Question

I have an array with these values:

Array
(
    [0] => Array
    (
        [0] => John
        [1] => 2012-03-29
        [2] => 1
    )
    [1] => Array
    (
        [0] => Doe
        [1] => 2012-03-30
        [2] => 1
    )
    [2] => Array
    (
        [0] => John
        [1] => 2012-03-31
        [2] => 2
    )
    [3] => Array
    (
        [0] => Doe
        [1] => 2012-03-31
        [2] => 5
    )
    [4] => Array
    (
        [0] => John
        [1] => 2012-04-02
        [2] => 5
    )
    [5] => Array
    (
        [0] => John
        [1] => 2012-04-02
        [2] => 21
    )
)

I’m trying to get the maximum value of array[][2] for each date in array[][1].

For example, in the array

Array[2][1] = 2012-03-31 | Array[2][2] = 2
Array[3][1] = 2012-03-31 | Array[3][2] = 5

Out of those two same days I want to keep Array[3] and discard Array[2].

I tried a foreach function:

foreach ($days as $k) {
    $max_vals[] = array($k[0], $k[1], max($k[2]));
}

But found that is not working with the days. How can I sort this array keeping the highest value for each day, and discard the rest?

Answer 1

The following code will iterate across each value in $data, either added newly encountered dates to the $cache or checking if it is larger.

$cache = array();
// Note: $data is the array you provided above, replace with your variable name.
foreach ($data as $k => $v) {
  if (isset($cache[$v[1]])) {
    if ($cache[$v[1]] < $v[2]) {
      $cache[$v[1]] = $v[2];
    }
  } else {
    $cache[$v[1]] = $v[2];
  }
}

When finished, $cache will be an array with the values being dates and the keys the max value of array[][2].