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ERROR: type should be string, got "https://projecteuler.net/problem=3\nProblem:\nThe prime factors of 13195 are 5, 7, 13 and 29.\nWhat is the largest prime factor of the number 600851475143?\nBecause this is a puzzle, I would prefer not to use canned Ruby methods. So here it goes...\nCurrent Logic:\nnum is the number we’re looking for prime factors of.\ncandidate is a potential prime factor\nsqrt is the square root of num \nuntil candidate >= sqrt\n\nI borrowed this idea from Sieves of Eratosthenes for finding prime numbers, where the algorithm checks for divisibility of every number up to the square root of num. candidate is the number to test if num has a divisor. \nif num % candidate == 0\n...\nend\n\nThe goal is to check if num is divisible by anything (has factors). \nIf num is not divisible by candidate, then candidate will increment by 1 until the until statement is true or until num is divisible by candidate.\nIf num is divisible by candidate, then we know candidate is prime and it gets inserted into prime_factor. Then recursion happens to test the newly defined num.\nprime_factors << num\n\nIf the until loop is true, then that num does NOT have a divisor and therefore is prime. As a result, it gets inserted into prime_factors.\nIssue:\nThe problem is not that it's timing out but rather that it's giving the wrong answer. It appears that my code loops more than needed. I added some logging to it. I am not sure why but I think it has something to do with the recursion piece. Admittedly, I never use recursion in my code and wanted to use it to expand my skill set. So recursion in general is fuzzy to me conceptually speaking. Any reading would be helpful too.\nWhat should happen:\nprime_factors = [2,2,19]\nprime_factors.last = 19 \nWhat actual happens:\nprime_factors = [2,2,19,19,38]\nprime_factors.last = 38\nThe whole code:\ndef largest_prime_factor(num,prime_factors)\n puts \"beg fx: num: #{num}, prime_factors: #{prime_factors}\n candidate = 2\n sqrt = Math.sqrt(num)\n loop_count = 0\n until candidate >= sqrt\n if num % candidate == 0\n num = num / candidate\n prime_factors << candidate\n largest_prime_factor(num,prime_factors)\n end\n candidate += 1\n loop_count +=1\n end\n puts \"outside loop: candidate >= sqrt is #{candidate >= sqrt} num: #{num}, prime_factors: #{prime_factors}, candidate: #{candidate}, sqrt: #{sqrt}, loop: #{loop_count}\" \n gets\n prime_factors << num\n prime_factors.last\nend \n\n\nA: So it looks like, as you have suggested, the issue is the recursion logic.\nJust because you call a function recursively doesn't mean that the \"parent\" stops working - he just sits and waits for the \"child\" to finish, and then keeps going. This is where this \"over looping\" is happening. The code is actually not over looping but rather, finishing up.\nYou can see this in your puts statement. Notice, after the loop stops, sqrt increases because script now running the parent code, not the after the recursive piece (child) finishes. \nFor the fix, I did 2 things:\n1. Create a Boolean that indicates that the code block has gone through recursion. If so, run this code, else... run something else.\n2. If candidate is not 2, then increment by 2. This skips testing all even numbers except for 2. There is no need to test for other even numbers since it's not a prime.\ndef largest_prime_factor(num,prime_factors,recursive)\n candidate = 2\n until candidate >= Math.sqrt(num)\n recursive = false\n if num % candidate == 0\n num = num / candidate\n recursive = true\n prime_factors << candidate\n largest_prime_factor(num,prime_factors,recursive)\n end\n break if recursive\n candidate == 2 ? candidate += 1 : candidate += 2\n end\n prime_factors << num unless recursive\n prime_factors.last\nend\n\n"