1 ================================================================================
2 
3                                     Arrays
4 
5   An array is a list of scalars. The list can include numbers, strings,
6   references, and file handles. Arrays start with the sigil @. For example:
7 
8 my @langs = ("perl", "C", "ksh");
9 
10   Here's the array @langs in table format:
11 
12   Index | Value
13   ------+-------
14     0   | "perl"
15     1   | "C"
16     2   | "ksh"
17 
18   The first element in the array, "perl", has index 0. The second element
19   "C" has index 1, and the third element "ksh" has index 2. Notice that
20   the first element starts at index 0, not 1.
21 
22   To get the first element "perl" in @langs, we write: $langs[0].
23   To get the second element "C" in @langs, we write: $langs[1].
24   To get the third element "ksh" in @langs, we write: $langs[2].
25 
26   Notice that when we retrieve a scalar from an array, the sigil changes to $.
27   If we want to get the entire array, we use the sigil @: @langs.
28 
29   To find the length of an array, use scalar(). scalar(@langs) is equal to 3.
30 
31 ================================================================================
32 
33                                  Push and Pop
34 
35   push(@array, LIST) lets you add LIST to the end of @array.
36 
37 	my @feedURLs;
38 	push(@feedURLs, "http://example.com/rss.xml");
39 
40   We add the URL to the end of @feedURLs and increase its length by 1.
41 
42   pop(@array, LIST) gives you the last element and removes it from @array.
43 
44 	my $url = pop(@feedURLs);
45 	process($url);
46 
47   We grab the last URL from @feedURLs, remove it from the array, and
48   then assign it to $url. Then, we process ($url).
49 
50 ================================================================================
51 
52                                  foreach Loops
53 
54 foreach my $lang (@langs) {
55 	print "Learn $lang, ";
56 }
57 
58   The foreach loop will *iterate* through each element in the array.
59   In this loop, it will get each string in the array @langs, replace
60   $lang with the string, then print "Learn $lang, "
61 
62   The above code will output: Learn perl, Learn C, Learn ksh, 
63 
64 ================================================================================
65 
66                                    Hashes
67 
68   A hash is like an array, except the index is called a key, and this key
69   is a string instead of a number.
70 
71 my %feedURLs = (
72 	"undeadly" => "http://undeadly.org/cgi?action=rss",
73 	"eff" => "https://www.eff.org/rss/updates.xml",
74 	"hackernews" => "https://news.ycombinator.com/rss",
75 );
76 
77       Key      | Value
78   -------------+-------------------------------------
79   "undeadly"   | "http://undeadly.org/cgi?action=rss"
80   "eff"        | "https://www.eff.org/rss/updates.xml"
81   "hackernews" | "https://news.ycombinator.com/rss"
82 
83   A hash contains *key-value pairs* because each key stores a value.
84   A hash is sometimes called a dictionary in other languages, because
85   the key-value pairs are similar to how a dictionary contains
86   terms and their definitions.
87 
88   Unlike an array, the keys of a hash are not numbers, so there is no real
89   order to the key-value pairs.
90 
91   To refer to a hash itself, we use the sigil %. But if we lookup the
92   value of a key, we use a $ because we're referring to a scalar.
93   $feedURLs{undeadly} will give us the value of the key "undeadly"
94 
95 ================================================================================
96 
97                                 Dumping Data
98 
99   When working with arrays and hashes, you may want to view the data stored
100   inside to help with debugging. We recommend using the module Data::Dumper
101   to dump the contents of arrays and hashes in a structured format:
102 
103 use Data::Dumper <data::Dumper>;
104 
105   Any time you want to dump an array or hash, pass a reference to a
106   hash or array:
107 
108 warn Dumper \@array;
109 warn Dumper \%feedURLs;
110 
111   For example:
112 
113 warn Dumper \%feedURLs;
114 
115 $VAR1 = {
116 	'undeadly' => 'http://undeadly.org/cgi?action=rss',
117 	'eff' => 'https://www.eff.org/rss/updates.xml',
118 	'hackernews' => 'https://news.ycombinator.com/rss',
119 }
120 
121 ================================================================================
122 
123                            Hashes: Keys and Values
124 
125   We use keys() to get a list of all the keys in a hash.
126   keys(%feedURLs) will give us an array with 3 elements:
127   ('undeadly', 'eff', 'hackernews')
128 
129   Note: keys in the hash have no predictable order to them.
130 
131   We use values() to get a list of all the values in a hash.
132   The values will match the same order as the keys. So
133   values(%feedURLs) will give us an array with 3 elements:
134   ('http://undeadly.org/cgi?action=rss',
135    'https://www.eff.org/rss/updates.xml',
136    'https://news.ycombinator.com/rss')
137 
138 ================================================================================
139 
140                                   News Bot
141 
142   Open up ~/thirdbot and follow the instructions to set up your third bot.
143 
144 ================================================================================