playground_5_euler1.rb

# 1) Go to http://projecteuler.net/ and sign up for an account

# General strategies
# 1) Try to simplify the problem
#   - break the problem up into smaller chunks
#   - substitute small number for really big numbers
#   - only go through one iteration of a loop
#   - try hardcoding variables first, then making them dynamic
# 2) Verbally describe what you want your program to do
# 3) Brainstorm tools that might be applicable to your problem
# 4) Write some pseudo code for the simplest possible case
# 5) Translate the pseudo code into Ruby
# 6) Gradually add complexity until you've solved the problem


# PROBLEM 1
##############################
# If we list all the natural numbers below 10 that are multiples of 3 or 5, we get 3, 5, 6 and 9. The sum of these multiples is 23.

# Find the sum of all the multiples of 3 or 5 below 1000.


# Tools
# - Use modulus to check for divisibility
#   ex) 5%3 should be 2
#   ex) 6%3 should be 0
# - Use find_all

# Strategies
# - If there is a remainder when using a modulus check, stick the number into an array
# - Use find_all with modulus in the condition


# Dan Park's solution
# sum = 0
# a = (1...1000).find_all { |i| i % 3 == 0 || i % 5 == 0 }
# puts a.each { |x| sum += x }
# puts sum


# Travis Loncar's solution
# sum = 0
# 1000.times { |num| num % 3 == 0 || num % 5 == 0 ? (sum += num) : () }
# puts sum




# 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.

# One approach
# start with an empty array
# have the first two elements be the first two elements in the sequence
# find the sum and add it on as the third element
# add one to the index that you're pulling out

# Another approach
# use a while loop
# store three numbers, for the three places we are using
# in the while loop, check if the current number is even
#   if so, add the number to the sum


# Jacob Hart's Solution
# #            First round   Second round
# number_a = 0 # 1           # 1
# number_b = 1 # 1           # 2
# sum = 0
# while number_b < 4_000_000 do
#   if number_b.even?
#     sum += number_b
#   end
#                         # First Second  Third
#   number_c = number_a   # c: 0  # c: 1  # c: 1
#   number_a = number_b   # a: 1  # a: 1  # a: 2
#   number_b += number_c  # b: 1  # b: 2  # b: 3
# end

# puts sum #=> 4613732




# Jacob's Solution with renamed variables
#
# two_before = 0
# one_before = 1
# current_num = two_before + one_before
# sum = 0
# while current_num < 4_000_000 do
#   if current_num.even?
#     sum += current_num
#   end

#   next_num = one_before + current_num
#   one_before = current_num
#   current_num = next_num
# end

# puts sum #=> 4613732



# Eric Chesbrough's solution
#
# array = []
# a1 = 0
# a2 = 1
# sum = 0

# while true
#   curr_term = a1 + a2
#   if (4_000_000 <= curr_term)
#     break
#   end
#   array += [curr_term]
#   a1 = a2
#   a2 = curr_term
# end
# array.find_all { |num| num % 2 == 0 ? (sum += num) : () }

# puts sum