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                                    Arrays

  An array is a list of scalars. The list can include numbers, strings,
  references, and file handles. Arrays start with the sigil @. For example:

my @langs = ("perl", "C", "ksh");

  Here's the array @langs in table format:

  Index | Value
  ------+-------
    0   | "perl"
    1   | "C"
    2   | "ksh"

  The first element in the array, "perl", has index 0. The second element
  "C" has index 1, and the third element "ksh" has index 2. Notice that
  the first element starts at index 0, not 1.

  To get the first element "perl" in @langs, we write: $langs[0].
  To get the second element "C" in @langs, we write: $langs[1].
  To get the third element "ksh" in @langs, we write: $langs[2].

  Notice that when we retrieve a scalar from an array, the sigil changes to $.
  If we want to get the entire array, we use the sigil @: @langs.

  To find the length of an array, use scalar(). scalar(@langs) is equal to 3.

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                                 Push and Pop

  push(@array, LIST) lets you add LIST to the end of @array.

	my @feedURLs;
	push(@feedURLs, "http://example.com/rss.xml");

  We add the URL to the end of @feedURLs and increase its length by 1.

  pop(@array, LIST) gives you the last element and removes it from @array.

	my $url = pop(@feedURLs);
	process($url);

  We grab the last URL from @feedURLs, remove it from the array, and
  then assign it to $url. Then, we process ($url).

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                                 foreach Loops

foreach my $lang (@langs) {
	print "Learn $lang, ";
}

  The foreach loop will *iterate* through each element in the array.
  In this loop, it will get each string in the array @langs, replace
  $lang with the string, then print "Learn $lang, "

  The above code will output: Learn perl, Learn C, Learn ksh, 

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                                   Hashes

  A hash is like an array, except the index is called a key, and this key
  is a string instead of a number.

my %feedURLs = (
	"undeadly" => "http://undeadly.org/cgi?action=rss",
	"eff" => "https://www.eff.org/rss/updates.xml",
	"hackernews" => "https://news.ycombinator.com/rss",
);

      Key      | Value
  -------------+-------------------------------------
  "undeadly"   | "http://undeadly.org/cgi?action=rss"
  "eff"        | "https://www.eff.org/rss/updates.xml"
  "hackernews" | "https://news.ycombinator.com/rss"

  A hash contains *key-value pairs* because each key stores a value.
  A hash is sometimes called a dictionary in other languages, because
  the key-value pairs are similar to how a dictionary contains
  terms and their definitions.

  Unlike an array, the keys of a hash are not numbers, so there is no real
  order to the key-value pairs.

  To refer to a hash itself, we use the sigil %. But if we lookup the
  value of a key, we use a $ because we're referring to a scalar.
  $feedURLs{undeadly} will give us the value of the key "undeadly"

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                                Dumping Data

  When working with arrays and hashes, you may want to view the data stored
  inside to help with debugging. We recommend using the module Data::Dumper
  to dump the contents of arrays and hashes in a structured format:

use Data::Dumper <data::Dumper>;

  Any time you want to dump an array or hash, pass a reference to a
  hash or array:

warn Dumper \@array;
warn Dumper \%feedURLs;

  For example:

warn Dumper \%feedURLs;

$VAR1 = {
	'undeadly' => 'http://undeadly.org/cgi?action=rss',
	'eff' => 'https://www.eff.org/rss/updates.xml',
	'hackernews' => 'https://news.ycombinator.com/rss',
}

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                           Hashes: Keys and Values

  We use keys() to get a list of all the keys in a hash.
  keys(%feedURLs) will give us an array with 3 elements:
  ('undeadly', 'eff', 'hackernews')

  Note: keys in the hash have no predictable order to them.

  We use values() to get a list of all the values in a hash.
  The values will match the same order as the keys. So
  values(%feedURLs) will give us an array with 3 elements:
  ('http://undeadly.org/cgi?action=rss',
   'https://www.eff.org/rss/updates.xml',
   'https://news.ycombinator.com/rss')

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                                  News Bot

  Open up ~/thirdbot and follow the instructions to set up your third bot.

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