algorithm - Generate all combinations of arbitrary alphabet up to arbitrary length

Question

Say I have an array of arbitrary size holding single characters. I want to compute all possible combinations of those characters up to an arbitrary length.

So lets say my array is [1, 2, 3]. The user-specified length is 2. Then the possible combinations are [11, 22, 33, 12, 13, 23, 21, 31, 32].

I'm having real trouble finding a suitable algorithm that allows arbitrary lengths and not just permutates the array. Oh and while speed is not absolutely critical, it should be reasonably fast too.

Answer 1

Just do an add with carry.

Say your array contained 4 symbols and you want ones of length 3.

Start with 000 (i.e. each symbol on your word = alphabet[0])

Then add up:

000 001 002 003 010 011 ...

The algorithm (given these indices) is just to increase the lowest number. If it reaches the number of symbols in your alphabet, increase the previous number (following the same rule) and set the current to 0.

C++ code:

int N_LETTERS = 4;
char alphabet[] = {'a', 'b', 'c', 'd'};

std::vector<std::string> get_all_words(int length)
{
  std::vector<int> index(length, 0);
  std::vector<std::string> words;

  while(true)
  {
    std::string word(length);
    for (int i = 0; i < length; ++i)
      word[i] = alphabet[index[i]];
    words.push_back(word);

    for (int i = length-1; ; --i)
    { 
      if (i < 0) return words;
      index[i]++;
      if (index[i] == N_LETTERS)
        index[i] = 0;
      else
        break;
    }
  }
}

Code is untested, but should do the trick.