Author Archives: joe

About joe

.NET developer and board game geek located in the greater Atlanta region.

Project Euler: Problem 35 in Ruby

I used my prime generator from Problem 27 for this one. It would have been faster to build the rotation into my generator, but it ran fine without it.

Problem 35

How many circular primes are there below one million?

require 'prime_generator'

primer = 1_000_000

def is_rot_prime? primer, chars
	chars.size.times do |i|
		chars = { |i| chars[i - 1] }
		return false if !primer.is_prime?(chars.join("").to_i)

count = 0
primer.stack.each do |n|
	count += 1 if is_rot_prime? primer, n.to_s.split("")

# subtract 1 because "1" doesn't count
puts count - 1

Speaking of the rotation, the ruby array initialize methods and negative indexers make it a cinch to rotate. How cool is this?

chars = { |i| chars[i - 1] }

Project Euler: Problem 34 in Ruby

Practically every Project Euler problem benefits from memoization, however the issues I ran into with #34 had nothing to do with my algorithm.

The first hurdle was figuring out the upper bound. After scratching around in my notebook, I figured any number I would be looking for would have 7 digits or less. That gives us an upper bound of 9! * 7.

The second hurde took me much, much longer to figure out.

Here’s the secret: 0! = 1

I had taken it for granted that 0! would (of course!) be 0, and I had pre-filled my cache with the number 0. I went round, and round, and round, and round, and round, and round before figuring out (quite accidentally) my error.

It doesn’t make any sense to me, but you can’t argue with math!

Problem 34

Find the sum of all numbers which are equal to the sum of the factorial of their digits.

# cache the digit factorials
factorials = [1]
(1..9).each do |i|
	factorials.push(i * factorials.last)

result = 0
(3..2_540_160).each do |n|
	sum = n.to_s.split("").inject(0) do |sum,n|
		sum + factorials[n.to_i]
	result += n if n == sum
puts result

Runs in just under a minute 🙂

Project Euler : Problem 36 in Ruby

I’ve been ill today, but there’s no better medicine than an easy Project Euler problem! I tried doing some bitwise magic, but in the end my simplest solution proved the fastest as well.

Problem 36

Find the sum of all numbers, less than one million, which are palindromic in base 10 and base 2.

def palindrome? s
	s == s.reverse

sum = (1..1_000_000).inject(0) do |sum, n|
	sum += palindrome?(n.to_s) && palindrome?(n.to_s 2) ? n : 0

puts sum

Project Euler: Problem 33 in Ruby

It’s fugly, but it works. The hardest part was understanding the question. If the longer description didn’t say that there were exactly 4 fractions, I might have gone crazy.

For reals.

Problem 33

Discover all the fractions with an unorthodox cancelling method.

top, bottom = 1, 1

(10..98).each do |i|
	((i/10)..9).each do |jt|
		jt *= 10
		(1..9).each do |jo|
			j = jt + jo
			next if i >= j
			if i % 10 == j / 10 && i.to_f / j == (i / 10).to_f / (j % 10)
				top *= i
				bottom *= j

puts bottom / bottom.gcd(top)

Find the Longest Palindrome in a String

I recently discovered, and one of the more recent posts dealt with one of the greplin programming challenges. I figured since people were posting their code there, then it wouldn’t be too bad for me to post mine here!

Find the Longest Palindrome in a string:

I doubt this is an optimal solution, but I like how it works:

text = "FourscoreandsevenyearsagoourfaathersbroughtforthonthiscontainentanewnationconceivedinzLibertyanddedicatedtothepropositionthatallmenarecreatedequalNowweareengagedinagreahtcivilwartestingwhetherthatnaptionoranynartionsoconceivedandsodedicatedcanlongendureWeareqmetonagreatbattlefiemldoftzhatwarWehavecometodedicpateaportionofthatfieldasafinalrestingplaceforthosewhoheregavetheirlivesthatthatnationmightliveItisaltogetherfangandproperthatweshoulddothisButinalargersensewecannotdedicatewecannotconsecratewecannothallowthisgroundThebravelmenlivinganddeadwhostruggledherehaveconsecrateditfaraboveourpoorponwertoaddordetractTgheworldadswfilllittlenotlenorlongrememberwhatwesayherebutitcanneverforgetwhattheydidhereItisforusthelivingrathertobededicatedheretotheulnfinishedworkwhichtheywhofoughtherehavethusfarsonoblyadvancedItisratherforustobeherededicatedtothegreattdafskremainingbeforeusthatfromthesehonoreddeadwetakeincreaseddevotiontothatcauseforwhichtheygavethelastpfullmeasureofdevotionthatweherehighlyresolvethatthesedeadshallnothavediedinvainthatthisnationunsderGodshallhaveanewbirthoffreedomandthatgovernmentofthepeoplebythepeopleforthepeopleshallnotperishfromtheearth"

def find_longest_palindrome s1, size
	longest = ""

	s1.size.times do |start|
		break if start + size > s1.size
		s2 = s1[start, size].reverse
		if s1.include? s2
			return s2

	find_longest_palindrome s1, size - 1

puts find_longest_palindrome(text, text.length)

I found the site via this post, 10 puzzle websites to sharpen your programming skills. Now that the wedding’s over (pics soon!) I can’t wait to get back to a regular extracurricular programming schedule!

Unit Testing in PowerShell with PSUnit

I did a bit of playing around with wcf, PowerShell and PSUnit the other day.

PSUnit is a unit-testing framework based on NUnit (or should I say JUnit). It was a bit of work to set up but I was really impressed by the speed, robustness, and ISE integration.

It doesn’t make much sense to test application logic through a web service, or even with PowerShell at all, but it’s nice to know there’s such a great tool available. I’ve been having some issues with MSTest and 64 bit assemblies and although I’ll probably end up going with NUnit, it was a fun and educational journey!

Thanks PSUnit Guys!


I had a minor bicycle malfunction that turned into an accident on the Cross Seminole Trail.

To pun it shortly: I fought the ground, and the ground won!


Think I look bad? You should have seen the other guy!

Project Euler : Problem 32 in Ruby

My solution to this is ugly, but like the tar baby the more I mess with it the worse it gets.

I originally tried to find all the 9 digit pandigitals to cycle through, but was able to cut down the processing by a TON after I figured out that there were only 2 possible digit combinations that could satisfy the problem. (x + xxxx = xxxx and xx + xxx = xxxx)


Problem #32

Find the sum of all products whose multiplicand/multiplier/product identity can be written as a 1 through 9 pandigital.

def combo size, current = 0, stack = [], results = {}

  return results[stack.join.to_i] = stack.clone if size == 0

  (1..9).each do |n|
    next if stack.include?(n)
    stack[current] = n
    combo(size - 1, current + 1, stack.clone, results)

  return results

def pandigitals a, b, c
  results = []

  $repo[a].each_pair do |a_num, a_arr|
    $repo[b].each_pair do |b_num, b_arr|
      product = a_num * b_num
      if $repo[c].include?(product)
      	c_arr = $repo[c][product]
        results.push(product) if (a_arr + b_arr + c_arr).uniq.length == 9

  return results

$repo = {
  1 => combo(1),
  2 => combo(2),
  3 => combo(3),
  4 => combo(4)

results = pandigitals(1, 4, 4) + pandigitals(2, 3, 4)

puts results.uniq.inject(:+)