My attempt at CodeEval.com's FizzBizz challenge.

CodeEval-FizzBizz.rb

# FizzBizz
# https://www.codeeval.com/open_challenges/1/
# Each line of input file: X Y Z
# where: X = first divisor, Y = second divisor, Z = number to count up to
# example:
# 3 5 10 => 1 2 F 4 B F 7 8 F B
# 2 7 15 => 1 F 3 F 5 F B F 9 F 11 F 13 FB 15

myfile = ARGV[0]

if File.exists?(myfile)
  File.open(myfile) do |file|
    file.each_line do |line|
      input = line.split(" ").map{ |e| e.to_i }
      next if input.size == 0
      output = []
      1.upto(input[2]) do |i|
        a_char = ""
        a_char << "F" if i % input[0] == 0
        a_char << "B" if i % input[1] == 0
        output << (a_char.empty? ? i : a_char)
      end
      p output.join(' ')
    end
  end
end

Doh! The error is that there are double quotes in the output. Replace the p output.join(' ') with puts output.join(' ') and CodeEval accepts the output.