Ruby Or-Equals ||= weirdness

david_albert_is_awesome_factorial.rb

@factorial_results = {}

def factorial(num)
  start = Time.now    
  raise "num has to be greater than 0" if num < 0    

  return 1 if num == 0    

  if @factorial_results.has_key?(num)    
    puts "found it!"        
    puts Time.now - start        
    return @factorial_results[num]        
  end      

  puts "calculating..."    
  result = 1    
  num.downto(1) do |i|    
    result = result * i        
  end      

  @factorial_results[num] = result    

  puts "done!"    

  puts Time.now - start    
  result    
end

ruby_memoization_strangeness.txt

Death-Lair-of-Killer-Bambi:moodphile stickycake$ pry
[1] pry(main)> qwer
NameError: undefined local variable or method `qwer' for main:Object
from (pry):1:in `__pry__'
[2] pry(main)> qwer || 5
NameError: undefined local variable or method `qwer' for main:Object
from (pry):2:in `__pry__'
[3] pry(main)> a = qwer || 5
NameError: undefined local variable or method `qwer' for main:Object
from (pry):3:in `__pry__'
[4] pry(main)> qwer = qwer || 5
=> 5