Ruby/Array/Push

# A stack is a LIFO (last in, first out) structure. 
# You can use an array like a stack by using the push and pop methods from the Array class. 
fruit = %w[ apple orange banana ]
fruit.pop # => "banana"
p fruit # => ["apple", "orange" ]
fruit.push "mango"
p fruit # => ["apple", "orange", "mango"]

p a = [1,2,3]                             # => [1, 2, 3]
p a << [4, 5, 6]                          # => [1, 2, 3, [4, 5, 6]]
p a << a                                  # => [1, 2, 3, [4, 5, 6], [...]]

a = [1, 2, 3]                           # => [1, 2, 3]
p a << 4.0                                # => [1, 2, 3, 4.0]
p a << "five"                             # => [1, 2, 3, 4.0, "five"]

# You can also use the push method, which is equivalent.
x = []
x << "Word"
puts x
x.push "Word"
puts x

x = []
x << "Word"
puts x