Examples of recursion in Ruby

recursion_examples.rb

# Template when looking at a recursive function:
#
# If base case, then end recursion
# else reducation case, and call recursively

# Example: Adding together all the elements of an array.
def sum(numbers_array)
  # Base Case: Is the array empty?
  if numbers_array.empty?
    return 0
  
  # Reduction Case: Shrink the numbers_array to get closer to the base case.
  else
    number = numbers_array.pop # numbers_array is now smaller
    return number + sum(numbers_array)
  end
end

sum([1,2,3]) # returns 6

# Example: Counting down
def countdown(n)
  # Base Case: Are we at zero yet?
  return if n.zero?
  puts n
  # Reducation Case: Shrink n to get closer to zero.
  countdown(n-1)
end
countdown(5)
# Outputs:
# 5
# 4
# 3
# 2
# 1