Using Ruby's inject to calculate a fibonacci number

Another twist to a common interview question. This time we’ll extend Ruby’s Integer base class so that we can do something like this:

  1. >> 0.fib
  2. => 0
  3. >> 1.fib
  4. => 1
  5. >> 29.fib
  6. => 514229


  1. class Integer
  2.  
  3. def fib
  4. return 0 if self == 0
  5. return 1 if self <= 2
  6. (self - 2).times.inject([1, 1]) { |memo, value| memo = [memo.last, (memo.first + memo.last)] }.last
  7. end
  9. end

The standard recursive answer will usually suffice in an interview:

  1. class Integer
  2.  
  3. def fib
  4. return self if self < 2
  5. (self - 2).fib + (self - 1).fib
  6. end
  7.  
  8. end

I’d just expect the candidate to be aware of the limitations of this approach. I like to pose this question during an interview.

Posted on Feb 11, 2011 | filed under Ruby | 0 comments

Setting a private class method in Ruby

In order to set the visibility on a class method in Ruby, you have to set it under the class’ Metaclass. Here’s an example:

  1. class Foo
  2. def self.public
  3. "public"
  4. end
  5.  
  6. private
  7.  
  8. def self.private
  9. "private"
  10. end
  11. end
  1. > Foo.public #=> "public"
  2. > Foo.private #=> "private"

One would expect a NoMethodError when calling private on class Foo, but that isn’t the case. However:

  1. class Bar
  2. # define methods under Bar's Metaclass
  3. class << self
  4. def public
  5. "publc"
  6. end
  7.  
  8. private
  9.  
  10. def private
  11. "private"
  12. end
  13. end
  14. end
  1. > Bar.public #=> "public"
  2. > Bar.private #=> NoMethodError: private method `private' called for Bar:Class
  3. > Bar.send(:private) #=> "private"

Posted on Feb 01, 2011 | filed under Ruby | 5 comments