Detailed explanation of PHP binary search

Binary search Also known as half search, the advantage is that the number of comparisons is less, the search speed is fast, and the average performance is good; its disadvantage is that the table to be looked up is required to be an ordered table, and insertionDeletiondifficulty. Therefore, the binary search method is suitable for ordered lists that do not change frequently but are searched frequently. First, assuming that the elements in the table are arranged in ascending order, compare the keyword recorded in the middle position of the table with the search keyword. If the two are equal, the search is successful; otherwise, use the middle position record to divide the table into two sub-tables, the first and last. If If the keyword recorded in the middle position is greater than the search keyword, then the previous sub-table will be searched further, otherwise the next sub-table will be searched further. Repeat the above process until a record that meets the conditions is found, making the search successful, or until the subtable does not exist, in which case the search fails.

Binary searchmethod requiresarrayis an ordered array

Assuming that our array is an increasing array, first we need to find the array The middle position.

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

two. 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, the value we need to change is the value of the end position. At this time, the value of the end position should be We're in the middle at this point.

three. On the other hand, if the middle value is less 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 again, because it is after the middle value, so the value we need to change is the starting position. value, the value of the starting position at this time should be our middle position at this time until we find the specified value.

Four. Or the intermediate value is equal to the original starting position, or the end position (in this case, the given value is not found), let's use code to implement it~

//循环实现
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;
}

//递归实现
/*
     * 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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