Arrays

# Build
xs = [1, 2, 3, 4]
ys = Array.new(5, 0)         # [0, 0, 0, 0, 0]
zs = Array.new(3) { |i| i * i }  # [0, 1, 4]
ws = %w[red green blue]       # %w skips quotes & commas

# Index + slice
xs[0]                          # 1
xs[-1]                         # 4 (negative wraps)
xs[1..2]                       # [2, 3] inclusive
xs[1...3]                      # [2, 3] exclusive end
xs.first(2)                    # [1, 2]
xs.last                        # 4
xs.take(2)
xs.drop(2)

# Mutators
xs << 5
xs.push(6)
xs.unshift(0)
xs.pop                         # 6
xs.shift                       # 0
xs.insert(1, 99)
xs.delete_at(0)

# Enumerable — the bread and butter
xs.map { |n| n * n }
xs.select { |n| n.even? }
xs.reject { |n| n.even? }
xs.reduce(:+)                  # sum
xs.sum
xs.min; xs.max; xs.minmax
xs.each_with_index { |n, i| puts "#{i}: #{n}" }
xs.group_by { |n| n % 3 }
xs.partition(&:even?)          # [[evens], [odds]]
xs.uniq
xs.sort_by { |n| -n }

# Multi-array
[1, 2, 3].zip([:a, :b, :c])    # [[1,:a],[2,:b],[3,:c]]
[[1, 2], [3, 4]].flatten        # [1, 2, 3, 4]

# Destructuring
first, *rest = [10, 20, 30, 40]   # first=10, rest=[20,30,40]

xs = [1, 2, 3]
xs << 4
puts xs.first, xs.last, xs.size