相关疑难解决方法(0)

在使用PHP一段时间后,我注意到并非所有PHP内置函数都如预期的那样快.考虑下面两个可能的函数实现,它使用缓存的素数数组来查找数字是否为素数.

//very slow for large $prime_array
$prime_array = array( 2, 3, 5, 7, 11, 13, .... 104729, ... );
$result_array = array();
foreach( $prime_array => $number ) {
    $result_array[$number] = in_array( $number, $large_prime_array );
}

//speed is much less dependent on size of $prime_array, and runs much faster.
$prime_array => array( 2 => NULL, 3 => NULL, 5 => NULL, 7 => NULL,
                       11 => NULL, 13 => NULL, .... 104729 => NULL, ... );
foreach( $prime_array => $number ) …