Question

I need a method written in Ruby which computes Variations. I have already written it in Java, but as I’m new to Ruby, there’s just something I’m missing about the Ruby implementation.

The method is supposed to do this:

method(1, "01") => ["0", "1"] <br>
method(2, "01") => ["00", "01", "10", "11"] ...so you get the idea.

note: in the Ruby implementation, I give params like this:

method(2, ["0", 1"])

but that’s not an issue.

My Java implementation:

public static List<String> Variations(int strength, String usableChars) {
    List<String> list = 
        new ArrayList<String>((int) Math.pow(usableChars.length(), strength));

    if (strength == 0) {
        list.add("");
    } else {
        List<String> l = Variations(strength - 1, usableChars);
        for (char c : usableChars.toCharArray()) {
            for (String s : l) {
                list.add(c + s);
            }
        }
    }
    return list;
}

And it’s working fine. But this is my Ruby implementation:

def Variation (strength, arrayOfString)
    array = Array.new(arrayOfString.size**strength)

    if strength == 0
        array << ""
    else
        a = Variation(strength-1, arrayOfString)
        for i in arrayOfString do
            for j in a do
                array << (i + j)
            end
        end
    end
    return array 
end

In this, I keep getting an error message

test.rb:10:in `Variation': can't convert nil into String (TypeError).

Answer 1

In Ruby, arrays grow automatically as needed. So change your array initialization from:

array = Array.new(arrayOfString.size**strength)

to

array = []

To enumerate over each character in a string, instead of

for i in arrayOfString do

do this:

arrayOfString.each_char do |i|

The final result:

#!/usr/bin/ruby1.8

def Variation (strength, arrayOfString)
  array = []
  if strength == 0
    array << ""
  else
    a = Variation(strength - 1, arrayOfString)
    arrayOfString.each_char do |i|
      for j in a do
        array << (i + j)
      end
    end
  end
  return array
end

p Variation(2, '01')    # => ["00", "01", "10", "11"]

each_char is in Ruby >= 1.8.7, or you can get it from the backports gem.