PHP array key values ​​are reversed, which has a great impact on performance PK

The performance impact of using array_flip() to reverse the array key values: the execution time increases by O(n^2) with the array size. Practical case: Get the key of an associative array by value.

PHP array key-value reversal: impact on performance and practical cases

In PHP, arrays are a commonly used Data structure, sometimes we need to reverse the key values ​​of an array. This is a common operation, but its impact on performance is often overlooked.

PHP provides the array_flip() function to do this. This function returns a new array in which the keys of the original array are reversed.

Let's compare the performance of using the array_flip() function on arrays of different sizes:

$array1 = range(0, 9999);
$array2 = range(0, 99999);
$array3 = range(0, 999999);

$time_start = microtime(true);
$flipped_array1 = array_flip($array1);
$time_end = microtime(true);
$elapsed_time1 = $time_end - $time_start;

$time_start = microtime(true);
$flipped_array2 = array_flip($array2);
$time_end = microtime(true);
$elapsed_time2 = $time_end - $time_start;

$time_start = microtime(true);
$flipped_array3 = array_flip($array3);
$time_end = microtime(true);
$elapsed_time3 = $time_end - $time_start;

echo "執行時間：\n";
echo "10,000 個元素：$elapsed_time1 秒\n";
echo "100,000 個元素：$elapsed_time2 秒\n";
echo "1,000,000 個元素：$elapsed_time3 秒\n";

From the results, we can see that using array_flip() The execution time of the function increases as the size of the array increases. This is because the array_flip() function uses a nested loop internally, and its complexity is O(n^2).

Practical case

A practical case using the array_flip() function is to reverse the key values ​​​​of an associative array so that the value can be passed to get the key. For example:

$associated_array = [
    "red" => "#FF0000",
    "blue" => "#0000FF",
    "green" => "#00FF00",
];

$flipped_array = array_flip($associated_array);

echo $flipped_array["#00FF00"]; // 輸出：green

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