Ruby array recap

array_recap.rb

arr = []                          # => []
arr = Array.new                   # => []
arr = Array.new(5, "t")           # => ["t", "t", "t", "t", "t"]
arr = Array.new(5) { |i| i **2 }  # => [0, 1, 4, 9, 16]

arr[3] = 'replacement'   # => "replacement"
arr                      # => [0, 1, 4, "replacement", 16]
arr.fetch 0              # => 0
arr.first                # => 0
arr.last                 # => 16
arr.each { |ele| p ele}  # => [0, 1, 4, "replacement", 16]
# Output:
# >> 0
# >> 1
# >> 4
# >> "replacement"
# >> 16

arr.push 'item'  # => [0, 1, 4, "replacement", 16, "item"]
arr.pop          # => "item"

arr.unshift 'another'  # => ["another", 0, 1, 4, "replacement", 16]
arr.shift              # => "another"
arr                    # => [0, 1, 4, "replacement", 16]

arr[0] = '4'             # => "4"
arr.delete('4')          # => "4"
arr.delete('not_there')  # => nil
arr.delete_at(3)         # => 16
arr                      # => [1, 4, "replacement"]
#Note that delete operates in-place. It returns the item deleted not a copy of the resulting array

arr=[0,1,2,3,4,5,6]  # => [0, 1, 2, 3, 4, 5, 6]
arr.drop(3)          # => [3, 4, 5, 6]
# i.e. the return value is the array without the first three elements
arr                  # => [0, 1, 2, 3, 4, 5, 6]
# but the original array is unaffected
# So delete is in-place but drop returns a copy of the array with the items dropped

arr=[0,1,2,3,4,5,6]              # => [0, 1, 2, 3, 4, 5, 6]
arr.drop_while { |ele| ele < 5}  # => [5, 6]

arr.select { |ele| ele > 4}  # => [5, 6]
arr.map { |ele| ele * 2}     # => [0, 2, 4, 6, 8, 10, 12]

arr.reduce(:+)                      # => 21
arr.reduce(2, :+)  # => 23
arr.reduce { |sum, ele| sum + ele }  # => 21
arr.reduce(20) { |sum, ele| sum + ele } # => 41
#Note that inject may also be used in place of reduce