How can I iterate through every possible combination of n playing cards

Question

How can I loop through all combinations of n playing cards in a standard deck of 52 cards?

Answer 1

This combinations iterator class is derived from the previous answers posted here.

I did some benchmarks and it is a least 3x faster than any next_combination() function you would have used before.

I wrote the code in MetaTrader mql4 to do testing of triangular arbitrage trading in forex. I think you can port it easily to Java or C++.

class CombinationsIterator
{
private:
	int input_array[];
	int index_array[];
	int m_indices;      // K
	int m_elements;     // N

public:
	CombinationsIterator(int &src_data[], int k)
	{
		m_indices = k;
		m_elements = ArraySize(src_data);
		ArrayCopy(input_array, src_data);
		ArrayResize(index_array, m_indices);

		// create initial combination (0..k-1)
		for (int i = 0; i < m_indices; i++)
		{
			index_array[i] = i;
		}
	}

	// https://stackoverflow.com/questions/5076695
	// bool next_combination(int &item[], int k, int N)
	bool advance()
	{
		int N = m_elements;
		for (int i = m_indices - 1; i >= 0; --i)
		{
			if (index_array[i] < --N)
			{
				++index_array[i];
				for (int j = i + 1; j < m_indices; ++j)
				{
					index_array[j] = index_array[j - 1] + 1;
				}
				return true;
			}
		}
		return false;
	}

	void get(int &items[])
	{
		// fill items[] from input array
		for (int i = 0; i < m_indices; i++)
		{
			items[i] = input_array[index_array[i]];
		}
	}
};

//+------------------------------------------------------------------+
//|                                                                  |
//+------------------------------------------------------------------+
// driver program to test above class

#define N 5
#define K 3

void OnStart()
{
	int x[N] = {1, 2, 3, 4, 5};

	CombinationsIterator comboIt(x, K);

	int items[K];

	do
	{
		comboIt.get(items);

		printf("%s", ArrayToString(items));

	} while (comboIt.advance());

}

Output:

1 2 3 
1 2 4 
1 2 5 
1 3 4 
1 3 5 
1 4 5 
2 3 4 
2 3 5 
2 4 5 
3 4 5