PHP check if array has duplicates

Question

Ten players and I need to draw player pairs for two rounds. This works fine but problem is that there could be same match in first and second round. Ie:

8 - 10
2 - 5
4 - 3
9 - 7 !!
6 - 1

2 - 4
5 - 10
8 - 1
3 - 6
7 - 9 !!

As you see there's same match twice 9 vs. 7 and 7 vs. 9. That is what I should prevent. Same players should'nt face each other in first two rounds. How to prevent it? This code creates draw:

$lkm = 10;

//-------------- first round:    
$eka = range(1, $lkm);
shuffle($eka);

         for ($key = 0; $key <= $lkm/2-1; $key++) {
                echo $eka[$key] . " - " . $eka[$lkm/2+$key] . "<br>";
            }


//-------------- second round:
echo "<br>";
$toka = range(1, $lkm);
shuffle($toka);

         for ($key = 0; $key <= $lkm/2-1; $key++) {
                echo $toka[$key] . " - " . $toka[$lkm/2+$key] . "<br>";
            }

Will be the total of players even all the time?

Eddie
– Eddie

2018-05-06 16:33:36 +00:00
Commented May 6, 2018 at 16:33 — Eddie
– Eddie, Commented May 6, 2018 at 16:33
In this system yes.

Temporary-Failure
– Temporary-Failure

2018-05-06 16:36:11 +00:00
Commented May 6, 2018 at 16:36 — Temporary-Failure
– Temporary-Failure, Commented May 6, 2018 at 16:36

Eddie · Accepted Answer · 2018-05-06 16:52:22Z

Using shuffle you will lose control on the placement of the array elements. Except of course if shuffle and shuffle it again.

My suggestion is, shuffle once and use different matching logic for round 1 and 2.

Example:

You have the shuffled array:

[0] => 2
[1] => 5
[2] => 10
[3] => 6
[4] => 9
[5] => 4
[6] => 1
[7] => 8
[8] => 3
[9] => 7

On round 1, you can use the adjacent elements to pair. Like:

[0] => 2  -| Pair
[1] => 5  -|

[2] => 10 -| Pair
[3] => 6  -|

[4] => 9  -| Pair
[5] => 4  -|

..... And so on

On round 2: First-half paired with the second half. Elements 0 - 4 is the first half, 5 - 9 is the second. So pair 0 and 5, 1 and 6 etc

[0] => 2------|
[1] => 5      |Pair
[2] => 10     |
[3] => 6 -----|--------|
[4] => 9      |        |Pair
[5] => 4------|        |
[6] => 1               |
[7] => 8               |
[8] => 3---------------|
[9] => 7

Here is the code:

$lkm = 10;                  
$players = range(1, $lkm);
shuffle($players);  //Shuffle only once

//Round 1 - Adjacent in order 1 vs 2, 3 vs 4
$round1 = array();
for ( $key = 0; $key < $lkm; $key = $key + 2 ) {
    //$players[$key] Vs $players[$key + 1]
    $round1[] = array( $players[$key], $players[$key + 1]);
}

//Round 2 - First half vs 2nd half.
$round2 = array();
for ( $key = 0; $key < ($lkm / 2) ; $key++) {
    //$players[$key] Vs $players[ $key + ($lkm / 2) ]
    $round2[] = array( $players[$key], $players[ $key + ($lkm / 2) ]);
}

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