php，一些有效的算法

I have an array in php

$arr = array(
    array('id' => 1, 'par' => 5),
    array('id' => 2, 'par' => 5),
    array('id' => 3, 'par' => 5),
    array('id' => 4, 'par' => 7),
    array('id' => 5, 'par' => 7),
    array('id' => 6, 'par' => 7),
    array('id' => 7, 'par' => 9),
    array('id' => 8, 'par' => 9),
    ...
);

Can anybody know an effective algorithm to get a first indeks of an element which has property $arr[x]['par'] == 7. How to get the first x from the array containing 2000 elements?

Thank you

I used a binary search

$par = 7;
$a = 0;$i = -1;
$b = count($arr);
while(true) {
    if($par == $arr[$a]['par']) { $i = $a; break; }
    if($par == $arr[$m]['par']) { $i = $m; break; }
    if($par == $arr[$b]['par']) { $i = $b; break; }
    if($a == $m || $m == $b) break;
    if($arr[$a]['par'] < $par && $par < $arr[$m]['par']) {
        $b = $m; $m = floor(($a+$b)/2);
    }
    if($arr[$m]['par'] < $parent && $parent < $arr[$b]['par']) {
        $a = $m; $m = floor(($a+$b)/2);
    }
}

That example was more slow, than a $i=0;while($i < $n && $arr[$i]['par'] != $par) $i++; Can be used the array_search instead?

I'm not sure if using an iterator is any faster than doing it by "hand", but you could use a RecursiveArrayIterator - http://php.net/manual/en/class.recursivearrayiterator.php

$arr = array(
    array('id' => 1, 'par' => 5),
    array('id' => 2, 'par' => 5),
    array('id' => 3, 'par' => 5),
    array('id' => 4, 'par' => 7),
    array('id' => 5, 'par' => 7),
    array('id' => 6, 'par' => 7),
    array('id' => 7, 'par' => 9),
    array('id' => 8, 'par' => 9),
);

$arrayIterator = new RecursiveIteratorIterator(new RecursiveArrayIterator($arr));
foreach ($arrayIterator as $subKey=>$subValue) {
        if ($subKey == 'par' && $subValue == 9) {
                $validArray = iterator_to_array($arrayIterator->getSubIterator());
                $id = $validArray['id'];  // This is your return array
                break;
        }
}

But, to be honest, doing it by hand would probably be a lot easier to understand and debug, and for 2000 records - why bother with anything more complex than:

foreach($arr as $subArray) {
    if ($subArray['par'] == 9) {
            $id = $subArray['id'];
            break;
    }
}

If you were handling more records, or had the popularity of Facebook, then start to get serious. But sometimes keeping it simple is the best way.