Simple understanding of PHP bubble sort

There are many kinds of sorting involved in PHP, and Bubble sorting is probably the most troublesome sorting I use. I still don’t understand it after doing it for a long time. This article briefly talks about PHP bubble sorting. The sorting is very suitable for people like me who don’t know much about bubble sorting!

1. Bubble algorithm

I have read several articles about bubbling, but always for each loop# The boundary value idea of ​​## is relatively general.

It is not easy for novices to remember. I usually memorize it by heart.

But for algorithms, it is easy to memorize them by heart, so it is easy to forget after a long time, so I wrote it once and tried to write down every idea as much as possible, so that it is easier to understand. Once you understand it, it will deepen your image and it will not be easy to forget.

Bubble algorithm, the core is

1. Loop comparison

Compare the size of two adjacent arrays each time, and then put the largest array At the end, if all comparisons are cycled once, the largest number in the array will be placed at the end of the array

2. Then repeat the cycle (repeat the above comparison cycle): when looping at this time The last value does not need to participate in the loop, because it has been determined to be the largest one. In other words,

repeats the loop, and the comparison array becomes smaller and smaller. Finally there is only one array element left. End of loop

Look at the code level:

$arr=array(5,4,3,6,7,1,2,10,8,9);

Start with pairwise comparison

if($arr[$i]>$arr[$i+1]){//相邻比较 这个应该比较容易理解吧
        $tem=$arr[$i];
        $arr[$i]=$arr[$i+1];
        $arr[$i+1]=$tem;
    }

The above code is to compare the adjacent value sizes of two arrays , put the big ones at the back. It can be seen that the largest $i is the previous digit of array

index , otherwise $arr[$i+1] does not exist and cannot be compared. That is to say, $i<count($ar)-1;$i’s initial value is 0

Let’s start with the inner comparison loop

General

for loopWrite like this

for($i=0;$i<$xx;$i++){
    if($arr[$i]>$arr[$i+1]){//相邻比较 这个应该比较容易理解吧
        $tem=$arr[$i];
        $arr[$i]=$arr[$i+1];
        $arr[$i+1]=$tem;
    }
}　

As mentioned above, each comparison loop starts from the first element at the beginning, so the starting value of $i is 0, repeat the loop once, and loop next time Just one less element to compare. Here $xx will be getting smaller and smaller; then how to determine this $xx?

$xx What is the value of the first cycle? The above pairwise comparison leads to the conclusion count($ar)-1

$xx What should the last value be? The conclusion drawn from the above pairwise comparison is 1.

That is to say, $xx is count($arr)-$k $k is the cumulative comparison loop, which can be changed to (repeating the loop once per cycle)

for($i=0;$i<count($arr)-$k;$i++){
    if($arr[$i]>$arr[$i+1]){//相邻比较
        $tem=$arr[$i];
        $arr[$i]=$arr[$i+1];
        $arr[$i+1]=$tem;
    }
}　

Look at this $k again Think about it, doesn’t $k increase cumulatively with repeated cycles? Add the repeated loop code as follows

Let’s take a look at how $k determines the boundary value. As mentioned above, the boundary value of $xx is count($arr)-1 to 1. It has been defined. Then the mathematical algorithm $k The boundary value comes out as 1 to count($arr)-1

and is written into the loop, which is $k=1;$k

The summary is

for($k=1;$k<count($arr);$k++){
    for($i=0;$i<count($arr)-$k;$i++){ 
        if($arr[$i]>$[$i+1]){
            $temparr= $arr[$i];
            $arr[$i] =$arr[$i+1];
            $arr[$i+1] = $temparr;
        }
    }
}

I hope everyone can understand it. It is better to practice more. Sometimes we can understand the knowledge that we don’t understand by writing it twice more. Got it!

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