the first
  value will be lost and only the second will work
• stoplightyellow speed up
• print stoplightyellow\n
• prints go faster
• However, different keys can refer to the same
  value without any problem.

4
Alternative Way of Loading Hashes

• A hash really is an array with alternating keys
  and values. You could load a hash by simply
  writing the keys and values the same way as you
  would load an array
• stoplight (red, stop, yellow,
  caution, green, go)
• This method is a bit annoying, because it is easy
  to lose track of keys and values. A better way
  is to use the gt operator (big arrow), which is
  really just a synonym for a comma
• stoplight ( red gt stop,
• yellow gt
  caution,
• green gt go )
• Note the use of newlines here--makes reading
  the code easier.

5
Hash Operators

• keys gives a list of all the keys used in the
  hash. Heres a common use
• foreach key (keys stoplight)
• print key stands for
  stoplightkey\n
• Note that the keys are not returned in a useful
  order. If you want them sorted you could write
• foreach (sort keys stoplight)
• or
• foreach (sort a ltgt b keys
  stoplight)
• Similarly, values gives a list of all the
  values, in some unusual order.
• each is an operator that returns a 2 element
  list consisting of the key and the value. It
  needs to do this in tandem with while
• while ( (key, value) each stoplight)
  
• print key value\n

6
More Hash Operators

• Removing elements in a hash is done with
  delete
• delete stoplightred
• both key and value are removed
• Testing for existence with exists
• exists stoplightred) returns true if
  that key-value pair exists, and false if it
  doesnt.

7
Counting

• Heres a simple use of hashes to get word counts.
  Input is a file of words, one per line.
• while (word ltSTDINgt)
• chomp word
• word_hashword
• Note that word_hash was created implicitly,
  without ever being explicitly declared. This is
  a standard Perl feature, but I will shortly
  discourage its use.
• Also, a nice feature of using hashes for counting
  is that each value is automatically set to 1 the
  first time a new key is encountered. That is,
  you dont have to initialize each key-value pair
  Perl does it for you automatically.