0002-Even-Fibonacci-Numbers.py

# Even Fibonacci numbers
# 
# Problem 2
# 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.


# Define function
def even_fibonacci_sum(less_than = 100):
  # set the first and second numbers of the Fibonacci sequence
  first_number = 1
  second_number = 2
  number_sequence = [first_number, second_number]
  next_number = first_number + second_number
  
  while next_number < less_than:
    # add the next number to the sequence
    number_sequence.append(next_number)
    # compute the next number in the sequence
    first_number = second_number
    second_number = next_number
    next_number = first_number + second_number
  
  # create function to detect even numbers
  even_numbers = lambda x: x % 2 == 0
  # filter for even numbers
  even_fibonacci_numbers = list(filter(even_numbers, number_sequence))
  
  # return the sum
  print("The sum of even Fibonacci numbers under", "{:,}".format(less_than), "is:\n", "{:,}".format(sum(even_fibonacci_numbers)))
  
even_fibonacci_sum(4000000)
# The sum of even Fibonacci numbers under 4,000,000 is:
#  4,613,732