Arrays act like stacks

a = [1, 2, 3]
a.push "four" #=> [1, 2, 3, "four"]
a.pop         #=> "four"
a             #=> [1, 2, 3]

or like queues

a = [1, 2, 3]
a.push(4)        #=> [1, 2, 3, 4]
a.shift          #=> 1
a                #=> [1, 2, 3]

a.unshift "zero" #=> ["zero", 1, 2, 3]

• push means "enqueue"; shift means "dequeue"

or like sets

a = [1, 2, 3]
a.include?(2)  #=> true

(technically an array is not a set because it doesn't enforce uniqueness)

first and last

a[0]          #=> 1
a.first       #=> 1
a[a.size-1]   #=> 3
a.last        #=> 3

Adding to an array

a = []
a << "x"       #=> ["x"]

<< (pronounced "shovel") does a push -- adds an item to the end of an array -- DESTRUCTIVE!

a = []
a + ["x"]       #=> ["x"]

+ does a concat -- adds two arrays together and makes a new array -- safe!

plus-equals

• for Strings and Arrays, plus-equals uses concat, not append
• that means it makes a copy, but then reassigns the variable
• so it feels destructive, but isn't

a = ["x", "y"]

a + "z"        #=> can't convert String into Array
a + ["z"]      #=> ["x", "y", "z"] (but a is unchanged)
a += ["z"]     #=> ["x", "y", "z"] (and a is changed)

a = b = ["x"]
a += ["y"]
a #=> ["x", "y"]
b #=> ["x"]

size isn't everything

• size and length are synonyms
• both give you the number of items in the array

Accessing array items

Arrays are zero-indexed

fruit = ["apple", "banana", "cherry"]
fruit[0] #=> "apple"
fruit[2] #=> "cherry"
fruit[3] #=> nil

Question: which is better?

zero-based indexing

or

one-based indexing?

Zero Is Better Than One

• If you use 0-based indexing

  • and you always use less than, not <=
  • then you never have to add or subtract one
  • so you have less change of an off-by-one error

• Think of the index as pointing to the space between items

• This allows consistent length and looping semantics
  • It's always "less than the limit"
  • and "the limit is the size"
  • i.e. a[0,3] has 3 things in it, indexed 0,1,2

Fun with Array Indexes

fruit = ["apple", "banana", "cherry", "date"]

negative indexes count from the back

fruit[-1] #=> "date"
fruit[-3] #=> "banana"

range indexes

fruit[1..3]  #=> ["banana", "cherry", "date"]
fruit[1...3] #=> ["banana", "cherry"]

getting out of bounds

>> a = ["apple", "banana", "cherry"]
=> ["apple", "banana", "cherry"]
>> a.length
=> 3
>> a[5]
=> nil
>> a.size
=> 3
>> a
=> ["apple", "banana", "cherry"]

moral: getting past the end returns nil, not error

setting out of bounds

>> a
=> ["apple", "banana", "cherry"]
>> a[9] = "jicama"
=> "jicama"
>> a.size
=> 10
>> a
=> ["apple", "banana", "cherry", nil, nil,
    nil, nil, nil, nil, "jicama"]

moral: setting past the end autofills with nil

Multidimensional arrays (aka matrices)

• No special matrix syntax
• Have to be built up "by hand" as arrays of arrays

times_table = []
4.times do |x|
  times_table[x] = []
  4.times do |y|
    times_table[x][y] = x * y
  end
end

>> times_table
=> [[0, 0, 0, 0], [0, 1, 2, 3], [0, 2, 4, 6], [0, 3, 6, 9]]
>> times_table[2][3]
=> 6
>> times_table[2]
=> [0, 2, 4, 6]

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