def nil_array(number)
  Array.new(number) { |index| index = nil }
end

def first_element(array)
  array[0]
end

def third_element(array)
  array[2]
end

def last_three_elements(array)
  array.last(3)
end

def add_element(array)
  array.push(1)
end

def remove_last_element(array)
  array.pop
  array 
end

def remove_first_three_elements(array)
  # Step 1: remove the first three elements
  array.shift(3)
  # Step 2: return the array (because Step 1 returns the values of the elements removed)
  array
end

def array_concatenation(original, additional)
  # return an array adding the original and additional array together
  original + additional

end

def array_difference(original, comparison)
  # return an array of elements from the original array that are not in the comparison array
  original.difference(comparison)
end

def empty_array?(array)
  # return true if the array is empty
  array.empty?
end

def reverse(array)
  # return the reverse of the array
  array.reverse
end

def array_length(array)
  # return the length of the array
  array.length
end

def include?(array, value)
  # return true if the array includes the value
  array.include?(value)
end

def join(array, separator)
  # return the result of joining the array with the separator
  array.join(separator)
end
