Fab Fab - 7 months ago 21
Ruby Question

Sum of even Fibonacci numbers?

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, ...

By considering the terms in the Fibonacci sequence whose values do not exceed four million, find the sum of the even-valued terms.

a = [1,2]
upto = 4_000_000

while a[-2] + a[-1] < upto
a << a[-2] + a[-1]
end

sum = 0
a.each { |x| sum += x if x.even? }

puts "The result is #{sum}"


How does this work? What is
a[-2]
? What does that even mean? The negative second index of a or a minus 2? Thanks for the explanation.

Answer

This is negative index. a[-1] and a[-2] are the last two elements of the array a.

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