How to implement binary search method in php

PHP implements binary search method

The array required by the binary search method is an ordered array, assuming that our array is an increasing Array, first, we need to find the middle position of the array.

1. To know the middle position, you need to know the starting position and the end position, and then take out the value of the middle position to compare with our value.

2. If the middle value is greater than our given value, it means that our value is before the middle position. At this time, it needs to be divided into two again, because it is before the middle, so the value we need to change is the value of the end position. The value of the end position at this time should be our middle position at this time.

3. On the contrary, if the middle value is smaller than the value we give, it means that the given value is after the middle position. At this time, the value of the latter part needs to be divided into two parts again, because it is after the middle value, so we The value that needs to be changed is the value of the starting position, which should be our middle position at this time until we find the specified value.

4. Or the intermediate value is equal to the initial starting position, or the end position (in this case, the given value is not found)

Let’s use code to implement it

1. Loop implementation

function getValue($num,$arr)
{
//查找数组的中间位置
$length=count($arr);
$start=0;
$end=$length;
$middle=floor(($start+$end)/2);
//循环判断
while($start>$end-1)
{
if($arr[middle]==$num)
{
return middle+1;
}elseif($arr[middle]<$num)
{
//如果当前要查找的值比当前数组的中间值还要打，那么意味着该值在数组的后半段
//所以起始位置变成当前的middle的值，end位置不变。
$start=$middle;
$middle=floor(($start+$end)/2);
}else{
//反之
$end=$middle;
$middle=floor(($start+$end)/2);
}}
return false;
}

2. Recursive implementation

/*
     * 从数组中获取元素值
     * @param1 int $num，要查找的目标值
     * @param2 array $arr，要查找的数组
     * @param3 int $start，查找的起始位置
     * @param4 int $end，查找的结束位置
     * @return mixed，找到了返回位置，没找到返回false
     */
     function getValue4($num,$arr,$start = 0,$end = 100){
        //采用二分法查找
        $middle = floor(($end + $start) / 2);

        //判断
        if($arr[$middle] == $num){
            //已经找到了，递归的出口
            return $middle + 1;
        }elseif($arr[$middle] < $num){
            //要查找的元素在数组的后半段
            $start = $middle + 1;
            //边界值
            if($start >= $end){
                //没有找到，但是已经超出边界值，递归出口
                return false;
            }
            //调用自己去查找：递归点
            return getValue4($num,$arr,$start,$end);    //getValue4($num,$arr,51,100)
        }else{
            //要查找的元素在数组的前半段
            $end = $middle - 1;
            //判断边界值
            if($end < 0)return false;

            //调用自己：递归点
            return getValue4($num,$arr,$start,$end);    //getValue4($num,$arr,0,49)
        }

        //都没有找到
        return false;
     }

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