请帮助我理解这段代码

Question

I have a java project done for school. This is a piece of code which i am having a hard time to understand its logic. Please shed some light on it.

for(int i = 0; i< leftbut.length; i++){
          int randomNumber =(int)(Math.random()*leftbut.length);
             tempNum = leftbut[randomNumber];
            leftbut[randomNumber] = leftbut[i];
            leftbut[i]=tempNum;      

     }

The leftbut in this case is actually an array of 9 buttons. This code is supposed to shuffle the 9 buttons in different positions. I just cant understand how this code works.

Answer 1

The code generate a random permutation of the original array.

However, note that this is biased - it does not generate all permutations in uniform distribution. This thread discusses what is the affect of this bias.

To overcome this issue - you might want to have a look on fisher yates shuffle (which main difference is, to generate a random number in range [i,n) in each iteration instead of in range [0,n).)

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EDIT:
You might better understand it if you encapsulate the assignments in a method:

private static void swap(int[] array, int i, int j) { 
       tempNum = array[j];
       array[j] = array[i];
       array[i]=tempNum; 
}

Now, the code will be simpler to follow:

for(int i = 0; i< leftbut.length; i++) {
          //choose a random index in the array
          int randomNumber =(int)(Math.random()*leftbut.length);
          //swap the element in index i with the random element chosen in the array
          swap(leftbut, i, randomNumber);
}

The idea is you swap() each element in the array with a random index. This random index is chosen randomly from the array, and its index is denoted as randomNumber .
Since you only swap() items around, you can easily prove that the output array is a permutation of the original.

Answer 2

它仅9次随机交换为leftbut数组的元素。

Answer 3

for(int i = 0; i< leftbut.length; i++){

Is a loop, it inizialize variable i to 0, and increment it each loop by 1

int randomNumber =(int)(Math.random()*leftbut.length);

declare the integer variable randomNumber and assign a random value in range 0 - length of array

 tempNum = leftbut[randomNumber];         
 leftbut[randomNumber] = leftbut[i];
 leftbut[i]=tempNum;  

this actually invert the 2 buttons position in the array, the value i become the random one and viceversa