def fib(limit =nil)
f1 = 0
f2 = 1
while (not limit or f2 <= limit)
yield f2
f1,f2 = f2, f1+f2
end
end
evens=[]
fib { |x| if x%2==0 : evens.push(x) end ; if x>4000000: break end}
puts evens.inject {|sum, n| sum+n}
I learned about the awesomeness of 'yield' by doing this problem, so it was totally worth it. yield let me calculate as many fibonacci terms as I needed, but not more, and without having to calculate any term more than once.
