ruby 课堂笔记

Posted 2021-05-16

module Enumerable

  def my_each
    for i in self
      yield(i)
    end  
  end

  def my_reduce(*args, &my_block)

    arg_count = args.length

    if !block_given?

      if arg_count == 1 
        operator, init_value = args.first, self.first

      elsif arg_count == 2 
        init_value, operator = args
      end

      self.my_each do |i|
        init_value = eval("init_value #{operator} #{i}")
      end
      return init_value
    end

    if block_given?

      if arg_count == 0 
        init_value = self.first

      elsif arg_count == 1
        init_value = args.first
      end
      
      self.my_each do |i|
        init_value = my_block.call(init_value, i)
      end
      return init_value
    
    end
    
  end 
end



  p [*1..5].my_reduce(0) {|sum, i| sum + i}
  p (1..5).my_reduce(0, :+)
  p (1..5).my_reduce(:+)
  p (1..5).my_reduce(:*)

# - first pass comments
## - second pass comments


def prime_factors number, factors=[] # solving for 21

  # given line 25/33, number may eventually equal one
  # since a number divided by itself is 1, aka prime
  return factors if number == 1 

  # starting from 2, finds the next factor in a range
  # from 2 to the number we are finding factors for
  # in this case, next factor is the prime # 3
  next_factor = (2..number).find do |possible_factor| # the first factor found will be prime
    number % possible_factor == 0
  end

  # save the factor, 3
  factors << next_factor
  ## the second time through, 7 gets put here too

  #finds the opposing factor ie: 21= 3 * 7
  # our method will find 3, next attemp will be
  # to factor down 7, if we can
  next_attempt = number / next_factor
  ## 7/7 is 1; we will hit our base case at the
  ## beginning of our 3rd time through

  # we pass in our collection of factors, 
  # if it is prime, we will return the factors
  # array which we have been collecting 
  # throughout the stack
  prime_factors( next_attempt , factors ) 
end

p prime_factors 21

# hopefully the descriptive names eliminate the need for comment

def search(collection, the_item_for_which_i_am_searching, the_lower_bounds=0, the_upper_bounds=collection.length)

  the_middle_index = (the_lower_bounds + the_upper_bounds) / 2
  the_middle_item  = collection[the_middle_index]

  return the_middle_index if the_middle_item == the_item_for_which_i_am_searching
  return -1 if the_upper_bounds <= the_lower_bounds

  if the_item_for_which_i_am_searching < the_middle_item
    return search( collection, the_item_for_which_i_am_searching, the_lower_bounds, the_middle_index-1 )
  else
    return search( collection, the_item_for_which_i_am_searching, the_middle_index+1, the_upper_bounds )
  end

end

# bad variable names; for contrast
#
# def search(ary, i, l=0, r=ary.length)
#   mid = (l + r) / 2
#   return mid if ary[mid] == i
#   return -1 if r <= l
#   return i < ary[mid] ? search( ary, i, l, mid-1 ) : search( ary, i, mid+1, r )
# end


p search([1,2,3,5,6,7,8], 4) == -1
p search([1,2,3,5,6,7,8], 0) == -1
p search([1,2,3,5,6,7,8], 9) == -1
p search([1,2,3,5,6,7,8], 6) ==  4