Generate combinations with repeated symbols?

I would like to generate fixed size sequences contained a fixed number of repeated symbols. For example how to generate sequences of size N containing exactly p symbols of one type q symbols of another, etc... where p, q, etc. < N.

In my case I want to generate sequences of size 13 containing 3 symbols denoted x, 2 symbols denoted y and 2 symbols denoted z. Blank spaces are marked with '-'

So it would look like xxxyyzz------, x-xx--y-y-zz- ... etc.

There are C(13, 3)*C(10,2)*C(8,2) ways of generating such sequences but I have no idea how to generate them.

Technically how do you describe this problem? Combinations with repetitions? Where can I find a description of algorithms to solve this? Does Knuth book describe this algorithms? Most of the solutions I find deal with generating permutations or combinations but none deal with repeated elements.

Thanks.

edited Jul 13 2011 at 16:01

asked Jul 13 2011 at 15:05

unknown (google)
21●2

I think this might be more for math.SE Consider multinomial coefficients: en.wikipedia.org/wiki/… – Daniel Moskovich Jul 13 2011 at 17:15

Thanks for the pointer. It led to me to the correct technical description of this problem which is the enumeration of permutations of a multiset. Now I need to find an algorithm to generate this permutations. – unknown (google) Jul 13 2011 at 21:22

There is quite a literature on this. A google search on "permutations of a multiset generation" will give many hits. – Richard Stanley Jul 14 2011 at 0:29