gistfile1.rb

#----------------
# euler2.rb
# Thomson Nguyen
#----------------
# I try to learn ruby by solving Project Euler 
# puzzles. (http://projecteuler.net)
#
# Feel free to take a look at my progress and 
# comment, but don't laugh!
# 
# All code subject to MIT License, blah blah
#
#
# Each new term in the Fibonacci sequence is 
# generated by adding the previous two terms. 
# By starting with 1 and 2, the first 10 terms 
# will be:
#
# 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
#
# Find the sum of all the even-valued terms in 
# the sequence which do not exceed four million.
#

# So at first I tried a recursive iterative approach:

def fib(n)
  return n if (0..1).include? n
  fib(n-1) + fib(n-2) if n > 1
end

def fibsum(k)
  i=0
  running=0
while i <= k
  running = running + fib(i)
  i += 1
end
return running-1 # Adhering to problem convention
end  

# But stopped after realizing it would take forever
# to calculate large sums!

# A better way to appraoch this is to use an array to
# keep all of our fib numbers in one place:

def fibarr(n)
  return n if (0..1).include? n
  array = [0, 1]
  n.times do 
    array.push(array[-1] + array[-2]) 
  end
  return array[2...n]
end

# Now we have a linear-time computation of fib numbers 
# in an array. inject is a great rubyism that we can 
# use to sum up the entries in this array:

class Array
  def sum 
    inject() { |sum,element| sum+element }
     end 
  end

# Now we can filter for even numbers and check the 
# 4 million condition...but at this point, I remembered
# every third number in the fib sequence is even.
# 

sum=0;a=1;b=1;c=a+b
 
while c < 4000000 
   sum=sum+c 
   a=b+c 
   b=c+a 
   c=a+b 
end

puts sum