PHP efficiently generates a unique random number

PHP encapsulation method to generate a random array

/*
* array unique_rand( int $min, int $max, int $num )
* 生成一定数量的不重复随机数
* $min 和 $max: 指定随机数的范围
* $num: 指定生成数量
*/
function unique_rand($min, $max, $num) {
//初始化变量为0
$count = 0;
//建一个新数组
$return = array();
while ($count < $num) {
//在一定范围内随机生成一个数放入数组中
$return[] = mt_rand($min, $max);
//去除数组中的重复值用了“翻翻法”，就是用array_flip()把数组的key和value交换两次。这种做法比用 array_unique() 快得多。
$return = array_flip(array_flip($return));
//将数组的数量存入变量count中
$count = count($return);
}
//为数组赋予新的键名
shuffle($return);
return $return;
}

This method obtains random numbers each time and stores them in the array before deduplicating them... Inefficient... I don’t want to use

PHP is efficient Generate m non-repeating random numbers in the n range (m<=n)

Note: It is also mentioned in the book "Programming Pearls", titled "How to efficiently generate m n ranges" Non-repeating random numbers within (m<=n)"

This algorithm very cleverly takes the position of the random number (the subscript of the array), instead of taking the random number itself, each time After getting a random number, it will be excluded from the value range, and next time it will only be picked from the remaining numbers. The random number selection can be completed in one traversal, which is very efficient.

function rand_num($num='200'){
for($i=0;$i<$num;$i++){
$n[$i] = $i;
}
for($i=0;$i<$num;$i++){
$rand = mt_rand($i,$num-1);
//数组 随机数交换下标
if($n[$i] == $i){
$n[$i] = $n[$rand];
$n[$rand] = $i;
}
}
}

1. The first step is to assign a value to each number in the array in the order of its subscript, and obtain an array of $num numeric key values ​​arranged in corresponding order.

2.The second step starts to take the random number $rand in the range [i, $num-1], and uses the obtained random number $rand as the current value in the array. The position key i corresponds to the subscript value $rand, and the value corresponding to the subscript of the key $rand in the array is replaced with i. This is actually a cross-exchange of array key values. The meaning is to exclude the generated random number from the value range [i,$num-1], and next time it will take the value from the remaining numbers [i 1,num-1].

3.The third step, in order to avoid repeated values, only perform alternating operations on the unchanged key-value pairs, that is, at the position where the original array is arranged sequentially (key == value) Perform alternating operations.

4.End.

As shown above, obtain 10 random number partial examples.

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