如何在不重复的情况下有效地迭代数组中的对？

Question

Assume I have 3 teams that each need to play each other once. How can one efficiently iterate through them to schedule matches without duplicates?

    $teams = range('a', 'c');
    $games = array();
    foreach ($teams as $t1)
        foreach ($teams as $t2)
            if ($t1 != $t2)
                $games[] = $t1 . ' vs. ' . $t2;
    print_r($games);

Results in:

    Array
    (
        [0] => a vs. b
        [1] => a vs. c
        [2] => b vs. a  <-- Duplicate of [0]
        [3] => b vs. c
        [4] => c vs. a  <-- Duplicate of [1]
        [5] => c vs. b  <-- Duplicate of [3]
    )

How can one efficiently avoid the duplicate matches?

The answer should yield:

    Array
    (
        [0] => a vs. b
        [1] => a vs. c
        [2] => b vs. c
    )

Answer 1

Generically:

Container C;
for (i = 0; i < C.size; ++i)
{
  for (j = i + 1; j < C.size; ++j)
  {
    /* have unique unordered set {i, j} */
  }
}

If you want the diagonal entries (i, i), start at j = i.