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

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